Nodes

CSTNode and its subclasses cover Python’s full grammar in a whitespace-sensitive fashion, forming LibCST’s concrete syntax tree.

Many of these nodes are designed to behave similarly to Python’s abstract syntax tree.

CSTNode

The base node type which all other nodes derive from.

class libcst.CSTNode

validate_types_shallow() → None

Compares the type annotations on a node’s fields with those field’s actual values at runtime. Raises a TypeError is a mismatch is found.

Only validates the current node, not any of it’s children. For a recursive version, see validate_types_deep().

If you’re using a static type checker (highly recommended), this is useless. However, if your code doesn’t use a static type checker, or if you’re unable to statically type your code for some reason, you can use this method to help validate your tree.

Some (non-typing) validation is done unconditionally during the construction of a node. That validation does not overlap with the work that validate_types_deep() does.

validate_types_deep() → None

Like validate_types_shallow(), but recursively validates the whole tree.

property children: Sequence[CSTNode]

The immediate (not transitive) child CSTNodes of the current node. Various properties on the nodes, such as string values, will not be visited if they are not a subclass of CSTNode.

Iterable properties of the node (e.g. an IndentedBlock’s body) will be flattened into the children’s sequence.

The children will always be returned in the same order that they appear lexically in the code.

visit(visitor: CSTTransformer | CSTVisitor) → _CSTNodeSelfT | RemovalSentinel | FlattenSentinel[_CSTNodeSelfT]

Visits the current node, its children, and all transitive children using the given visitor’s callbacks.

with_changes(**changes: Any) → _CSTNodeSelfT

A convenience method for performing mutation-like operations on immutable nodes. Creates a new object of the same type, replacing fields with values from the supplied keyword arguments.

For example, to update the test of an if conditional, you could do:

def leave_If(self, original_node: cst.If, updated_node: cst.If) -> cst.If:
    new_node = updated_node.with_changes(test=new_conditional)
    return new_node

new_node will have the same body, orelse, and whitespace fields as updated_node, but with the updated test field.

The accepted arguments match the arguments given to __init__, however there are no required or positional arguments.

TODO: This API is untyped. There’s probably no sane way to type it using pyre’s current feature-set, but we should still think about ways to type this or a similar API in the future.

deep_clone() → _CSTNodeSelfT

Recursively clone the entire tree. The created tree is a new tree has the same representation but different identity.

>>> tree = cst.parse_expression("1+2")

>>> tree.deep_clone() == tree
False

>>> tree == tree
True

>>> tree.deep_equals(tree.deep_clone())
True

deep_equals(other: CSTNode) → bool

Recursively inspects the entire tree under self and other to determine if the two trees are equal by representation instead of identity (==).

deep_replace(old_node: CSTNode, new_node: CSTNodeT) → _CSTNodeSelfT | CSTNodeT

Recursively replaces any instance of old_node with new_node by identity. Use this to avoid nested with_changes blocks when you are replacing one of a node’s deep children with a new node. Note that if you have previously modified the tree in a way that old_node appears more than once as a deep child, all instances will be replaced.

deep_remove(old_node: CSTNode) → _CSTNodeSelfT | RemovalSentinel

Recursively removes any instance of old_node by identity. Note that if you have previously modified the tree in a way that old_node appears more than once as a deep child, all instances will be removed.

with_deep_changes(old_node: CSTNode, **changes: Any) → _CSTNodeSelfT

A convenience method for applying with_changes to a child node. Use this to avoid chains of with_changes or combinations of deep_replace and with_changes.

The accepted arguments match the arguments given to the child node’s __init__.

TODO: This API is untyped. There’s probably no sane way to type it using pyre’s current feature-set, but we should still think about ways to type this or a similar API in the future.

classmethod field(*args: object, **kwargs: object) → Any

A helper that allows us to easily use CSTNodes in dataclass constructor defaults without accidentally aliasing nodes by identity across multiple instances.

Module

A node that represents an entire python module.

class libcst.Module

Contains some top-level information inferred from the file letting us set correct defaults when printing the tree about global formatting rules. All code parsed with parse_module() will be encapsulated in a module.

body: Sequence[SimpleStatementLine | BaseCompoundStatement]

A list of zero or more statements that make up this module.

header: Sequence[EmptyLine]

Normally any whitespace/comments are assigned to the next node visited, but Module is a special case, and comments at the top of the file tend to refer to the module itself, so we assign them to the Module instead of the first statement in the body.

footer: Sequence[EmptyLine]

Any trailing whitespace/comments found after the last statement.

encoding: str

The file’s encoding format. When parsing a bytes object, this value may be inferred from the contents of the parsed source code. When parsing a str, this value defaults to "utf-8".

This value affects how bytes encodes the source code.

default_indent: str

The indentation of the file, expressed as a series of tabs and/or spaces. This value is inferred from the contents of the parsed source code by default.

default_newline: str

The newline of the file, expressed as \n, \r\n, or \r. This value is inferred from the contents of the parsed source code by default.

has_trailing_newline: bool

Whether the module has a trailing newline or not.

visit(visitor: CSTTransformer | CSTVisitor) → _ModuleSelfT

Returns the result of running a visitor over this module.

Module overrides the default visitor entry point to resolve metadata dependencies declared by ‘visitor’.

property code: str

The string representation of this module, respecting the inferred indentation and newline type.

property bytes: bytes

The bytes representation of this module, respecting the inferred indentation and newline type, using the current encoding.

code_for_node(node: CSTNode) → str

Generates the code for the given node in the context of this module. This is a method of Module, not CSTNode, because we need to know the module’s default indentation and newline formats.

property config_for_parsing: PartialParserConfig

Generates a parser config appropriate for passing to a parse_expression() or parse_statement() call. This is useful when using either parser function to generate code from a string template. By using a generated parser config instead of the default, you can guarantee that trees generated from both statement and expression strings have the same inferred defaults for things like newlines, indents and similar:

module = cst.parse_module("pass\n")
expression = cst.parse_expression("1 + 2", config=module.config_for_parsing)

get_docstring(clean: bool = True) → str | None

Returns a inspect.cleandoc() cleaned docstring if the docstring is available, None otherwise.

Expressions

An expression is anything that represents a value (e.g. it could be returned from a function). All expressions subclass from BaseExpression.

Expression can be parsed with parse_expression() or as part of a statement or module using parse_statement() or parse_module().

class libcst.BaseExpression

An base class for all expressions. BaseExpression contains no fields.

Names and Object Attributes

class libcst.Name

A simple variable name. Names are typically used in the context of a variable access, an assignment, or a deletion.

Dotted variable names (a.b.c) are represented with Attribute nodes, and subscripted variable names (a[b]) are represented with Subscript nodes.

value: str

The variable’s name (or “identifier”) as a string.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.Attribute

An attribute reference, such as x.y.

Note that in the case of x.y.z, the outer attribute will have an attr of z and the value will be another Attribute referencing the y attribute on x:

Attribute(
    value=Attribute(
        value=Name("x")
        attr=Name("y")
    ),
    attr=Name("z"),
)

value: BaseExpression

An expression which, when evaluated, will produce an object with attr as an attribute.

attr: Name

The name of the attribute being accessed on the value object.

dot: Dot

A separating dot. If there’s whitespace between the value and attr, this dot owns it.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

Operations and Comparisons

Operation and Comparison nodes combine one or more expressions with an operator.

class libcst.UnaryOperation

Any generic unary expression, such as not x or -x. UnaryOperation nodes apply a BaseUnaryOp to an expression.

operator: BaseUnaryOp

The unary operator that applies some operation (e.g. negation) to the expression.

expression: BaseExpression

The expression that should be transformed (e.g. negated) by the operator to create a new value.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.BinaryOperation

An operation that combines two expression such as x << y or y + z. BinaryOperation nodes apply a BaseBinaryOp to an expression.

Binary operations do not include operations performed with BaseBooleanOp nodes, such as and or or. Instead, those operations are provided by BooleanOperation.

It also does not include support for comparision operators performed with BaseCompOp, such as <, >=, ==, is, or in. Instead, those operations are provided by Comparison.

left: BaseExpression

The left hand side of the operation.

operator: BaseBinaryOp

The actual operator such as << or + that combines the left and right expressions.

right: BaseExpression

The right hand side of the operation.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.BooleanOperation

An operation that combines two booleans such as x or y or z and w BooleanOperation nodes apply a BaseBooleanOp to an expression.

Boolean operations do not include operations performed with BaseBinaryOp nodes, such as + or <<. Instead, those operations are provided by BinaryOperation.

It also does not include support for comparision operators performed with BaseCompOp, such as <, >=, ==, is, or in. Instead, those operations are provided by Comparison.

left: BaseExpression

The left hand side of the operation.

operator: BaseBooleanOp

The actual operator such as and or or that combines the left and right expressions.

right: BaseExpression

The right hand side of the operation.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.Comparison

A comparison between multiple values such as x < y, x < y < z, or x in [y, z]. These comparisions typically result in boolean values.

Unlike BinaryOperation and BooleanOperation, comparisons are not restricted to a left and right child. Instead they can contain an arbitrary number of ComparisonTarget children.

x < y < z is not equivalent to (x < y) < z or x < (y < z). Instead, it’s roughly equivalent to x < y and y < z.

For more details, see Python’s documentation on comparisons.

# x < y < z

Comparison(
    Name("x"),
    [
        ComparisonTarget(LessThan(), Name("y")),
        ComparisonTarget(LessThan(), Name("z")),
    ],
)

left: BaseExpression

The first value in the full sequence of values to compare. This value will be compared against the first value in comparisions.

comparisons: Sequence[ComparisonTarget]

Pairs of BaseCompOp operators and expression values to compare. These come after left. Each value is compared against the value before and after itself in the sequence.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.ComparisonTarget

A target for a Comparison. Owns the comparison operator and the value to the right of the operator.

operator: BaseCompOp

A comparison operator such as <, >=, ==, is, or in.

comparator: BaseExpression

The right hand side of the comparison operation.

Control Flow

class libcst.Asynchronous

Used by asynchronous function definitions, as well as async for and async with.

whitespace_after: SimpleWhitespace

Any space that appears directly after this async keyword.

class libcst.Await

An await expression. Await expressions are only valid inside the body of an asynchronous FunctionDef or (as of Python 3.7) inside of an asynchronous GeneratorExp nodes.

expression: BaseExpression

The actual expression we need to wait for.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_after_await: BaseParenthesizableWhitespace

Whitespace that appears after the async keyword, but before the inner expression.

class libcst.Yield

A yield expression similar to yield x or yield from fun().

To learn more about the ways that yield can be used in generators, refer to Python’s language reference.

value: BaseExpression | From | None

The value yielded from the generator, in the case of a From clause, a sub-generator to iterate over.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_after_yield: BaseParenthesizableWhitespace | MaybeSentinel

Whitespace after the yield keyword, but before the value.

class libcst.From

A from x stanza in a Yield or Raise.

item: BaseExpression

The expression that we are yielding/raising from.

whitespace_before_from: BaseParenthesizableWhitespace | MaybeSentinel

The whitespace at the very start of this node.

whitespace_after_from: BaseParenthesizableWhitespace

The whitespace after the from keyword, but before the item.

class libcst.IfExp

An if expression of the form body if test else orelse.

If statements are provided by If and Else nodes.

test: BaseExpression

The test to perform.

body: BaseExpression

The expression to evaluate when the test is true.

orelse: BaseExpression

The expression to evaluate when the test is false.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_before_if: BaseParenthesizableWhitespace

Whitespace after the body expression, but before the if keyword.

whitespace_after_if: BaseParenthesizableWhitespace

Whitespace after the if keyword, but before the test clause.

whitespace_before_else: BaseParenthesizableWhitespace

Whitespace after the test expression, but before the else keyword.

whitespace_after_else: BaseParenthesizableWhitespace

Whitespace after the else keyword, but before the orelse expression.

Lambdas and Function Calls

class libcst.Lambda

A lambda expression that creates an anonymous function.

Lambda(
    params=Parameters([Param(Name("arg"))]),
    body=Ellipsis(),
)

Represents the following code:

lambda arg: ...

Named functions statements are provided by FunctionDef.

params: Parameters

The arguments to the lambda. This is similar to the arguments on a FunctionDef, however lambda arguments are not allowed to have an Annotation.

body: BaseExpression

The value that the lambda computes and returns when called.

colon: Colon

The colon separating the parameters from the body.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_after_lambda: BaseParenthesizableWhitespace | MaybeSentinel

Whitespace after the lambda keyword, but before any argument or the colon.

class libcst.Call

An expression representing a function call, such as do_math(1, 2) or picture.post_on_instagram().

Function calls consist of a function name and a sequence of arguments wrapped in Arg nodes.

func: BaseExpression

The expression resulting in a callable that we are to call. Often a Name or Attribute.

args: Sequence[Arg]

The arguments to pass to the resulting callable. These may be a mix of positional arguments, keyword arguments, or “starred” arguments.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation. These are not the parenthesis before and after the list of args, but rather arguments around the entire call expression, such as (( do_math(1, 2) )).

whitespace_after_func: BaseParenthesizableWhitespace

Whitespace after the func name, but before the opening parenthesis.

whitespace_before_args: BaseParenthesizableWhitespace

Whitespace after the opening parenthesis but before the first argument (if there are any). Whitespace after the last argument but before the closing parenthesis is owned by the last Arg if it exists.

class libcst.Arg

A single argument to a Call.

This supports named keyword arguments in the form of keyword=value and variable argument expansion using *args or **kwargs syntax.

value: BaseExpression

The argument expression itself, not including a preceding keyword, or any of the surrounding the value, like a comma or asterisks.

keyword: Name | None

Optional keyword for the argument.

equal: AssignEqual | MaybeSentinel

The equal sign used to denote assignment if there is a keyword.

comma: Comma | MaybeSentinel

Any trailing comma.

star: Literal['', '*', '**']

A string with zero, one, or two asterisks appearing before the name. These are expanded into variable number of positional or keyword arguments.

whitespace_after_star: BaseParenthesizableWhitespace

Whitespace after the star (if it exists), but before the keyword or value (if no keyword is provided).

whitespace_after_arg: BaseParenthesizableWhitespace

Whitespace after this entire node. The Comma node (if it exists) may also store some trailing whitespace.

Literal Values

class libcst.Ellipsis

An ellipsis .... When used as an expression, it evaluates to the Ellipsis constant. Ellipsis are often used as placeholders in code or in conjunction with SubscriptElement.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

Numbers

class libcst.BaseNumber

A type such as Integer, Float, or Imaginary that can be used anywhere that you need to explicitly take any number type.

class libcst.Integer

value: str

A string representation of the integer, such as "100000" or 100_000.

To convert this string representation to an int, use the calculated property evaluated_value.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

property evaluated_value: int

Return an ast.literal_eval() evaluated int of value.

class libcst.Float

value: str

A string representation of the floating point number, such as "0.05", ".050", or "5e-2".

To convert this string representation to an float, use the calculated property evaluated_value.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

property evaluated_value: float

Return an ast.literal_eval() evaluated float of value.

class libcst.Imaginary

value: str

A string representation of the imaginary (complex) number, such as "2j".

To convert this string representation to an complex, use the calculated property evaluated_value.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

property evaluated_value: complex

Return an ast.literal_eval() evaluated complex of value.

Strings

class libcst.BaseString

A type that can be used anywhere that you need to take any string. This includes SimpleString, ConcatenatedString, and FormattedString.

class libcst.SimpleString

Any sort of literal string expression that is not a FormattedString (f-string), including triple-quoted multi-line strings.

value: str

The texual representation of the string, including quotes, prefix characters, and any escape characters present in the original source code , such as r"my string\n". To remove the quotes and interpret any escape characters, use the calculated property evaluated_value.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precidence dictation.

property prefix: str

Returns the string’s prefix, if any exists. The prefix can be r, u, b, br or rb.

property quote: Literal['"', "'", '"""', "'''"]

Returns the quotation used to denote the string. Can be either ', ", ''' or """.

property raw_value: str

Returns the raw value of the string as it appears in source, without the beginning or end quotes and without the prefix. This is often useful when constructing transforms which need to manipulate strings in source code.

property evaluated_value: str | bytes

Return an ast.literal_eval() evaluated str of value.

class libcst.ConcatenatedString

Represents an implicitly concatenated string, such as:

"abc" "def" == "abcdef"

Warning

This is different from two strings joined in a BinaryOperation with an Add operator, and is sometimes viewed as an antifeature of Python.

left: SimpleString | FormattedString

String on the left of the concatenation.

right: SimpleString | FormattedString | ConcatenatedString

String on the right of the concatenation.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precidence dictation.

whitespace_between: BaseParenthesizableWhitespace

Whitespace between the left and right substrings.

property evaluated_value: str | bytes | None

Return an ast.literal_eval() evaluated str of recursively concatenated left and right if and only if both left and right are composed by SimpleString or ConcatenatedString (FormattedString cannot be evaluated).

Formatted Strings (f-strings)

class libcst.FormattedString

An “f-string”. These formatted strings are string literals prefixed by the letter “f”. An f-string may contain interpolated expressions inside curly braces ({ and }).

F-strings are defined in PEP 498 and documented in Python’s language reference.

>>> import libcst as cst
>>> cst.parse_expression('f"ab{cd}ef"')
FormattedString(
    parts=[
        FormattedStringText(
            value='ab',
        ),
        FormattedStringExpression(
            expression=Name(
                value='cd',
                lpar=[],
                rpar=[],
            ),
            conversion=None,
            format_spec=None,
            whitespace_before_expression=SimpleWhitespace(
                value='',
            ),
            whitespace_after_expression=SimpleWhitespace(
                value='',
            ),
        ),
        FormattedStringText(
            value='ef',
        ),
    ],
    start='f"',
    end='"',
    lpar=[],
    rpar=[],
)

parts: Sequence[BaseFormattedStringContent]

A formatted string is composed as a series of FormattedStringText and FormattedStringExpression parts.

start: str

The string prefix and the leading quote, such as f", F', fr", or f""".

end: Literal['"', "'", '"""', "'''"]

The trailing quote. This must match the type of quote used in start.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precidence dictation.

property prefix: str

Returns the string’s prefix, if any exists. The prefix can be f, fr, or rf.

property quote: Literal['"', "'", '"""', "'''"]

Returns the quotation used to denote the string. Can be either ', ", ''' or """.

class libcst.BaseFormattedStringContent

The base type for FormattedStringText and FormattedStringExpression. A FormattedString is composed of a sequence of BaseFormattedStringContent parts.

class libcst.FormattedStringText

Part of a FormattedString that is not inside curly braces ({ or }). For example, in:

f"ab{cd}ef"

ab and ef are FormattedStringText nodes, but {cd} is a FormattedStringExpression.

value: str

The raw string value, including any escape characters present in the source code, not including any enclosing quotes.

class libcst.FormattedStringExpression

Part of a FormattedString that is inside curly braces ({ or }), including the surrounding curly braces. For example, in:

f"ab{cd}ef"

{cd} is a FormattedStringExpression, but ab and ef are FormattedStringText nodes.

An f-string expression may contain conversion and format_spec suffixes that control how the expression is converted to a string. See Python’s language reference for details.

expression: BaseExpression

The expression we will evaluate and render when generating the string.

conversion: str | None

An optional conversion specifier, such as !s, !r or !a.

format_spec: Sequence[BaseFormattedStringContent] | None

An optional format specifier following the format specification mini-language.

whitespace_before_expression: BaseParenthesizableWhitespace

Whitespace after the opening curly brace ({), but before the expression.

whitespace_after_expression: BaseParenthesizableWhitespace

Whitespace after the expression, but before the conversion, format_spec and the closing curly brace (}). Python does not allow whitespace inside or after a conversion or format_spec.

equal: AssignEqual | None

Equal sign for formatted string expression uses self-documenting expressions, such as f"{x=}". See the Python 3.8 release notes.

Collections

Simple Collections

class libcst.Tuple

An immutable literal tuple. Tuples are often (but not always) parenthesized.

Tuple([
    Element(Integer("1")),
    Element(Integer("2")),
    StarredElement(Name("others")),
])

generates the following code:

(1, 2, *others)

elements: Sequence[BaseElement]

A sequence containing all the Element and StarredElement nodes in the tuple.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.BaseList

A base class for List and ListComp, which both result in a list object when evaluated.

lbracket: LeftSquareBracket = Field(name=None,type=None,default=<dataclasses._MISSING_TYPE object>,default_factory=<function CSTNode.field.<locals>.<lambda>>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=<dataclasses._MISSING_TYPE object>,_field_type=None)

rbracket: RightSquareBracket = Field(name=None,type=None,default=<dataclasses._MISSING_TYPE object>,default_factory=<function CSTNode.field.<locals>.<lambda>>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=<dataclasses._MISSING_TYPE object>,_field_type=None)

Brackets surrounding the list.

lpar: Sequence[LeftParen] = ()

rpar: Sequence[RightParen] = ()

Sequence of parenthesis for precedence dictation.

class libcst.List

A mutable literal list.

List([
    Element(Integer("1")),
    Element(Integer("2")),
    StarredElement(Name("others")),
])

generates the following code:

[1, 2, *others]

List comprehensions are represented with a ListComp node.

elements: Sequence[BaseElement]

A sequence containing all the Element and StarredElement nodes in the list.

lbracket: LeftSquareBracket

rbracket: RightSquareBracket

Brackets surrounding the list.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.BaseSet

An abstract base class for Set and SetComp, which both result in a set object when evaluated.

class libcst.Set

A mutable literal set.

Set([
    Element(Integer("1")),
    Element(Integer("2")),
    StarredElement(Name("others")),
])

generates the following code:

{1, 2, *others}

Set comprehensions are represented with a SetComp node.

elements: Sequence[BaseElement]

A sequence containing all the Element and StarredElement nodes in the set.

lbrace: LeftCurlyBrace

rbrace: RightCurlyBrace

Braces surrounding the set.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

Simple Collection Elements

class libcst.BaseElement

An element of a literal list, tuple, or set. For elements of a literal dict, see BaseDictElement.

class libcst.Element

A simple value in a literal List, Tuple, or Set. These a literal collection may also contain a StarredElement.

If you’re using a literal Dict, see DictElement instead.

value: BaseExpression

comma: Comma | MaybeSentinel

A trailing comma. By default, we’ll only insert a comma if one is required.

class libcst.StarredElement

A starred *value element that expands to represent multiple values in a literal List, Tuple, or Set.

If you’re using a literal Dict, see StarredDictElement instead.

If this node owns parenthesis, those parenthesis wrap the leading asterisk, but not the trailing comma. For example:

StarredElement(
    cst.Name("el"),
    comma=cst.Comma(),
    lpar=[cst.LeftParen()],
    rpar=[cst.RightParen()],
)

will generate:

(*el),

value: BaseExpression

comma: Comma | MaybeSentinel

A trailing comma. By default, we’ll only insert a comma if one is required.

lpar: Sequence[LeftParen]

Parenthesis at the beginning of the node, before the leading asterisk.

rpar: Sequence[RightParen]

Parentheses after the value, but before a comma (if there is one).

whitespace_before_value: BaseParenthesizableWhitespace

Whitespace between the leading asterisk and the value expression.

Dictionaries

class libcst.BaseDict

An abstract base class for Dict and DictComp, which both result in a dict object when evaluated.

class libcst.Dict

A literal dictionary. Key-value pairs are stored in elements using DictElement nodes.

It’s possible to expand one dictionary into another, as in {k: v, **expanded}. Expanded elements are stored as StarredDictElement nodes.

Dict([
    DictElement(Name("k1"), Name("v1")),
    DictElement(Name("k2"), Name("v2")),
    StarredDictElement(Name("expanded")),
])

generates the following code:

{k1: v1, k2: v2, **expanded}

elements: Sequence[BaseDictElement]

lbrace: LeftCurlyBrace

rbrace: RightCurlyBrace

Braces surrounding the set or dict.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

Dictionary Elements

class libcst.BaseDictElement

An element of a literal dict. For elements of a list, tuple, or set, see BaseElement.

class libcst.DictElement

A simple key: value pair that represents a single entry in a literal Dict. Dict nodes may also contain a StarredDictElement.

If you’re using a literal List, Tuple, or Set, see Element instead.

key: BaseExpression

value: BaseExpression

comma: Comma | MaybeSentinel

A trailing comma. By default, we’ll only insert a comma if one is required.

whitespace_before_colon: BaseParenthesizableWhitespace

Whitespace after the key, but before the colon in key : value.

whitespace_after_colon: BaseParenthesizableWhitespace

Whitespace after the colon, but before the value in key : value.

class libcst.StarredDictElement

A starred **value element that expands to represent multiple values in a literal Dict.

If you’re using a literal List, Tuple, or Set, see StarredElement instead.

Unlike StarredElement, this node does not own left or right parenthesis, but the value field may still contain parenthesis. This is due to some asymmetry in Python’s grammar.

value: BaseExpression

comma: Comma | MaybeSentinel

A trailing comma. By default, we’ll only insert a comma if one is required.

whitespace_before_value: BaseParenthesizableWhitespace

Whitespace between the leading asterisks and the value expression.

Comprehensions

class libcst.BaseComp

A base class for all comprehension and generator expressions, including GeneratorExp, ListComp, SetComp, and DictComp.

for_in: CompFor

class libcst.BaseSimpleComp

The base class for ListComp, SetComp, and GeneratorExp. DictComp is not a BaseSimpleComp, because it uses key and value.

elt: BaseExpression

The expression evaluated during each iteration of the comprehension. This lexically comes before the for_in clause, but it is semantically the inner-most element, evaluated inside the for_in clause.

for_in: CompFor

The for ... in ... if ... clause that lexically comes after elt. This may be a nested structure for nested comprehensions. See CompFor for details.

class libcst.GeneratorExp

A generator expression. elt represents the value yielded for each item in CompFor.iter.

All for ... in ... and if ... clauses are stored as a recursive CompFor data structure inside for_in.

elt: BaseExpression

The expression evaluated and yielded during each iteration of the generator.

for_in: CompFor

The for ... in ... if ... clause that comes after elt. This may be a nested structure for nested comprehensions. See CompFor for details.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parentheses for precedence dictation. Generator expressions must always be parenthesized. However, if a generator expression is the only argument inside a function call, the enclosing Call node may own the parentheses instead.

class libcst.ListComp

A list comprehension. elt represents the value stored for each item in CompFor.iter.

All for ... in ... and if ... clauses are stored as a recursive CompFor data structure inside for_in.

elt: BaseExpression

The expression evaluated and stored during each iteration of the comprehension.

for_in: CompFor

The for ... in ... if ... clause that comes after elt. This may be a nested structure for nested comprehensions. See CompFor for details.

lbracket: LeftSquareBracket

rbracket: RightSquareBracket

Brackets surrounding the list comprehension.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.SetComp

A set comprehension. elt represents the value stored for each item in CompFor.iter.

All for ... in ... and if ... clauses are stored as a recursive CompFor data structure inside for_in.

elt: BaseExpression

The expression evaluated and stored during each iteration of the comprehension.

for_in: CompFor

The for ... in ... if ... clause that comes after elt. This may be a nested structure for nested comprehensions. See CompFor for details.

lbrace: LeftCurlyBrace

rbrace: RightCurlyBrace

Braces surrounding the set comprehension.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

class libcst.DictComp

A dictionary comprehension. key and value represent the dictionary entry evaluated for each item.

All for ... in ... and if ... clauses are stored as a recursive CompFor data structure inside for_in.

key: BaseExpression

The key inserted into the dictionary during each iteration of the comprehension.

value: BaseExpression

The value associated with the key inserted into the dictionary during each iteration of the comprehension.

for_in: CompFor

The for ... in ... if ... clause that lexically comes after key and value. This may be a nested structure for nested comprehensions. See CompFor for details.

lbrace: LeftCurlyBrace

rbrace: RightCurlyBrace

Braces surrounding the dict comprehension.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_before_colon: BaseParenthesizableWhitespace

Whitespace after the key, but before the colon in key : value.

whitespace_after_colon: BaseParenthesizableWhitespace

Whitespace after the colon, but before the value in key : value.

class libcst.CompFor

One for clause in a BaseComp, or a nested hierarchy of for clauses.

Nested loops in comprehensions are difficult to get right, but they can be thought of as a flat representation of nested clauses.

elt for a in b for c in d if e can be thought of as:

for a in b:
    for c in d:
        if e:
            yield elt

And that would form the following CST:

ListComp(
    elt=Name("elt"),
    for_in=CompFor(
        target=Name("a"),
        iter=Name("b"),
        ifs=[],
        inner_comp_for=CompFor(
            target=Name("c"),
            iter=Name("d"),
            ifs=[
                CompIf(
                    test=Name("e"),
                ),
            ],
        ),
    ),
)

Normal for statements are provided by For.

target: BaseAssignTargetExpression

The target to assign a value to in each iteration of the loop. This is different from GeneratorExp.elt, ListComp.elt, SetComp.elt, and key and value in DictComp, because it doesn’t directly effect the value of resulting generator, list, set, or dict.

iter: BaseExpression

The value to iterate over. Every value in iter is stored in target.

ifs: Sequence[CompIf]

Zero or more conditional clauses that control this loop. If any of these tests fail, the target item is skipped.

if a if b if c

has similar semantics to:

if a and b and c

inner_for_in: CompFor | None

Another CompFor node used to form nested loops. Nested comprehensions can be useful, but they tend to be difficult to read and write. As a result they are uncommon.

asynchronous: Asynchronous | None

An optional async modifier that appears before the for keyword.

whitespace_before: BaseParenthesizableWhitespace

Whitespace that appears at the beginning of this node, before the for and async keywords.

whitespace_after_for: BaseParenthesizableWhitespace

Whitespace appearing after the for keyword, but before the target.

whitespace_before_in: BaseParenthesizableWhitespace

Whitespace appearing after the target, but before the in keyword.

whitespace_after_in: BaseParenthesizableWhitespace

Whitespace appearing after the in keyword, but before the iter.

class libcst.CompIf

A conditional clause in a CompFor, used as part of a generator or comprehension expression.

If the test fails, the current element in the CompFor will be skipped.

test: BaseExpression

An expression to evaluate. When interpreted, Python will coerce it to a boolean.

whitespace_before: BaseParenthesizableWhitespace

Whitespace before the if keyword.

whitespace_before_test: BaseParenthesizableWhitespace

Whitespace after the if keyword, but before the test expression.

Subscripts and Slices

class libcst.Subscript

A indexed subscript reference (Index) such as x[2], a Slice such as x[1:-1], or an extended slice (SubscriptElement) such as x[1:2, 3].

value: BaseExpression

The left-hand expression which, when evaluated, will be subscripted, such as x in x[2].

slice: Sequence[SubscriptElement]

The SubscriptElement to extract from the value.

lbracket: LeftSquareBracket

rbracket: RightSquareBracket

Brackets after the value surrounding the slice.

lpar: Sequence[LeftParen]

rpar: Sequence[RightParen]

Sequence of parenthesis for precedence dictation.

whitespace_after_value: BaseParenthesizableWhitespace

Whitespace after the value, but before the lbracket.

class libcst.BaseSlice

Any slice type that can slot into a SubscriptElement. This node is purely for typing.

class libcst.Index

Any index as passed to a Subscript. In x[2], this would be the 2 value.

value: BaseExpression

The index value itself.

star: Literal['*'] | None

An optional string with an asterisk appearing before the name. This is expanded into variable number of positional arguments. See PEP-646

whitespace_after_star: BaseParenthesizableWhitespace | None

Whitespace after the star (if it exists), but before the value.

class libcst.Slice

Any slice operation in a Subscript, such as 1:, 2:3:4, etc.

Note that the grammar does NOT allow parenthesis around a slice so they are not supported here.

lower: BaseExpression | None

The lower bound in the slice, if present

upper: BaseExpression | None

The upper bound in the slice, if present

step: BaseExpression | None

The step in the slice, if present

first_colon: Colon

The first slice operator

second_colon: Colon | MaybeSentinel

The second slice operator, usually omitted

class libcst.SubscriptElement

Part of a sequence of slices in a Subscript, such as 1:2, 3. This is not used in Python’s standard library, but it is used in some third-party libraries. For example, NumPy uses it to select values and ranges from multi-dimensional arrays.

slice: BaseSlice

A slice or index that is part of a subscript.

comma: Comma | MaybeSentinel

A separating comma, with any whitespace it owns.

Parenthesis, Brackets, and Braces

class libcst.LeftParen

Used by various nodes to denote a parenthesized section. This doesn’t own the whitespace to the left of it since this is owned by the parent node.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this left parenthesis.

class libcst.RightParen

Used by various nodes to denote a parenthesized section. This doesn’t own the whitespace to the right of it since this is owned by the parent node.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly after this left parenthesis.

class libcst.LeftSquareBracket

Used by various nodes to denote a subscript or list section. This doesn’t own the whitespace to the left of it since this is owned by the parent node.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this left square bracket.

class libcst.RightSquareBracket

Used by various nodes to denote a subscript or list section. This doesn’t own the whitespace to the right of it since this is owned by the parent node.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this right square bracket.

class libcst.LeftCurlyBrace

Used by various nodes to denote a dict or set. This doesn’t own the whitespace to the left of it since this is owned by the parent node.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this left curly brace.

class libcst.RightCurlyBrace

Used by various nodes to denote a dict or set. This doesn’t own the whitespace to the right of it since this is owned by the parent node.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this right curly brace.

Statements

Statements represent a “line of code” or a control structure with other lines of code, such as an If block.

All statements subclass from BaseSmallStatement or BaseCompoundStatement.

Statements can be parsed with parse_statement() or as part of a module using parse_module().

Simple Statements

Statements which at most have expressions as child attributes.

class libcst.BaseSmallStatement

Encapsulates a small statement, like del or pass, and optionally adds a trailing semicolon. A small statement is always contained inside a SimpleStatementLine or SimpleStatementSuite. This exists to simplify type definitions and isinstance checks.

semicolon: Semicolon | MaybeSentinel = 1

An optional semicolon that appears after a small statement. This is optional for the last small statement in a SimpleStatementLine or SimpleStatementSuite, but all other small statements inside a simple statement must contain a semicolon to disambiguate multiple small statements on the same line.

class libcst.AnnAssign

An assignment statement such as x: int = 5 or x: int. This only allows for one assignment target unlike Assign but it includes a variable annotation. Also unlike Assign, the assignment target is optional, as it is possible to annotate an expression without assigning to it.

target: BaseAssignTargetExpression

The target that is being annotated and possibly assigned to.

annotation: Annotation

The annotation for the target.

value: BaseExpression | None

The optional expression being assigned to the target.

equal: AssignEqual | MaybeSentinel

The equals sign used to denote assignment if there is a value.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Assert

An assert statement such as assert x > 5 or assert x > 5, 'Uh oh!'

test: BaseExpression

The test we are going to assert on.

msg: BaseExpression | None

The optional message to display if the test evaluates to a falsey value.

comma: Comma | MaybeSentinel

A comma separating test and message, if there is a message.

whitespace_after_assert: SimpleWhitespace

Whitespace appearing after the assert keyword and before the test.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Assign

An assignment statement such as x = y or x = y = z. Unlike AnnAssign, this does not allow type annotations but does allow for multiple targets.

targets: Sequence[AssignTarget]

One or more targets that are being assigned to.

value: BaseExpression

The expression being assigned to the targets.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.AugAssign

An augmented assignment statement, such as x += 5.

target: BaseAssignTargetExpression

Target that is being operated on and assigned to.

operator: BaseAugOp

The augmented assignment operation being performed.

value: BaseExpression

The value used with the above operator to calculate the new assignment.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Break

Represents a break statement, which is used to break out of a For or While loop early.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Continue

Represents a continue statement, which is used to skip to the next iteration in a For or While loop.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Del

Represents a del statement. del is always followed by a target.

target: BaseDelTargetExpression

The target expression will be deleted. This can be a name, a tuple, an item of a list, an item of a dictionary, or an attribute.

whitespace_after_del: SimpleWhitespace

The whitespace after the del keyword.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Expr

An expression used as a statement, where the result is unused and unassigned. The most common place you will find this is in function calls where the return value is unneeded.

value: BaseExpression

The expression itself. Python will evaluate the expression but not assign the result anywhere.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Global

A global statement.

names: Sequence[NameItem]

A list of one or more names.

whitespace_after_global: SimpleWhitespace

Whitespace appearing after the global keyword and before the first name.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Import

An import statement.

names: Sequence[ImportAlias]

One or more names that are being imported, with optional local aliases.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

whitespace_after_import: SimpleWhitespace

The whitespace that appears after the import keyword but before the first import alias.

class libcst.ImportFrom

A from x import y statement.

module: Attribute | Name | None

Name or Attribute node representing the module we’re importing from. This is optional as ImportFrom allows purely relative imports.

names: Sequence[ImportAlias] | ImportStar

One or more names that are being imported from the specified module, with optional local aliases.

relative: Sequence[Dot]

Sequence of Dot nodes indicating relative import level.

lpar: LeftParen | None

Optional open parenthesis for multi-line import continuation.

rpar: RightParen | None

Optional close parenthesis for multi-line import continuation.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

whitespace_after_from: SimpleWhitespace

The whitespace that appears after the from keyword but before the module and any relative import dots.

whitespace_before_import: SimpleWhitespace

The whitespace that appears after the module but before the import keyword.

whitespace_after_import: SimpleWhitespace

The whitespace that appears after the import keyword but before the first import name or optional left paren.

class libcst.Nonlocal

A nonlocal statement.

names: Sequence[NameItem]

A list of one or more names.

whitespace_after_nonlocal: SimpleWhitespace

Whitespace appearing after the global keyword and before the first name.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Pass

Represents a pass statement.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Raise

A raise exc or raise exc from cause statement.

exc: BaseExpression | None

The exception that we should raise.

cause: From | None

Optionally, a from cause clause to allow us to raise an exception out of another exception’s context.

whitespace_after_raise: SimpleWhitespace | MaybeSentinel

Any whitespace appearing between the raise keyword and the exception.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

class libcst.Return

Represents a return or a return x statement.

value: BaseExpression | None

The optional expression that will be evaluated and returned.

whitespace_after_return: SimpleWhitespace | MaybeSentinel

Optional whitespace after the return keyword before the optional value expression.

semicolon: Semicolon | MaybeSentinel

Optional semicolon when this is used in a statement line. This semicolon owns the whitespace on both sides of it when it is used.

Compound Statements

Statements that have one or more statement blocks as a child attribute.

class libcst.BaseCompoundStatement

Encapsulates a compound statement, like if True: pass or while True: pass. This exists to simplify type definitions and isinstance checks.

Compound statements contain (groups of) other statements; they affect or control the execution of those other statements in some way. In general, compound statements span multiple lines, although in simple incarnations a whole compound statement may be contained in one line.

—https://docs.python.org/3/reference/compound_stmts.html

body: BaseSuite

The body of this compound statement.

leading_lines: Sequence[EmptyLine]

Any empty lines or comments appearing before this statement.

class libcst.ClassDef

A class definition.

name: Name

The class name.

body: BaseSuite

The class body.

bases: Sequence[Arg]

Sequence of base classes this class inherits from.

keywords: Sequence[Arg]

Sequence of keywords, such as “metaclass”.

decorators: Sequence[Decorator]

Sequence of decorators applied to this class.

lpar: LeftParen | MaybeSentinel

Optional open parenthesis used when there are bases or keywords.

rpar: RightParen | MaybeSentinel

Optional close parenthesis used when there are bases or keywords.

leading_lines: Sequence[EmptyLine]

Leading empty lines and comments before the first decorator. We assume any comments before the first decorator are owned by the class definition itself. If there are no decorators, this will still contain all of the empty lines and comments before the class definition.

lines_after_decorators: Sequence[EmptyLine]

Empty lines and comments between the final decorator and the ClassDef node. In the case of no decorators, this will be empty.

whitespace_after_class: SimpleWhitespace

Whitespace after the class keyword and before the class name.

whitespace_after_name: SimpleWhitespace

Whitespace after the class name and before the type parameters or the opening parenthesis for the bases and keywords.

whitespace_before_colon: SimpleWhitespace

Whitespace after the closing parenthesis or class name and before the colon.

type_parameters: TypeParameters | None

An optional declaration of type parameters.

whitespace_after_type_parameters: SimpleWhitespace

Whitespace between type parameters and opening parenthesis for the bases and keywords.

get_docstring(clean: bool = True) → str | None

Returns a inspect.cleandoc() cleaned docstring if the docstring is available, None otherwise.

class libcst.For

A for target in iter statement.

target: BaseAssignTargetExpression

The target of the iterator in the for statement.

iter: BaseExpression

The iterable expression we will loop over.

body: BaseSuite

The suite that is wrapped with this statement.

orelse: Else | None

An optional else case which will be executed if there is no Break statement encountered while looping.

asynchronous: Asynchronous | None

Optional async modifier, if this is an async for statement.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this for statement.

whitespace_after_for: SimpleWhitespace

Whitespace after the for keyword and before the target.

whitespace_before_in: SimpleWhitespace

Whitespace after the target and before the in keyword.

whitespace_after_in: SimpleWhitespace

Whitespace after the in keyword and before the iter.

whitespace_before_colon: SimpleWhitespace

Whitespace after the iter and before the colon.

class libcst.FunctionDef

A function definition.

name: Name

The function name.

params: Parameters

The function parameters. Present even if there are no params.

body: BaseSuite

The function body.

decorators: Sequence[Decorator]

Sequence of decorators applied to this function. Decorators are listed in order that they appear in source (top to bottom) as apposed to the order that they are applied to the function at runtime.

returns: Annotation | None

An optional return annotation, if the function is annotated.

asynchronous: Asynchronous | None

Optional async modifier, if this is an async function.

leading_lines: Sequence[EmptyLine]

Leading empty lines and comments before the first decorator. We assume any comments before the first decorator are owned by the function definition itself. If there are no decorators, this will still contain all of the empty lines and comments before the function definition.

lines_after_decorators: Sequence[EmptyLine]

Empty lines and comments between the final decorator and the FunctionDef node. In the case of no decorators, this will be empty.

whitespace_after_def: SimpleWhitespace

Whitespace after the def keyword and before the function name.

whitespace_after_name: SimpleWhitespace

Whitespace after the function name and before the type parameters or the opening parenthesis for the parameters.

whitespace_before_params: BaseParenthesizableWhitespace

Whitespace after the opening parenthesis for the parameters but before the first param itself.

whitespace_before_colon: SimpleWhitespace

Whitespace after the closing parenthesis or return annotation and before the colon.

type_parameters: TypeParameters | None

An optional declaration of type parameters.

whitespace_after_type_parameters: SimpleWhitespace

Whitespace between the type parameters and the opening parenthesis for the (non-type) parameters.

get_docstring(clean: bool = True) → str | None

When docstring is available, returns a inspect.cleandoc() cleaned docstring. Otherwise, returns None.

class libcst.If

An if statement. test holds a single test expression.

elif clauses don’t have a special representation in the AST, but rather appear as extra If nodes within the orelse section of the previous one.

test: BaseExpression

The expression that, when evaluated, should give us a truthy/falsey value.

body: BaseSuite

The body of this compound statement.

orelse: If | Else | None

An optional elif or else clause. If signifies an elif block. Else signifies an else block. None signifies no else or elif block.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this compound statement line.

whitespace_before_test: SimpleWhitespace

The whitespace appearing after the if keyword but before the test expression.

whitespace_after_test: SimpleWhitespace

The whitespace appearing after the test expression but before the colon.

class libcst.Try

A regular try statement that cannot contain ExceptStar blocks. For try statements that can contain ExceptStar blocks, see TryStar.

body: BaseSuite

The suite that is wrapped with a try statement.

handlers: Sequence[ExceptHandler]

A list of zero or more exception handlers.

orelse: Else | None

An optional else case.

finalbody: Finally | None

An optional finally case.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this compound statement line.

whitespace_before_colon: SimpleWhitespace

The whitespace that appears after the try keyword but before the colon.

class libcst.While

A while statement.

test: BaseExpression

The test we will loop against.

body: BaseSuite

The suite that is wrapped with this statement.

orelse: Else | None

An optional else case which will be executed if there is no Break statement encountered while looping.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this while statement.

whitespace_after_while: SimpleWhitespace

Whitespace after the while keyword and before the test.

whitespace_before_colon: SimpleWhitespace

Whitespace after the test and before the colon.

class libcst.With

A with statement.

items: Sequence[WithItem]

A sequence of one or more items that evaluate to context managers.

body: BaseSuite

The suite that is wrapped with this statement.

asynchronous: Asynchronous | None

Optional async modifier if this is an async with statement.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this with statement.

lpar: LeftParen | MaybeSentinel

Optional open parenthesis for multi-line with bindings

rpar: RightParen | MaybeSentinel

Optional close parenthesis for multi-line with bindings

whitespace_after_with: SimpleWhitespace

Whitespace after the with keyword and before the first item.

whitespace_before_colon: SimpleWhitespace

Whitespace after the last item and before the colon.

Helper Nodes

Nodes that are used by various statements to represent some syntax, but are not statements on their own and cannot be used outside of the statements they belong with.

class libcst.Annotation

An annotation for a function (PEP 3107) or on a variable (PEP 526). Typically these are used in the context of type hints (PEP 484), such as:

# a variable with a type
good_ideas: List[str] = []

# a function with type annotations
def concat(substrings: Sequence[str]) -> str:
    ...

annotation: BaseExpression

The annotation’s value itself. This is the part of the annotation after the colon or arrow.

whitespace_before_indicator: BaseParenthesizableWhitespace | MaybeSentinel

whitespace_after_indicator: BaseParenthesizableWhitespace

class libcst.AsName

An as name clause inside an ExceptHandler, ImportAlias or WithItem node.

name: Name | Tuple | List

Identifier that the parent node will be aliased to.

whitespace_before_as: BaseParenthesizableWhitespace

Whitespace between the parent node and the as keyword.

whitespace_after_as: BaseParenthesizableWhitespace

Whitespace between the as keyword and the name.

class libcst.AssignTarget

A target for an Assign. Owns the equals sign and the whitespace around it.

target: BaseAssignTargetExpression

The target expression being assigned to.

whitespace_before_equal: SimpleWhitespace

The whitespace appearing before the equals sign.

whitespace_after_equal: SimpleWhitespace

The whitespace appearing after the equals sign.

class libcst.BaseAssignTargetExpression

An expression that’s valid on the left side of an assignment. That assignment may be part an Assign node, or it may be part of a number of other control structures that perform an assignment, such as a For loop.

Python’s grammar defines all expression as valid in this position, but the AST compiler further restricts the allowed types, which is what this type attempts to express.

This is similar to a BaseDelTargetExpression, but it also includes StarredElement as a valid node.

The set of valid nodes are defined as part of CPython’s AST context computation.

class libcst.BaseDelTargetExpression

An expression that’s valid on the right side of a Del statement.

Python’s grammar defines all expression as valid in this position, but the AST compiler further restricts the allowed types, which is what this type attempts to express.

This is similar to a BaseAssignTargetExpression, but it excludes StarredElement.

The set of valid nodes are defined as part of CPython’s AST context computation and as part of CPython’s bytecode compiler.

class libcst.Decorator

A single decorator that decorates a FunctionDef or a ClassDef.

decorator: BaseExpression

The decorator that will return a new function wrapping the parent of this decorator.

leading_lines: Sequence[EmptyLine]

Line comments and empty lines before this decorator. The parent FunctionDef or ClassDef node owns leading lines before the first decorator so that if the first decorator is removed, spacing is preserved.

whitespace_after_at: SimpleWhitespace

Whitespace after the @ and before the decorator expression itself.

trailing_whitespace: TrailingWhitespace

Optional trailing comment and newline following the decorator before the next line.

class libcst.Else

An else clause that appears optionally after an If, While, Try, or For statement.

This node does not match elif clauses in If statements. It also does not match the required else clause in an IfExp expression (a = if b else c).

body: BaseSuite

The body of else clause.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this compound statement line.

whitespace_before_colon: SimpleWhitespace

The whitespace appearing after the else keyword but before the colon.

class libcst.ExceptHandler

An except clause that appears optionally after a Try statement.

body: BaseSuite

The body of the except.

type: BaseExpression | None

The type of exception this catches. Can be a tuple in some cases, or None if the code is catching all exceptions.

name: AsName | None

The optional name that a caught exception is assigned to.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this compound statement line.

whitespace_after_except: SimpleWhitespace

The whitespace between the except keyword and the type attribute.

whitespace_before_colon: SimpleWhitespace

The whitespace after any type or name node (whichever comes last) and the colon.

class libcst.Finally

A finally clause that appears optionally after a Try statement.

body: BaseSuite

The body of the except.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this compound statement line.

whitespace_before_colon: SimpleWhitespace

The whitespace that appears after the finally keyword but before the colon.

class libcst.ImportAlias

An import, with an optional AsName. Used in both Import and ImportFrom to specify a single import out of another module.

name: Attribute | Name

Name or Attribute node representing the object we are importing.

asname: AsName | None

Local alias we will import the above object as.

comma: Comma | MaybeSentinel

Any trailing comma that appears after this import. This is optional for the last ImportAlias in a Import or ImportFrom, but all other import aliases inside an import must contain a comma to disambiguate multiple imports.

property evaluated_name: str

Returns the string name this ImportAlias represents.

property evaluated_alias: str | None

Returns the string name for any alias that this ImportAlias has. If there is no asname attribute, this returns None.

class libcst.NameItem

A single identifier name inside a Global or Nonlocal statement.

This exists because a list of names in a global or nonlocal statement need to be separated by a comma, which ends up owned by the NameItem node.

name: Name

Identifier name.

comma: Comma | MaybeSentinel

This is forbidden for the last NameItem in a Global/Nonlocal, but all other tems inside a global or nonlocal statement must contain a comma to separate them.

class libcst.Parameters

A function or lambda parameter list.

params: Sequence[Param]

Positional parameters, with or without defaults. Positional parameters with defaults must all be after those without defaults.

star_arg: Param | ParamStar | MaybeSentinel

kwonly_params: Sequence[Param]

Keyword-only params that may or may not have defaults.

star_kwarg: Param | None

Optional parameter that captures unspecified kwargs.

posonly_params: Sequence[Param]

Positional-only parameters, with or without defaults. Positional-only parameters with defaults must all be after those without defaults.

posonly_ind: ParamSlash | MaybeSentinel

Optional sentinel that dictates parameters preceeding are positional-only args.

class libcst.Param

A positional or keyword argument in a Parameters list. May contain an Annotation and, in some cases, a default.

name: Name

The parameter name itself.

annotation: Annotation | None

Any optional Annotation. These annotations are usually used as type hints.

equal: AssignEqual | MaybeSentinel

The equal sign used to denote assignment if there is a default.

default: BaseExpression | None

Any optional default value, used when the argument is not supplied.

comma: Comma | MaybeSentinel

A trailing comma. If one is not provided, MaybeSentinel will be replaced with a comma only if a comma is required.

star: str | MaybeSentinel

Zero, one, or two asterisks appearing before name for Param’s star_arg and star_kwarg.

whitespace_after_star: BaseParenthesizableWhitespace

The whitespace before name. It will appear after star when a star exists.

whitespace_after_param: BaseParenthesizableWhitespace

The whitespace after this entire node.

class libcst.ParamSlash

A sentinel indicator on a Parameters list to denote that the previous params are positional-only args.

This syntax is described in PEP 570.

comma: Comma | MaybeSentinel

Optional comma that comes after the slash. This comma doesn’t own the whitespace between / and ,.

whitespace_after: BaseParenthesizableWhitespace

Whitespace after the / character. This is captured here in case there is a comma.

class libcst.ParamStar

A sentinel indicator on a Parameters list to denote that the subsequent params are keyword-only args.

This syntax is described in PEP 3102.

comma: Comma

class libcst.WithItem

A single context manager in a With block, with an optional variable name.

item: BaseExpression

Expression that evaluates to a context manager.

asname: AsName | None

Variable to assign the context manager to, if it is needed in the With body.

comma: Comma | MaybeSentinel

This is forbidden for the last WithItem in a With, but all other items inside a with block must contain a comma to separate them.

Statement Blocks

Nodes that represent some group of statements.

class libcst.BaseSuite

A dummy base-class for both SimpleStatementSuite and IndentedBlock. This exists to simplify type definitions and isinstance checks.

A suite is a group of statements controlled by a clause. A suite can be one or more semicolon-separated simple statements on the same line as the header, following the header’s colon, or it can be one or more indented statements on subsequent lines.

—https://docs.python.org/3/reference/compound_stmts.html

body: Sequence[BaseStatement] | Sequence[BaseSmallStatement]

class libcst.SimpleStatementLine

A simple statement that’s part of an IndentedBlock or Module. A simple statement is a series of small statements joined together by semicolons.

This isn’t differentiated from a SimpleStatementSuite in the grammar, but because a SimpleStatementLine can own additional whitespace that a SimpleStatementSuite doesn’t have, we’re differentiating it in the CST.

body: Sequence[BaseSmallStatement]

Sequence of small statements. All but the last statement are required to have a semicolon.

leading_lines: Sequence[EmptyLine]

Sequence of empty lines appearing before this simple statement line.

trailing_whitespace: TrailingWhitespace

Any optional trailing comment and the final NEWLINE at the end of the line.

class libcst.SimpleStatementSuite

A simple statement that’s used as a suite. A simple statement is a series of small statements joined together by semicolons. A suite is the thing that follows the colon in a compound statement.

if test:<leading_whitespace><body><trailing_whitespace>

This isn’t differentiated from a SimpleStatementLine in the grammar, but because the two classes need to track different whitespace, we’re differentiating it in the CST.

body: Sequence[BaseSmallStatement]

Sequence of small statements. All but the last statement are required to have a semicolon.

leading_whitespace: SimpleWhitespace

The whitespace between the colon in the parent statement and the body.

trailing_whitespace: TrailingWhitespace

Any optional trailing comment and the final NEWLINE at the end of the line.

class libcst.IndentedBlock

Represents a block of statements beginning with an INDENT token and ending in a DEDENT token. Used as the body of compound statements, such as an if statement’s body.

A common alternative to an IndentedBlock is a SimpleStatementSuite, which can also be used as a BaseSuite, meaning that it can be used as the body of many compound statements.

An IndentedBlock always occurs after a colon in a BaseCompoundStatement, so it owns the trailing whitespace for the compound statement’s clause.

if test: # IndentedBlock's header
    body

body: Sequence[BaseStatement]

Sequence of statements belonging to this indented block.

header: TrailingWhitespace

Any optional trailing comment and the final NEWLINE at the end of the line.

indent: str | None

A string represents a specific indentation. A None value uses the modules’s default indentation. This is included because indentation is allowed to be inconsistent across a file, just not ambiguously.

footer: Sequence[EmptyLine]

Any trailing comments or lines after the dedent that are owned by this indented block. Statements own preceeding and same-line trailing comments, but not trailing lines, so it falls on IndentedBlock to own it. In the case that a statement follows an IndentedBlock, that statement will own the comments and lines that are at the same indent as the statement, and this IndentedBlock will own the comments and lines that are indented further.

Operators

Nodes that are used to signify an operation to be performed on a variable or value.

Unary Operators

Nodes that are used with UnaryOperation to perform some unary operation.

class libcst.BitInvert

class libcst.Minus

class libcst.Not

class libcst.Plus

A unary operator that can be used in a UnaryOperation expression.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

In addition, BaseUnaryOp is defined purely for typing and isinstance checks.

class libcst.BaseUnaryOp

Boolean Operators

Nodes that are used with BooleanOperation to perform some boolean operation.

class libcst.And

class libcst.Or

A boolean operator that can be used in a BooleanOperation expression.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

In addition, BaseBooleanOp is defined purely for typing and isinstance checks.

class libcst.BaseBooleanOp

Binary Operators

Nodes that are used with BinaryOperation to perform some binary operation.

class libcst.Add

class libcst.BitAnd

class libcst.BitOr

class libcst.BitXor

class libcst.Divide

class libcst.FloorDivide

class libcst.LeftShift

class libcst.MatrixMultiply

class libcst.Modulo

class libcst.Multiply

class libcst.Power

class libcst.RightShift

class libcst.Subtract

A binary operator that can be used in a BinaryOperation expression.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

In addition, BaseBinaryOp is defined purely for typing and isinstance checks.

class libcst.BaseBinaryOp

Comparison Operators

Nodes that are used with Comparison to perform some comparison operation.

class libcst.Equal

class libcst.GreaterThan

class libcst.GreaterThanEqual

class libcst.In

class libcst.Is

class libcst.LessThan

class libcst.LessThanEqual

A comparision operator that can be used in a Comparison expression.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

class libcst.NotEqual

A comparison operator that can be used in a Comparison expression.

This node defines a static value for convenience, but in reality due to PEP 401 it can be one of two values, both of which should be a NotEqual Comparison operator.

value: str

The actual text value of this operator. Can be either != or <>.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

class libcst.IsNot

class libcst.NotIn

A comparision operator that can be used in a Comparison expression.

This operator spans two tokens that must be separated by at least one space, so there is a third whitespace attribute to represent this.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_between: BaseParenthesizableWhitespace

Any space that appears between the not and in tokens.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

In addition, BaseCompOp is defined purely for typing and isinstance checks.

class libcst.BaseCompOp

Augmented Assignment Operators

Nodes that are used with AugAssign to perform some agumented assignment.

class libcst.AddAssign

class libcst.BitAndAssign

class libcst.BitOrAssign

class libcst.BitXorAssign

class libcst.DivideAssign

class libcst.FloorDivideAssign

class libcst.LeftShiftAssign

class libcst.MatrixMultiplyAssign

class libcst.ModuloAssign

class libcst.MultiplyAssign

class libcst.PowerAssign

class libcst.RightShiftAssign

class libcst.SubtractAssign

An augmented assignment operator that can be used in a AugAssign statement.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this operator.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this operator.

In addition, BaseAugOp is defined purely for typing and isinstance checks.

class libcst.BaseAugOp

Miscellaneous

Miscelaneous nodes that are purely syntactic trivia. While python requires these nodes in order to parse a module, statement or expression they do not carry any meaning on their own.

class libcst.AssignEqual

Used by AnnAssign to denote a single equal character when doing an assignment on top of a type annotation. Also used by Param and Arg to denote assignment of a default value, and by FormattedStringExpression to denote usage of self-documenting expressions.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this equal sign.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this equal sign.

class libcst.Colon

Used by Slice as a separator between subsequent expressions, and in Lambda to separate arguments and body.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this colon.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this colon.

class libcst.Comma

Syntactic trivia used as a separator between subsequent items in various parts of the grammar.

Some use-cases are:

• Import or ImportFrom.

• FunctionDef arguments.

• Tuple/List/Set/Dict elements.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this comma.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this comma.

class libcst.Dot

Used by Attribute as a separator between subsequent Name nodes.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this dot.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this dot.

class libcst.ImportStar

Used by ImportFrom to denote a star import instead of a list of importable objects.

class libcst.Semicolon

Used by any small statement (any subclass of BaseSmallStatement such as Pass) as a separator between subsequent nodes contained within a SimpleStatementLine or SimpleStatementSuite.

whitespace_before: BaseParenthesizableWhitespace

Any space that appears directly before this semicolon.

whitespace_after: BaseParenthesizableWhitespace

Any space that appears directly after this semicolon.

Whitespace

Nodes that encapsulate pure whitespace.

class libcst.Comment

A comment including the leading pound (#) character.

The leading pound character is included in the ‘value’ property (instead of being stripped) to help re-enforce the idea that whitespace immediately after the pound character may be significant. E.g:

# comment with whitespace at the start (usually preferred)
#comment without whitespace at the start (usually not desirable)

Usually wrapped in a TrailingWhitespace or EmptyLine node.

value: str

The comment itself. Valid values start with the pound (#) character followed by zero or more non-newline characters. Comments cannot include newlines.

class libcst.EmptyLine

Represents a line with only whitespace/comments. Usually statements will own any EmptyLine nodes above themselves, and a Module will own the document’s header/footer EmptyLine nodes.

indent: bool

An empty line doesn’t have to correspond to the current indentation level. For example, this happens when all trailing whitespace is stripped and there is an empty line between two statements.

whitespace: SimpleWhitespace

Extra whitespace after the indent, but before the comment.

comment: Comment | None

An optional comment appearing after the indent and extra whitespace.

newline: Newline

The newline character that terminates this empty line.

class libcst.Newline

Represents the newline that ends an EmptyLine or a statement (as part of TrailingWhitespace).

Other newlines may occur in the document after continuation characters (the backslash, \), but those newlines are treated as part of the SimpleWhitespace.

Optionally, a value can be specified in order to overwrite the module’s default newline. In general, this should be left as the default, which is None. This is allowed because python modules are permitted to mix multiple unambiguous newline markers.

value: str | None

A value of None indicates that the module’s default newline sequence should be used. A value of \n or \r\n indicates that the exact value specified will be used for this newline.

class libcst.ParenthesizedWhitespace

This is the kind of whitespace you might see inside a parenthesized expression or statement between two tokens when there is a newline without a line continuation (\) character.

https://docs.python.org/3/reference/lexical_analysis.html#implicit-line-joining

A parenthesized whitespace cannot be empty since it requires at least one TrailingWhitespace. If you have whitespace that does not contain comments or newlines, use SimpleWhitespace instead.

first_line: TrailingWhitespace

The whitespace that comes after the previous node, up to and including the end-of-line comment and newline.

empty_lines: Sequence[EmptyLine]

Any lines after the first that contain only indentation and/or comments.

indent: bool

Whether or not the final simple whitespace is indented regularly.

last_line: SimpleWhitespace

Extra whitespace after the indent, but before the next node.

property empty: bool

Indicates that this node is empty (zero whitespace characters). For ParenthesizedWhitespace this will always be False.

class libcst.SimpleWhitespace

This is the kind of whitespace you might see inside the body of a statement or expression between two tokens. This is the most common type of whitespace.

A simple whitespace cannot contain a newline character unless it is directly preceeded by a line continuation character (\). It can contain zero or more spaces or tabs. If you need a newline character without a line continuation character, use ParenthesizedWhitespace instead.

Simple whitespace is often non-semantic (optional), but in cases where whitespace solves a grammar ambiguity between tokens (e.g. if test, versus iftest), it has some semantic value.

An example SimpleWhitespace containing a space, a line continuation, a newline and another space is as follows:

SimpleWhitespace(r" \\n ")

value: str

Actual string value of the simple whitespace. A legal value contains only space, \f and \t characters, and optionally a continuation (\) followed by a newline (\n or \r\n).

property empty: bool

Indicates that this node is empty (zero whitespace characters).

class libcst.TrailingWhitespace

The whitespace at the end of a line after a statement. If a line contains only whitespace, EmptyLine should be used instead.

whitespace: SimpleWhitespace

Any simple whitespace before any comment or newline.

comment: Comment | None

An optional comment appearing after any simple whitespace.

newline: Newline

The newline character that terminates this trailing whitespace.

class libcst.BaseParenthesizableWhitespace

This is the kind of whitespace you might see inside the body of a statement or expression between two tokens. This is the most common type of whitespace.

The list of allowed characters in a whitespace depends on whether it is found inside a parenthesized expression or not. This class allows nodes which can be found inside or outside a (), [] or {} section to accept either whitespace form.

https://docs.python.org/3/reference/lexical_analysis.html#implicit-line-joining

Parenthesizable whitespace may contain a backslash character (\), when used as a line-continuation character. While the continuation character isn’t technically “whitespace”, it serves the same purpose.

Parenthesizable whitespace is often non-semantic (optional), but in cases where whitespace solves a grammar ambiguity between tokens (e.g. if test, versus iftest), it has some semantic value.

abstract property empty: bool

Indicates that this node is empty (zero whitespace characters).

Maybe Sentinel

class libcst.MaybeSentinel

A MaybeSentinel value is used as the default value for some attributes to denote that when generating code (when Module.code is evaluated) we should optionally include this element in order to generate valid code.

MaybeSentinel is only used for “syntactic trivia” that most users shouldn’t care much about anyways, like commas, semicolons, and whitespace.

For example, a function call’s Arg.comma value defaults to MaybeSentinel.DEFAULT. A comma is required after every argument, except for the last one. If a comma is required and Arg.comma is a MaybeSentinel, one is inserted.

This makes manual node construction easier, but it also means that we safely add arguments to a preexisting function call without manually fixing the commas:

>>> import libcst as cst
>>> fn_call = cst.parse_expression("fn(1, 2)")
>>> new_fn_call = fn_call.with_changes(
...     args=[*fn_call.args, cst.Arg(cst.Integer("3"))]
... )
>>> dummy_module = cst.parse_module("")  # we need to use Module.code_for_node
>>> dummy_module.code_for_node(fn_call)
'fn(1, 2)'
>>> dummy_module.code_for_node(new_fn_call)
'fn(1, 2, 3)'

Notice that a comma was automatically inserted after the second argument. Since the original second argument had no comma, it was initialized to MaybeSentinel.DEFAULT. During the code generation of the second argument, a comma was inserted to ensure that the resulting code is valid.

Warning

While this sentinel is used in place of nodes, it is not a CSTNode, and will not be visited by a CSTVisitor.

Some other libraries, like RedBaron, take other approaches to this problem. RedBaron’s tree is mutable (LibCST’s tree is immutable), and so they’re able to solve this problem with “proxy lists”. Both approaches come with different sets of tradeoffs.

DEFAULT = 1