— Abstract syntax trees

ast
— Abstract syntax trees¶
Source code: Lib/ast.py
The ast
module helps Python applications to process trees of the Python
abstract syntax grammar. The abstract syntax itself might change with each
Python release; this module helps to find out programmatically what the current
grammar looks like.
An abstract syntax tree can be generated by passing ast.PyCF_ONLY_AST
as
a flag to the compile()
built-in function, or using the parse()
helper provided in this module. The result will be a tree of objects whose
classes all inherit from ast.AST
. An abstract syntax tree can be
compiled into a Python code object using the built-in compile()
function.
Abstract grammar¶
The abstract grammar is currently defined as follows:
-- ASDL's 4 builtin types are:
-- identifier, int, string, constant
module Python
{
mod = Module(stmt* body, type_ignore* type_ignores)
| Interactive(stmt* body)
| Expression(expr body)
| FunctionType(expr* argtypes, expr returns)
stmt = FunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? type_comment, type_param* type_params)
| AsyncFunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? type_comment, type_param* type_params)
| ClassDef(identifier name,
expr* bases,
keyword* keywords,
stmt* body,
expr* decorator_list,
type_param* type_params)
| Return(expr? value)
| Delete(expr* targets)
| Assign(expr* targets, expr value, string? type_comment)
| TypeAlias(expr name, type_param* type_params, expr value)
| AugAssign(expr target, operator op, expr value)
-- 'simple' indicates that we annotate simple name without parens
| AnnAssign(expr target, expr annotation, expr? value, int simple)
-- use 'orelse' because else is a keyword in target languages
| For(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
| AsyncFor(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
| While(expr test, stmt* body, stmt* orelse)
| If(expr test, stmt* body, stmt* orelse)
| With(withitem* items, stmt* body, string? type_comment)
| AsyncWith(withitem* items, stmt* body, string? type_comment)
| Match(expr subject, match_case* cases)
| Raise(expr? exc, expr? cause)
| Try(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
| TryStar(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
| Assert(expr test, expr? msg)
| Import(alias* names)
| ImportFrom(identifier? module, alias* names, int? level)
| Global(identifier* names)
| Nonlocal(identifier* names)
| Expr(expr value)
| Pass | Break | Continue
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
-- BoolOp() can use left & right?
expr = BoolOp(boolop op, expr* values)
| NamedExpr(expr target, expr value)
| BinOp(expr left, operator op, expr right)
| UnaryOp(unaryop op, expr operand)
| Lambda(arguments args, expr body)
| IfExp(expr test, expr body, expr orelse)
| Dict(expr?* keys, expr* values)
| Set(expr* elts)
| ListComp(expr elt, comprehension* generators)
| SetComp(expr elt, comprehension* generators)
| DictComp(expr key, expr value, comprehension* generators)
| GeneratorExp(expr elt, comprehension* generators)
-- the grammar constrains where yield expressions can occur
| Await(expr value)
| Yield(expr? value)
| YieldFrom(expr value)
-- need sequences for compare to distinguish between
-- x < 4 < 3 and (x < 4) < 3
| Compare(expr left, cmpop* ops, expr* comparators)
| Call(expr func, expr* args, keyword* keywords)
| FormattedValue(expr value, int conversion, expr? format_spec)
| Interpolation(expr value, constant str, int conversion, expr? format_spec)
| JoinedStr(expr* values)
| TemplateStr(expr* values)
| Constant(constant value, string? kind)
-- the following expression can appear in assignment context
| Attribute(expr value, identifier attr, expr_context ctx)
| Subscript(expr value, expr slice, expr_context ctx)
| Starred(expr value, expr_context ctx)
| Name(identifier id, expr_context ctx)
| List(expr* elts, expr_context ctx)
| Tuple(expr* elts, expr_context ctx)
-- can appear only in Subscript
| Slice(expr? lower, expr? upper, expr? step)
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
expr_context = Load | Store | Del
boolop = And | Or
operator = Add | Sub | Mult | MatMult | Div | Mod | Pow | LShift
| RShift | BitOr | BitXor | BitAnd | FloorDiv
unaryop = Invert | Not | UAdd | USub
cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn
comprehension = (expr target, expr iter, expr* ifs, int is_async)
excepthandler = ExceptHandler(expr? type, identifier? name, stmt* body)
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
arguments = (arg* posonlyargs, arg* args, arg? vararg, arg* kwonlyargs,
expr?* kw_defaults, arg? kwarg, expr* defaults)
arg = (identifier arg, expr? annotation, string? type_comment)
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
-- keyword arguments supplied to call (NULL identifier for **kwargs)
keyword = (identifier? arg, expr value)
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
-- import name with optional 'as' alias.
alias = (identifier name, identifier? asname)
attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
withitem = (expr context_expr, expr? optional_vars)
match_case = (pattern pattern, expr? guard, stmt* body)
pattern = MatchValue(expr value)
| MatchSingleton(constant value)
| MatchSequence(pattern* patterns)
| MatchMapping(expr* keys, pattern* patterns, identifier? rest)
| MatchClass(expr cls, pattern* patterns, identifier* kwd_attrs, pattern* kwd_patterns)
| MatchStar(identifier? name)
-- The optional "rest" MatchMapping parameter handles capturing extra mapping keys
| MatchAs(pattern? pattern, identifier? name)
| MatchOr(pattern* patterns)
attributes (int lineno, int col_offset, int end_lineno, int end_col_offset)
type_ignore = TypeIgnore(int lineno, string tag)
type_param = TypeVar(identifier name, expr? bound, expr? default_value)
| ParamSpec(identifier name, expr? default_value)
| TypeVarTuple(identifier name, expr? default_value)
attributes (int lineno, int col_offset, int end_lineno, int end_col_offset)
}
Node classes¶
- class ast.AST¶
This is the base of all AST node classes. The actual node classes are derived from the
Parser/Python.asdl
file, which is reproduced above. They are defined in the_ast
C module and re-exported inast
.There is one class defined for each left-hand side symbol in the abstract grammar (for example,
ast.stmt
orast.expr
). In addition, there is one class defined for each constructor on the right-hand side; these classes inherit from the classes for the left-hand side trees. For example,ast.BinOp
inherits fromast.expr
. For production rules with alternatives (aka “sums”), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.- _fields¶
Each concrete class has an attribute
_fields
which gives the names of all child nodes.Each instance of a concrete class has one attribute for each child node, of the type as defined in the grammar. For example,
ast.BinOp
instances have an attributeleft
of typeast.expr
.If these attributes are marked as optional in the grammar (using a question mark), the value might be
None
. If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST withcompile()
.
- _field_types¶
The
_field_types
attribute on each concrete class is a dictionary mapping field names (as also listed in_fields
) to their types.>>> ast.TypeVar._field_types {'name': <class 'str'>, 'bound': ast.expr | None, 'default_value': ast.expr | None}
Added in version 3.13.
- lineno¶
- col_offset¶
- end_lineno¶
- end_col_offset¶
Instances of
ast.expr
andast.stmt
subclasses havelineno
,col_offset
,end_lineno
, andend_col_offset
attributes. Thelineno
andend_lineno
are the first and last line numbers of source text span (1-indexed so the first line is line 1) and thecol_offset
andend_col_offset
are the corresponding UTF-8 byte offsets of the first and last tokens that generated the node. The UTF-8 offset is recorded because the parser uses UTF-8 internally.Note that the end positions are not required by the compiler and are therefore optional. The end offset is after the last symbol, for example one can get the source segment of a one-line expression node using
source_line[node.col_offset : node.end_col_offset]
.
The constructor of a class
ast.T
parses its arguments as follows:If there are positional arguments, there must be as many as there are items in
T._fields
; they will be assigned as attributes of these names.If there are keyword arguments, they will set the attributes of the same names to the given values.
For example, to create and populate an
ast.UnaryOp
node, you could usenode = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0), lineno=0, col_offset=0)
If a field that is optional in the grammar is omitted from the constructor, it defaults to
None
. If a list field is omitted, it defaults to the empty list. If a field of typeast.expr_context
is omitted, it defaults toLoad()
. If any other field is omitted, aDeprecationWarning
is raised and the AST node will not have this field. In Python 3.15, this condition will raise an error.
Changed in version 3.8: Class ast.Constant
is now used for all constants.
Changed in version 3.9: Simple indices are represented by their value, extended slices are represented as tuples.
Changed in version 3.14: The __repr__()
output of AST
nodes includes
the values of the node fields.
Deprecated since version 3.8, removed in version 3.14: Previous versions of Python provided the AST classes ast.Num
,
ast.Str
, ast.Bytes
, ast.NameConstant
and
ast.Ellipsis
, which were deprecated in Python 3.8. These classes
were removed in Python 3.14, and their functionality has been replaced with
ast.Constant
.
Deprecated since version 3.9: Old classes ast.Index
and ast.ExtSlice
are still
available, but they will be removed in future Python releases.
In the meantime, instantiating them will return an instance of
a different class.
Deprecated since version 3.13, will be removed in version 3.15: Previous versions of Python allowed the creation of AST nodes that were missing required fields. Similarly, AST node constructors allowed arbitrary keyword arguments that were set as attributes of the AST node, even if they did not match any of the fields of the AST node. This behavior is deprecated and will be removed in Python 3.15.
Note
The descriptions of the specific node classes displayed here were initially adapted from the fantastic Green Tree Snakes project and all its contributors.
Root nodes¶
- class ast.Module(body, type_ignores)¶
A Python module, as with file input. Node type generated by
ast.parse()
in the default"exec"
mode.body
is alist
of the module’s Statements.type_ignores
is alist
of the module’s type ignore comments; seeast.parse()
for more details.>>> print(ast.dump(ast.parse('x = 1'), indent=4)) Module( body=[ Assign( targets=[ Name(id='x', ctx=Store())], value=Constant(value=1))])
- class ast.Expression(body)¶
A single Python expression input. Node type generated by
ast.parse()
when mode is"eval"
.body
is a single node, one of the expression types.>>> print(ast.dump(ast.parse('123', mode='eval'), indent=4)) Expression( body=Constant(value=123))
- class ast.Interactive(body)¶
A single interactive input, like in Interactive Mode. Node type generated by
ast.parse()
when mode is"single"
.body
is alist
of statement nodes.>>> print(ast.dump(ast.parse('x = 1; y = 2', mode='single'), indent=4)) Interactive( body=[ Assign( targets=[ Name(id='x', ctx=Store())], value=Constant(value=1)), Assign( targets=[ Name(id='y', ctx=Store())], value=Constant(value=2))])
- class ast.FunctionType(argtypes, returns)¶
A representation of an old-style type comments for functions, as Python versions prior to 3.5 didn’t support PEP 484 annotations. Node type generated by
ast.parse()
when mode is"func_type"
.Such type comments would look like this:
def sum_two_number(a, b): # type: (int, int) -> int return a + b
argtypes
is alist
of expression nodes.returns
is a single expression node.>>> print(ast.dump(ast.parse('(int, str) -> List[int]', mode='func_type'), indent=4)) FunctionType( argtypes=[ Name(id='int', ctx=Load()), Name(id='str', ctx=Load())], returns=Subscript( value=Name(id='List', ctx=Load()), slice=Name(id='int', ctx=Load()), ctx=Load()))
Added in version 3.8.
Literals¶
- class ast.Constant(value)¶
A constant value. The
value
attribute of theConstant
literal contains the Python object it represents. The values represented can be instances ofstr
,bytes
,int
,float
,complex
, andbool
, and the constantsNone
andEllipsis
.>>> print(ast.dump(ast.parse('123', mode='eval'), indent=4)) Expression( body=Constant(value=123))
- class ast.FormattedValue(value, conversion, format_spec)¶
Node representing a single formatting field in an f-string. If the string contains a single formatting field and nothing else the node can be isolated otherwise it appears in
JoinedStr
.value
is any expression node (such as a literal, a variable, or a function call).conversion
is an integer:format_spec
is aJoinedStr
node representing the formatting of the value, orNone
if no format was specified. Bothconversion
andformat_spec
can be set at the same time.
- class ast.JoinedStr(values)¶
An f-string, comprising a series of
FormattedValue
andConstant
nodes.>>> print(ast.dump(ast.parse('f"sin({a}) is {sin(a):.3}"', mode='eval'), indent=4)) Expression( body=JoinedStr( values=[ Constant(value='sin('), FormattedValue( value=Name(id='a', ctx=Load()), conversion=-1), Constant(value=') is '), FormattedValue( value=Call( func=Name(id='sin', ctx=Load()), args=[ Name(id='a', ctx=Load())]), conversion=-1, format_spec=JoinedStr( values=[ Constant(value='.3')]))]))
- class ast.TemplateStr(values, /)¶
Added in version 3.14.
Node representing a template string literal, comprising a series of
Interpolation
andConstant
nodes. These nodes may be any order, and do not need to be interleaved.>>> expr = ast.parse('t"{name} finished {place:ordinal}"', mode='eval') >>> print(ast.dump(expr, indent=4)) Expression( body=TemplateStr( values=[ Interpolation( value=Name(id='name', ctx=Load()), str='name', conversion=-1), Constant(value=' finished '), Interpolation( value=Name(id='place', ctx=Load()), str='place', conversion=-1, format_spec=JoinedStr( values=[ Constant(value='ordinal')]))]))
- class ast.Interpolation(value, str, conversion, format_spec=None)¶
Added in version 3.14.
Node representing a single interpolation field in a template string literal.
value
is any expression node (such as a literal, a variable, or a function call). This has the same meaning asFormattedValue.value
.str
is a constant containing the text of the interpolation expression.If
str
is set toNone
, thenvalue
is used to generate code when callingast.unparse()
. This no longer guarantees that the generated code is identical to the original and is intended for code generation.conversion
is an integer:-1: no conversion
97 (
ord('a')
):!a
ASCII
conversion114 (
ord('r')
):!r
repr()
conversion115 (
ord('s')
):!s
string
conversion
This has the same meaning as
FormattedValue.conversion
.format_spec
is aJoinedStr
node representing the formatting of the value, orNone
if no format was specified. Bothconversion
andformat_spec
can be set at the same time. This has the same meaning asFormattedValue.format_spec
.
- class ast.List(elts, ctx)¶
- class ast.Tuple(elts, ctx)¶
A list or tuple.
elts
holds a list of nodes representing the elements.ctx
isStore
if the container is an assignment target (i.e.(x,y)=something
), andLoad
otherwise.>>> print(ast.dump(ast.parse('[1, 2, 3]', mode='eval'), indent=4)) Expression( body=List( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)], ctx=Load())) >>> print(ast.dump(ast.parse('(1, 2, 3)', mode='eval'), indent=4)) Expression( body=Tuple( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)], ctx=Load()))
- class ast.Set(elts)¶
A set.
elts
holds a list of nodes representing the set’s elements.>>> print(ast.dump(ast.parse('{1, 2, 3}', mode='eval'), indent=4)) Expression( body=Set( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)]))
- class ast.Dict(keys, values)¶
A dictionary.
keys
andvalues
hold lists of nodes representing the keys and the values respectively, in matching order (what would be returned when callingdictionary.keys()
anddictionary.values()
).When doing dictionary unpacking using dictionary literals the expression to be expanded goes in the
values
list, with aNone
at the corresponding position inkeys
.>>> print(ast.dump(ast.parse('{"a":1, **d}', mode='eval'), indent=4)) Expression( body=Dict( keys=[ Constant(value='a'), None], values=[ Constant(value=1), Name(id='d', ctx=Load())]))
Variables¶
- class ast.Name(id, ctx)¶
A variable name.
id
holds the name as a string, andctx
is one of the following types.
- class ast.Load¶
- class ast.Store¶
- class ast.Del¶
Variable references can be used to load the value of a variable, to assign a new value to it, or to delete it. Variable references are given a context to distinguish these cases.
>>> print(ast.dump(ast.parse('a'), indent=4)) Module( body=[ Expr( value=Name(id='a', ctx=Load()))]) >>> print(ast.dump(ast.parse('a = 1'), indent=4)) Module( body=[ Assign( targets=[ Name(id='a', ctx=Store())], value=Constant(value=1))]) >>> print(ast.dump(ast.parse('del a'), indent=4)) Module( body=[ Delete( targets=[ Name(id='a', ctx=Del())])])
- class ast.Starred(value, ctx)¶
A
*var
variable reference.value
holds the variable, typically aName
node. This type must be used when building aCall
node with*args
.>>> print(ast.dump(ast.parse('a, *b = it'), indent=4)) Module( body=[ Assign( targets=[ Tuple( elts=[ Name(id='a', ctx=Store()), Starred( value=Name(id='b', ctx=Store()), ctx=Store())], ctx=Store())], value=Name(id='it', ctx=Load()))])
Expressions¶
- class ast.Expr(value)¶
When an expression, such as a function call, appears as a statement by itself with its return value not used or stored, it is wrapped in this container.
value
holds one of the other nodes in this section, aConstant
, aName
, aLambda
, aYield
orYieldFrom
node.>>> print(ast.dump(ast.parse('-a'), indent=4)) Module( body=[ Expr( value=UnaryOp( op=USub(), operand=Name(id='a', ctx=Load())))])
- class ast.UnaryOp(op, operand)¶
A unary operation.
op
is the operator, andoperand
any expression node.
- class ast.UAdd¶
- class ast.USub¶
- class ast.Not¶
- class ast.Invert¶
Unary operator tokens.
Not
is thenot
keyword,Invert
is the~
operator.>>> print(ast.dump(ast.parse('not x', mode='eval'), indent=4)) Expression( body=UnaryOp( op=Not(), operand=Name(id='x', ctx=Load())))
- class ast.BinOp(left, op, right)¶
A binary operation (like addition or division).
op
is the operator, andleft
andright
are any expression nodes.>>> print(ast.dump(ast.parse('x + y', mode='eval'), indent=4)) Expression( body=BinOp( left=Name(id='x', ctx=Load()), op=Add(), right=Name(id='y', ctx=Load())))
- class ast.Add¶
- class ast.Sub¶
- class ast.Mult¶
- class ast.Div¶
- class ast.FloorDiv¶
- class ast.Mod¶
- class ast.Pow¶
- class ast.LShift¶
- class ast.RShift¶
- class ast.BitOr¶
- class ast.BitXor¶
- class ast.BitAnd¶
- class ast.MatMult¶
Binary operator tokens.
- class ast.BoolOp(op, values)¶
A boolean operation, ‘or’ or ‘and’.
op
isOr
orAnd
.values
are the values involved. Consecutive operations with the same operator, such asa or b or c
, are collapsed into one node with several values.This doesn’t include
not
, which is aUnaryOp
.>>> print(ast.dump(ast.parse('x or y', mode='eval'), indent=4)) Expression( body=BoolOp( op=Or(), values=[ Name(id='x', ctx=Load()), Name(id='y', ctx=Load())]))
- class ast.Compare(left, ops, comparators)¶
A comparison of two or more values.
left
is the first value in the comparison,ops
the list of operators, andcomparators
the list of values after the first element in the comparison.>>> print(ast.dump(ast.parse('1 <= a < 10', mode='eval'), indent=4)) Expression( body=Compare( left=Constant(value=1), ops=[ LtE(), Lt()], comparators=[ Name(id='a', ctx=Load()), Constant(value=10)]))
- class ast.Eq¶
- class ast.NotEq¶
- class ast.Lt¶
- class ast.LtE¶
- class ast.Gt¶
- class ast.GtE¶
- class ast.Is¶
- class ast.IsNot¶
- class ast.In¶
- class ast.NotIn¶
Comparison operator tokens.
- class ast.Call(func, args, keywords)¶
A function call.
func
is the function, which will often be aName
orAttribute
object. Of the arguments:args
holds a list of the arguments passed by position.keywords
holds a list ofkeyword
objects representing arguments passed by keyword.
The
args
andkeywords
arguments are optional and default to empty lists.>>> print(ast.dump(ast.parse('func(a, b=c, *d, **e)', mode='eval'), indent=4)) Expression( body=Call( func=Name(id='func', ctx=Load()), args=[ Name(id='a', ctx=Load()), Starred( value=Name(id='d', ctx=Load()), ctx=Load())], keywords=[ keyword( arg='b', value=Name(id='c', ctx=Load())), keyword( value=Name(id='e', ctx=Load()))]))
- class ast.keyword(arg, value)¶
A keyword argument to a function call or class definition.
arg
is a raw string of the parameter name,value
is a node to pass in.
- class ast.IfExp(test, body, orelse)¶
An expression such as
a if b else c
. Each field holds a single node, so in the following example, all three areName
nodes.>>> print(ast.dump(ast.parse('a if b else c', mode='eval'), indent=4)) Expression( body=IfExp( test=Name(id='b', ctx=Load()), body=Name(id='a', ctx=Load()), orelse=Name(id='c', ctx=Load())))
- class ast.Attribute(value, attr, ctx)¶
Attribute access, e.g.
d.keys
.value
is a node, typically aName
.attr
is a bare string giving the name of the attribute, andctx
isLoad
,Store
orDel
according to how the attribute is acted on.>>> print(ast.dump(ast.parse('snake.colour', mode='eval'), indent=4)) Expression( body=Attribute( value=Name(id='snake', ctx=Load()), attr='colour', ctx=Load()))
- class ast.NamedExpr(target, value)¶
A named expression. This AST node is produced by the assignment expressions operator (also known as the walrus operator). As opposed to the
Assign
node in which the first argument can be multiple nodes, in this case bothtarget
andvalue
must be single nodes.>>> print(ast.dump(ast.parse('(x := 4)', mode='eval'), indent=4)) Expression( body=NamedExpr( target=Name(id='x', ctx=Store()), value=Constant(value=4)))
Added in version 3.8.
Subscripting¶
- class ast.Subscript(value, slice, ctx)¶
A subscript, such as
l[1]
.value
is the subscripted object (usually sequence or mapping).slice
is an index, slice or key. It can be aTuple
and contain aSlice
.ctx
isLoad
,Store
orDel
according to the action performed with the subscript.>>> print(ast.dump(ast.parse('l[1:2, 3]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Tuple( elts=[ Slice( lower=Constant(value=1), upper=Constant(value=2)), Constant(value=3)], ctx=Load()), ctx=Load()))
- class ast.Slice(lower, upper, step)¶
Regular slicing (on the form
lower:upper
orlower:upper:step
). Can occur only inside the slice field ofSubscript
, either directly or as an element ofTuple
.>>> print(ast.dump(ast.parse('l[1:2]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Slice( lower=Constant(value=1), upper=Constant(value=2)), ctx=Load()))
Comprehensions¶
- class ast.ListComp(elt, generators)¶
- class ast.SetComp(elt, generators)¶
- class ast.GeneratorExp(elt, generators)¶
- class ast.DictComp(key, value, generators)¶
List and set comprehensions, generator expressions, and dictionary comprehensions.
elt
(orkey
andvalue
) is a single node representing the part that will be evaluated for each item.generators
is a list ofcomprehension
nodes.>>> print(ast.dump( ... ast.parse('[x for x in numbers]', mode='eval'), ... indent=4, ... )) Expression( body=ListComp( elt=Name(id='x', ctx=Load()), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), is_async=0)])) >>> print(ast.dump( ... ast.parse('{x: x**2 for x in numbers}', mode='eval'), ... indent=4, ... )) Expression( body=DictComp( key=Name(id='x', ctx=Load()), value=BinOp( left=Name(id='x', ctx=Load()), op=Pow(), right=Constant(value=2)), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), is_async=0)])) >>> print(ast.dump( ... ast.parse('{x for x in numbers}', mode='eval'), ... indent=4, ... )) Expression( body=SetComp( elt=Name(id='x', ctx=Load()), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), is_async=0)]))
- class ast.comprehension(target, iter, ifs, is_async)¶
One
for
clause in a comprehension.target
is the reference to use for each element - typically aName
orTuple
node.iter
is the object to iterate over.ifs
is a list of test expressions: eachfor
clause can have multipleifs
.is_async
indicates a comprehension is asynchronous (using anasync for
instead offor
). The value is an integer (0 or 1).>>> print(ast.dump(ast.parse('[ord(c) for line in file for c in line]', mode='eval'), ... indent=4)) # Multiple comprehensions in one. Expression( body=ListComp( elt=Call( func=Name(id='ord', ctx=Load()), args=[ Name(id='c', ctx=Load())]), generators=[ comprehension( target=Name(id='line', ctx=Store()), iter=Name(id='file', ctx=Load()), is_async=0), comprehension( target=Name(id='c', ctx=Store()), iter=Name(id='line', ctx=Load()), is_async=0)])) >>> print(ast.dump(ast.parse('(n**2 for n in it if n>5 if n<10)', mode='eval'), ... indent=4)) # generator comprehension Expression( body=GeneratorExp( elt=BinOp( left=Name(id='n', ctx=Load()), op=Pow(), right=Constant(value=2)), generators=[ comprehension( target=Name(id='n', ctx=Store()), iter=Name(id='it', ctx=Load()), ifs=[ Compare( left=Name(id='n', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=5)]), Compare( left=Name(id='n', ctx=Load()), ops=[ Lt()], comparators=[ Constant(value=10)])], is_async=0)])) >>> print(ast.dump(ast.parse('[i async for i in soc]', mode='eval'), ... indent=4)) # Async comprehension Expression( body=ListComp( elt=Name(id='i', ctx=Load()), generators=[ comprehension( target=Name(id='i', ctx=Store()), iter=Name(id='soc', ctx=Load()), is_async=1)]))
Statements¶
- class ast.Assign(targets, value, type_comment)¶
An assignment.
targets
is a list of nodes, andvalue
is a single node.Multiple nodes in
targets
represents assigning the same value to each. Unpacking is represented by putting aTuple
orList
withintargets
.- type_comment¶
type_comment
is an optional string with the type annotation as a comment.
>>> print(ast.dump(ast.parse('a = b = 1'), indent=4)) # Multiple assignment Module( body=[ Assign( targets=[ Name(id='a', ctx=Store()), Name(id='b', ctx=Store())], value=Constant(value=1))]) >>> print(ast.dump(ast.parse('a,b = c'), indent=4)) # Unpacking Module( body=[ Assign( targets=[ Tuple( elts=[ Name(id='a', ctx=Store()), Name(id='b', ctx=Store())], ctx=Store())], value=Name(id='c', ctx=Load()))])
- class ast.AnnAssign(target, annotation, value, simple)¶
An assignment with a type annotation.
target
is a single node and can be aName
, anAttribute
or aSubscript
.annotation
is the annotation, such as aConstant
orName
node.value
is a single optional node.simple
is always either 0 (indicating a “complex” target) or 1 (indicating a “simple” target). A “simple” target consists solely of aName
node that does not appear between parentheses; all other targets are considered complex. Only simple targets appear in the__annotations__
dictionary of modules and classes.>>> print(ast.dump(ast.parse('c: int'), indent=4)) Module( body=[ AnnAssign( target=Name(id='c', ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=1)]) >>> print(ast.dump(ast.parse('(a): int = 1'), indent=4)) # Annotation with parenthesis Module( body=[ AnnAssign( target=Name(id='a', ctx=Store()), annotation=Name(id='int', ctx=Load()), value=Constant(value=1), simple=0)]) >>> print(ast.dump(ast.parse('a.b: int'), indent=4)) # Attribute annotation Module( body=[ AnnAssign( target=Attribute( value=Name(id='a', ctx=Load()), attr='b', ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=0)]) >>> print(ast.dump(ast.parse('a[1]: int'), indent=4)) # Subscript annotation Module( body=[ AnnAssign( target=Subscript( value=Name(id='a', ctx=Load()), slice=Constant(value=1), ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=0)])
- class ast.AugAssign(target, op, value)¶
Augmented assignment, such as
a += 1
. In the following example,target
is aName
node forx
(with theStore
context),op
isAdd
, andvalue
is aConstant
with value for 1.The
target
attribute cannot be of classTuple
orList
, unlike the targets ofAssign
.>>> print(ast.dump(ast.parse('x += 2'), indent=4)) Module( body=[ AugAssign( target=Name(id='x', ctx=Store()), op=Add(), value=Constant(value=2))])
- class ast.Raise(exc, cause)¶
A
raise
statement.exc
is the exception object to be raised, normally aCall
orName
, orNone
for a standaloneraise
.cause
is the optional part fory
inraise x from y
.>>> print(ast.dump(ast.parse('raise x from y'), indent=4)) Module( body=[ Raise( exc=Name(id='x', ctx=Load()), cause=Name(id='y', ctx=Load()))])
- class ast.Assert(test, msg)¶
An assertion.
test
holds the condition, such as aCompare
node.msg
holds the failure message.>>> print(ast.dump(ast.parse('assert x,y'), indent=4)) Module( body=[ Assert( test=Name(id='x', ctx=Load()), msg=Name(id='y', ctx=Load()))])
- class ast.Delete(targets)¶
Represents a
del
statement.targets
is a list of nodes, such asName
,Attribute
orSubscript
nodes.>>> print(ast.dump(ast.parse('del x,y,z'), indent=4)) Module( body=[ Delete( targets=[ Name(id='x', ctx=Del()), Name(id='y', ctx=Del()), Name(id='z', ctx=Del())])])
- class ast.Pass¶
A
pass
statement.>>> print(ast.dump(ast.parse('pass'), indent=4)) Module( body=[ Pass()])
- class ast.TypeAlias(name, type_params, value)¶
A type alias created through the
type
statement.name
is the name of the alias,type_params
is a list of type parameters, andvalue
is the value of the type alias.>>> print(ast.dump(ast.parse('type Alias = int'), indent=4)) Module( body=[ TypeAlias( name=Name(id='Alias', ctx=Store()), value=Name(id='int', ctx=Load()))])
Added in version 3.12.
Other statements which are only applicable inside functions or loops are described in other sections.
Imports¶
- class ast.Import(names)¶
An import statement.
names
is a list ofalias
nodes.>>> print(ast.dump(ast.parse('import x,y,z'), indent=4)) Module( body=[ Import( names=[ alias(name='x'), alias(name='y'), alias(name='z')])])
- class ast.ImportFrom(module, names, level)¶
Represents
from x import y
.module
is a raw string of the ‘from’ name, without any leading dots, orNone
for statements such asfrom . import foo
.level
is an integer holding the level of the relative import (0 means absolute import).>>> print(ast.dump(ast.parse('from y import x,y,z'), indent=4)) Module( body=[ ImportFrom( module='y', names=[ alias(name='x'), alias(name='y'), alias(name='z')], level=0)])
- class ast.alias(name, asname)¶
Both parameters are raw strings of the names.
asname
can beNone
if the regular name is to be used.>>> print(ast.dump(ast.parse('from ..foo.bar import a as b, c'), indent=4)) Module( body=[ ImportFrom( module='foo.bar', names=[ alias(name='a', asname='b'), alias(name='c')], level=2)])
Control flow¶
Note
Optional clauses such as else
are stored as an empty list if they’re
not present.
- class ast.If(test, body, orelse)¶
An
if
statement.test
holds a single node, such as aCompare
node.body
andorelse
each hold a list of nodes.elif
clauses don’t have a special representation in the AST, but rather appear as extraIf
nodes within theorelse
section of the previous one.>>> print(ast.dump(ast.parse(""" ... if x: ... ... ... elif y: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ If( test=Name(id='x', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ If( test=Name(id='y', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])])])
- class ast.For(target, iter, body, orelse, type_comment)¶
A
for
loop.target
holds the variable(s) the loop assigns to, as a singleName
,Tuple
,List
,Attribute
orSubscript
node.iter
holds the item to be looped over, again as a single node.body
andorelse
contain lists of nodes to execute. Those inorelse
are executed if the loop finishes normally, rather than via abreak
statement.- type_comment¶
type_comment
is an optional string with the type annotation as a comment.
>>> print(ast.dump(ast.parse(""" ... for x in y: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ For( target=Name(id='x', ctx=Store()), iter=Name(id='y', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])])
- class ast.While(test, body, orelse)¶
A
while
loop.test
holds the condition, such as aCompare
node.>>> print(ast.dump(ast.parse(""" ... while x: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ While( test=Name(id='x', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])])
- class ast.Break¶
- class ast.Continue¶
The
break
andcontinue
statements.>>> print(ast.dump(ast.parse("""\ ... for a in b: ... if a > 5: ... break ... else: ... continue ... ... """), indent=4)) Module( body=[ For( target=Name(id='a', ctx=Store()), iter=Name(id='b', ctx=Load()), body=[ If( test=Compare( left=Name(id='a', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=5)]), body=[ Break()], orelse=[ Continue()])])])
- class ast.Try(body, handlers, orelse, finalbody)¶
try
blocks. All attributes are list of nodes to execute, except forhandlers
, which is a list ofExceptHandler
nodes.>>> print(ast.dump(ast.parse(""" ... try: ... ... ... except Exception: ... ... ... except OtherException as e: ... ... ... else: ... ... ... finally: ... ... ... """), indent=4)) Module( body=[ Try( body=[ Expr( value=Constant(value=Ellipsis))], handlers=[ ExceptHandler( type=Name(id='Exception', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))]), ExceptHandler( type=Name(id='OtherException', ctx=Load()), name='e', body=[ Expr( value=Constant(value=Ellipsis))])], orelse=[ Expr( value=Constant(value=Ellipsis))], finalbody=[ Expr( value=Constant(value=Ellipsis))])])
- class ast.TryStar(body, handlers, orelse, finalbody)¶
try
blocks which are followed byexcept*
clauses. The attributes are the same as forTry
but theExceptHandler
nodes inhandlers
are interpreted asexcept*
blocks rather thenexcept
.>>> print(ast.dump(ast.parse(""" ... try: ... ... ... except* Exception: ... ... ... """), indent=4)) Module( body=[ TryStar( body=[ Expr( value=Constant(value=Ellipsis))], handlers=[ ExceptHandler( type=Name(id='Exception', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.11.
- class ast.ExceptHandler(type, name, body)¶
A single
except
clause.type
is the exception type it will match, typically aName
node (orNone
for a catch-allexcept:
clause).name
is a raw string for the name to hold the exception, orNone
if the clause doesn’t haveas foo
.body
is a list of nodes.>>> print(ast.dump(ast.parse("""\ ... try: ... a + 1 ... except TypeError: ... pass ... """), indent=4)) Module( body=[ Try( body=[ Expr( value=BinOp( left=Name(id='a', ctx=Load()), op=Add(), right=Constant(value=1)))], handlers=[ ExceptHandler( type=Name(id='TypeError', ctx=Load()), body=[ Pass()])])])
- class ast.With(items, body, type_comment)¶
A
with
block.items
is a list ofwithitem
nodes representing the context managers, andbody
is the indented block inside the context.- type_comment¶
type_comment
is an optional string with the type annotation as a comment.
- class ast.withitem(context_expr, optional_vars)¶
A single context manager in a
with
block.context_expr
is the context manager, often aCall
node.optional_vars
is aName
,Tuple
orList
for theas foo
part, orNone
if that isn’t used.>>> print(ast.dump(ast.parse("""\ ... with a as b, c as d: ... something(b, d) ... """), indent=4)) Module( body=[ With( items=[ withitem( context_expr=Name(id='a', ctx=Load()), optional_vars=Name(id='b', ctx=Store())), withitem( context_expr=Name(id='c', ctx=Load()), optional_vars=Name(id='d', ctx=Store()))], body=[ Expr( value=Call( func=Name(id='something', ctx=Load()), args=[ Name(id='b', ctx=Load()), Name(id='d', ctx=Load())]))])])
Pattern matching¶
- class ast.Match(subject, cases)¶
A
match
statement.subject
holds the subject of the match (the object that is being matched against the cases) andcases
contains an iterable ofmatch_case
nodes with the different cases.Added in version 3.10.
- class ast.match_case(pattern, guard, body)¶
A single case pattern in a
match
statement.pattern
contains the match pattern that the subject will be matched against. Note that theAST
nodes produced for patterns differ from those produced for expressions, even when they share the same syntax.The
guard
attribute contains an expression that will be evaluated if the pattern matches the subject.body
contains a list of nodes to execute if the pattern matches and the result of evaluating the guard expression is true.>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] if x>0: ... ... ... case tuple(): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchAs(name='x')]), guard=Compare( left=Name(id='x', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=0)]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchClass( cls=Name(id='tuple', ctx=Load())), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchValue(value)¶
A match literal or value pattern that compares by equality.
value
is an expression node. Permitted value nodes are restricted as described in the match statement documentation. This pattern succeeds if the match subject is equal to the evaluated value.>>> print(ast.dump(ast.parse(""" ... match x: ... case "Relevant": ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchValue( value=Constant(value='Relevant')), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchSingleton(value)¶
A match literal pattern that compares by identity.
value
is the singleton to be compared against:None
,True
, orFalse
. This pattern succeeds if the match subject is the given constant.>>> print(ast.dump(ast.parse(""" ... match x: ... case None: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSingleton(value=None), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchSequence(patterns)¶
A match sequence pattern.
patterns
contains the patterns to be matched against the subject elements if the subject is a sequence. Matches a variable length sequence if one of the subpatterns is aMatchStar
node, otherwise matches a fixed length sequence.>>> print(ast.dump(ast.parse(""" ... match x: ... case [1, 2]: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchValue( value=Constant(value=1)), MatchValue( value=Constant(value=2))]), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchStar(name)¶
Matches the rest of the sequence in a variable length match sequence pattern. If
name
is notNone
, a list containing the remaining sequence elements is bound to that name if the overall sequence pattern is successful.>>> print(ast.dump(ast.parse(""" ... match x: ... case [1, 2, *rest]: ... ... ... case [*_]: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchValue( value=Constant(value=1)), MatchValue( value=Constant(value=2)), MatchStar(name='rest')]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchSequence( patterns=[ MatchStar()]), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchMapping(keys, patterns, rest)¶
A match mapping pattern.
keys
is a sequence of expression nodes.patterns
is a corresponding sequence of pattern nodes.rest
is an optional name that can be specified to capture the remaining mapping elements. Permitted key expressions are restricted as described in the match statement documentation.This pattern succeeds if the subject is a mapping, all evaluated key expressions are present in the mapping, and the value corresponding to each key matches the corresponding subpattern. If
rest
is notNone
, a dict containing the remaining mapping elements is bound to that name if the overall mapping pattern is successful.>>> print(ast.dump(ast.parse(""" ... match x: ... case {1: _, 2: _}: ... ... ... case {**rest}: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchMapping( keys=[ Constant(value=1), Constant(value=2)], patterns=[ MatchAs(), MatchAs()]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchMapping(rest='rest'), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchClass(cls, patterns, kwd_attrs, kwd_patterns)¶
A match class pattern.
cls
is an expression giving the nominal class to be matched.patterns
is a sequence of pattern nodes to be matched against the class defined sequence of pattern matching attributes.kwd_attrs
is a sequence of additional attributes to be matched (specified as keyword arguments in the class pattern),kwd_patterns
are the corresponding patterns (specified as keyword values in the class pattern).This pattern succeeds if the subject is an instance of the nominated class, all positional patterns match the corresponding class-defined attributes, and any specified keyword attributes match their corresponding pattern.
Note: classes may define a property that returns self in order to match a pattern node against the instance being matched. Several builtin types are also matched that way, as described in the match statement documentation.
>>> print(ast.dump(ast.parse(""" ... match x: ... case Point2D(0, 0): ... ... ... case Point3D(x=0, y=0, z=0): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchClass( cls=Name(id='Point2D', ctx=Load()), patterns=[ MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0))]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchClass( cls=Name(id='Point3D', ctx=Load()), kwd_attrs=[ 'x', 'y', 'z'], kwd_patterns=[ MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0))]), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchAs(pattern, name)¶
A match “as-pattern”, capture pattern or wildcard pattern.
pattern
contains the match pattern that the subject will be matched against. If the pattern isNone
, the node represents a capture pattern (i.e a bare name) and will always succeed.The
name
attribute contains the name that will be bound if the pattern is successful. Ifname
isNone
,pattern
must also beNone
and the node represents the wildcard pattern.>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] as y: ... ... ... case _: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchAs( pattern=MatchSequence( patterns=[ MatchAs(name='x')]), name='y'), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchAs(), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
- class ast.MatchOr(patterns)¶
A match “or-pattern”. An or-pattern matches each of its subpatterns in turn to the subject, until one succeeds. The or-pattern is then deemed to succeed. If none of the subpatterns succeed the or-pattern fails. The
patterns
attribute contains a list of match pattern nodes that will be matched against the subject.>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] | (y): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchOr( patterns=[ MatchSequence( patterns=[ MatchAs(name='x')]), MatchAs(name='y')]), body=[ Expr( value=Constant(value=Ellipsis))])])])
Added in version 3.10.
Type annotations¶
- class ast.TypeIgnore(lineno, tag)¶
A
# type: ignore
comment located at lineno. tag is the optional tag specified by the form# type: ignore <tag>
.>>> print(ast.dump(ast.parse('x = 1 # type: ignore', type_comments=True), indent=4)) Module( body=[ Assign( targets=[ Name(id='x', ctx=Store())], value=Constant(value=1))], type_ignores=[ TypeIgnore(lineno=1, tag='')]) >>> print(ast.dump(ast.parse('x: bool = 1 # type: ignore[assignment]', type_comments=True), indent=4)) Module( body=[ AnnAssign( target=Name(id='x', ctx=Store()), annotation=Name(id='bool', ctx=Load()), value=Constant(value=1), simple=1)], type_ignores=[ TypeIgnore(lineno=1, tag='[assignment]')])
Note
TypeIgnore
nodes are not generated when the type_comments parameter is set toFalse
(default). Seeast.parse()
for more details.Added in version 3.8.
Type parameters¶
Type parameters can exist on classes, functions, and type aliases.
- class ast.TypeVar(name, bound, default_value)¶
A
typing.TypeVar
.name
is the name of the type variable.bound
is the bound or constraints, if any. Ifbound
is aTuple
, it represents constraints; otherwise it represents the bound.default_value
is the default value; if theTypeVar
has no default, this attribute will be set toNone
.>>> print(ast.dump(ast.parse("type Alias[T: int = bool] = list[T]"), indent=4)) Module( body=[ TypeAlias( name=Name(id='Alias', ctx=Store()), type_params=[ TypeVar( name='T', bound=Name(id='int', ctx=Load()), default_value=Name(id='bool', ctx=Load()))], value=Subscript( value=Name(id='list', ctx=Load()), slice=Name(id='T', ctx=Load()), ctx=Load()))])
Added in version 3.12.
Changed in version 3.13: Added the default_value parameter.
- class ast.ParamSpec(name, default_value)¶
A
typing.ParamSpec
.name
is the name of the parameter specification.default_value
is the default value; if theParamSpec
has no default, this attribute will be set toNone
.>>> print(ast.dump(ast.parse("type Alias[**P = [int, str]] = Callable[P, int]"), indent=4)) Module( body=[ TypeAlias( name=Name(id='Alias', ctx=Store()), type_params=[ ParamSpec( name='P', default_value=List( elts=[ Name(id='int', ctx=Load()), Name(id='str', ctx=Load())], ctx=Load()))], value=Subscript( value=Name(id='Callable', ctx=Load()), slice=Tuple( elts=[ Name(id='P', ctx=Load()), Name(id='int', ctx=Load())], ctx=Load()), ctx=Load()))])
Added in version 3.12.
Changed in version 3.13: Added the default_value parameter.
- class ast.TypeVarTuple(name, default_value)¶
A
typing.TypeVarTuple
.name
is the name of the type variable tuple.default_value
is the default value; if theTypeVarTuple
has no default, this attribute will be set toNone
.>>> print(ast.dump(ast.parse("type Alias[*Ts = ()] = tuple[*Ts]"), indent=4)) Module( body=[ TypeAlias( name=Name(id='Alias', ctx=Store()), type_params=[ TypeVarTuple( name='Ts', default_value=Tuple(ctx=Load()))], value=Subscript( value=Name(id='tuple', ctx=Load()), slice=Tuple( elts=[ Starred( value=Name(id='Ts', ctx=Load()), ctx=Load())], ctx=Load()), ctx=Load()))])
Added in version 3.12.
Changed in version 3.13: Added the default_value parameter.
Function and class definitions¶
- class ast.FunctionDef(name, args, body, decorator_list, returns, type_comment, type_params)¶
A function definition.
name
is a raw string of the function name.args
is anarguments
node.body
is the list of nodes inside the function.decorator_list
is the list of decorators to be applied, stored outermost first (i.e. the first in the list will be applied last).returns
is the return annotation.type_params
is a list of type parameters.
- type_comment¶
type_comment
is an optional string with the type annotation as a comment.
Changed in version 3.12: Added
type_params
.
- class ast.Lambda(args, body)¶
lambda
is a minimal function definition that can be used inside an expression. UnlikeFunctionDef
,body
holds a single node.>>> print(ast.dump(ast.parse('lambda x,y: ...'), indent=4)) Module( body=[ Expr( value=Lambda( args=arguments( args=[ arg(arg='x'), arg(arg='y')]), body=Constant(value=Ellipsis)))])
- class ast.arguments(posonlyargs, args, vararg, kwonlyargs, kw_defaults, kwarg, defaults)¶
The arguments for a function.
posonlyargs
,args
andkwonlyargs
are lists ofarg
nodes.vararg
andkwarg
are singlearg
nodes, referring to the*args, **kwargs
parameters.kw_defaults
is a list of default values for keyword-only arguments. If one isNone
, the corresponding argument is required.defaults
is a list of default values for arguments that can be passed positionally. If there are fewer defaults, they correspond to the last n arguments.
- class ast.arg(arg, annotation, type_comment)¶
A single argument in a list.
arg
is a raw string of the argument name;annotation
is its annotation, such as aName
node.- type_comment¶
type_comment
is an optional string with the type annotation as a comment
>>> print(ast.dump(ast.parse("""\ ... @decorator1 ... @decorator2 ... def f(a: 'annotation', b=1, c=2, *d, e, f=3, **g) -> 'return annotation': ... pass ... """), indent=4)) Module( body=[ FunctionDef( name='f', args=arguments( args=[ arg( arg='a', annotation=Constant(value='annotation')), arg(arg='b'), arg(arg='c')], vararg=arg(arg='d'), kwonlyargs=[ arg(arg='e'), arg(arg='f')], kw_defaults=[ None, Constant(value=3)], kwarg=arg(arg='g'), defaults=[ Constant(value=1), Constant(value=2)]), body=[ Pass()], decorator_list=[ Name(id='decorator1', ctx=Load()), Name(id='decorator2', ctx=Load())], returns=Constant(value='return annotation'))])
- class ast.Return(value)¶
A
return
statement.>>> print(ast.dump(ast.parse('return 4'), indent=4)) Module( body=[ Return( value=Constant(value=4))])
- class ast.Yield(value)¶
- class ast.YieldFrom(value)¶
A
yield
oryield from
expression. Because these are expressions, they must be wrapped in anExpr
node if the value sent back is not used.>>> print(ast.dump(ast.parse('yield x'), indent=4)) Module( body=[ Expr( value=Yield( value=Name(id='x', ctx=Load())))]) >>> print(ast.dump(ast.parse('yield from x'), indent=4)) Module( body=[ Expr( value=YieldFrom( value=Name(id='x', ctx=Load())))])
- class ast.Global(names)¶
- class ast.Nonlocal(names)¶
global
andnonlocal
statements.names
is a list of raw strings.>>> print(ast.dump(ast.parse('global x,y,z'), indent=4)) Module( body=[ Global( names=[ 'x', 'y', 'z'])]) >>> print(ast.dump(ast.parse('nonlocal x,y,z'), indent=4)) Module( body=[ Nonlocal( names=[ 'x', 'y', 'z'])])
- class ast.ClassDef(name, bases, keywords, body, decorator_list, type_params)¶
A class definition.
name
is a raw string for the class namebases
is a list of nodes for explicitly specified base classes.keywords
is a list ofkeyword
nodes, principally for ‘metaclass’. Other keywords will be passed to the metaclass, as per PEP 3115.body
is a list of nodes representing the code within the class definition.decorator_list
is a list of nodes, as inFunctionDef
.type_params
is a list of type parameters.
>>> print(ast.dump(ast.parse("""\ ... @decorator1 ... @decorator2 ... class Foo(base1, base2, metaclass=meta): ... pass ... """), indent=4)) Module( body=[ ClassDef( name='Foo', bases=[ Name(id='base1', ctx=Load()), Name(id='base2', ctx=Load())], keywords=[ keyword( arg='metaclass', value=Name(id='meta', ctx=Load()))], body=[ Pass()], decorator_list=[ Name(id='decorator1', ctx=Load()), Name(id='decorator2', ctx=Load())])])
Changed in version 3.12: Added
type_params
.
Async and await¶
- class ast.AsyncFunctionDef(name, args, body, decorator_list, returns, type_comment, type_params)¶
An
async def
function definition. Has the same fields asFunctionDef
.Changed in version 3.12: Added
type_params
.
- class ast.Await(value)¶
An
await
expression.value
is what it waits for. Only valid in the body of anAsyncFunctionDef
.
>>> print(ast.dump(ast.parse("""\
... async def f():
... await other_func()
... """), indent=4))
Module(
body=[
AsyncFunctionDef(
name='f',
args=arguments(),
body=[
Expr(
value=Await(
value=Call(
func=Name(id='other_func', ctx=Load()))))])])
- class ast.AsyncFor(target, iter, body, orelse, type_comment)¶
- class ast.AsyncWith(items, body, type_comment)¶
async for
loops andasync with
context managers. They have the same fields asFor
andWith
, respectively. Only valid in the body of anAsyncFunctionDef
.
Note
When a string is parsed by ast.parse()
, operator nodes (subclasses
of ast.operator
, ast.unaryop
, ast.cmpop
,
ast.boolop
and ast.expr_context
) on the returned tree
will be singletons. Changes to one will be reflected in all other
occurrences of the same value (for example, ast.Add
).
ast
helpers¶
Apart from the node classes, the ast
module defines these utility functions
and classes for traversing abstract syntax trees:
- ast.parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None, optimize=-1)¶
Parse the source into an AST node. Equivalent to
compile(source, filename, mode, flags=FLAGS_VALUE, optimize=optimize)
, whereFLAGS_VALUE
isast.PyCF_ONLY_AST
ifoptimize <= 0
andast.PyCF_OPTIMIZED_AST
otherwise.If
type_comments=True
is given, the parser is modified to check and return type comments as specified by PEP 484 and PEP 526. This is equivalent to addingast.PyCF_TYPE_COMMENTS
to the flags passed tocompile()
. This will report syntax errors for misplaced type comments. Without this flag, type comments will be ignored, and thetype_comment
field on selected AST nodes will always beNone
. In addition, the locations of# type: ignore
comments will be returned as thetype_ignores
attribute ofModule
(otherwise it is always an empty list).In addition, if
mode
is'func_type'
, the input syntax is modified to correspond to PEP 484 “signature type comments”, e.g.(str, int) -> List[str]
.Setting
feature_version
to a tuple(major, minor)
will result in a “best-effort” attempt to parse using that Python version’s grammar. For example, settingfeature_version=(3, 9)
will attempt to disallow parsing ofmatch
statements. Currentlymajor
must equal to3
. The lowest supported version is(3, 7)
(and this may increase in future Python versions); the highest issys.version_info[0:2]
. “Best-effort” attempt means there is no guarantee that the parse (or success of the parse) is the same as when run on the Python version corresponding tofeature_version
.If source contains a null character (
\0
),ValueError
is raised.Warning
Note that successfully parsing source code into an AST object doesn’t guarantee that the source code provided is valid Python code that can be executed as the compilation step can raise further
SyntaxError
exceptions. For instance, the sourcereturn 42
generates a valid AST node for a return statement, but it cannot be compiled alone (it needs to be inside a function node).In particular,
ast.parse()
won’t do any scoping checks, which the compilation step does.Warning
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.
Changed in version 3.8: Added
type_comments
,mode='func_type'
andfeature_version
.Changed in version 3.13: The minimum supported version for
feature_version
is now(3, 7)
. Theoptimize
argument was added.
- ast.unparse(ast_obj)¶
Unparse an
ast.AST
object and generate a string with code that would produce an equivalentast.AST
object if parsed back withast.parse()
.Warning
The produced code string will not necessarily be equal to the original code that generated the
ast.AST
object (without any compiler optimizations, such as constant tuples/frozensets).Warning
Trying to unparse a highly complex expression would result with
RecursionError
.Added in version 3.9.
- ast.literal_eval(node_or_string)¶
Evaluate an expression node or a string containing only a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans,
None
andEllipsis
.This can be used for evaluating strings containing Python values without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.
This function had been documented as “safe” in the past without defining what that meant. That was misleading. This is specifically designed not to execute Python code, unlike the more general
eval()
. There is no namespace, no name lookups, or ability to call out. But it is not free from attack: A relatively small input can lead to memory exhaustion or to C stack exhaustion, crashing the process. There is also the possibility for excessive CPU consumption denial of service on some inputs. Calling it on untrusted data is thus not recommended.Warning
It is possible to crash the Python interpreter due to stack depth limitations in Python’s AST compiler.
It can raise
ValueError
,TypeError
,SyntaxError
,MemoryError
andRecursionError
depending on the malformed input.Changed in version 3.2: Now allows bytes and set literals.
Changed in version 3.9: Now supports creating empty sets with
'set()'
.Changed in version 3.10: For string inputs, leading spaces and tabs are now stripped.
- ast.get_docstring(node, clean=True)¶
Return the docstring of the given node (which must be a
FunctionDef
,AsyncFunctionDef
,ClassDef
, orModule
node), orNone
if it has no docstring. If clean is true, clean up the docstring’s indentation withinspect.cleandoc()
.Changed in version 3.5:
AsyncFunctionDef
is now supported.
- ast.get_source_segment(source, node, *, padded=False)¶
Get source code segment of the source that generated node. If some location information (
lineno
,end_lineno
,col_offset
, orend_col_offset
) is missing, returnNone
.If padded is
True
, the first line of a multi-line statement will be padded with spaces to match its original position.Added in version 3.8.
- ast.fix_missing_locations(node)¶
When you compile a node tree with
compile()
, the compiler expectslineno
andcol_offset
attributes for every node that supports them. This is rather tedious to fill in for generated nodes, so this helper adds these attributes recursively where not already set, by setting them to the values of the parent node. It works recursively starting at node.
- ast.increment_lineno(node, n=1)¶
Increment the line number and end line number of each node in the tree starting at node by n. This is useful to “move code” to a different location in a file.
- ast.copy_location(new_node, old_node)¶
Copy source location (
lineno
,col_offset
,end_lineno
, andend_col_offset
) from old_node to new_node if possible, and return new_node.
- ast.iter_fields(node)¶
Yield a tuple of
(fieldname, value)
for each field innode._fields
that is present on node.
- ast.iter_child_nodes(node)¶
Yield all direct child nodes of node, that is, all fields that are nodes and all items of fields that are lists of nodes.
- ast.walk(node)¶
Recursively yield all descendant nodes in the tree starting at node (including node itself), in no specified order. This is useful if you only want to modify nodes in place and don’t care about the context.
- class ast.NodeVisitor¶
A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the
visit()
method.This class is meant to be subclassed, with the subclass adding visitor methods.
- visit(node)¶
Visit a node. The default implementation calls the method called
self.visit_classname
where classname is the name of the node class, orgeneric_visit()
if that method doesn’t exist.
- generic_visit(node)¶
This visitor calls
visit()
on all children of the node.Note that child nodes of nodes that have a custom visitor method won’t be visited unless the visitor calls
generic_visit()
or visits them itself.
- visit_Constant(node)¶
Handles all constant nodes.
Don’t use the
NodeVisitor
if you want to apply changes to nodes during traversal. For this a special visitor exists (NodeTransformer
) that allows modifications.Deprecated since version 3.8, removed in version 3.14: Methods
visit_Num()
,visit_Str()
,visit_Bytes()
,visit_NameConstant()
andvisit_Ellipsis()
will not be called in Python 3.14+. Add thevisit_Constant()
method instead to handle all constant nodes.
- class ast.NodeTransformer¶
A
NodeVisitor
subclass that walks the abstract syntax tree and allows modification of nodes.The
NodeTransformer
will walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method isNone
, the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.Here is an example transformer that rewrites all occurrences of name lookups (
foo
) todata['foo']
:class RewriteName(NodeTransformer): def visit_Name(self, node): return Subscript( value=Name(id='data', ctx=Load()), slice=Constant(value=node.id), ctx=node.ctx )
Keep in mind that if the node you’re operating on has child nodes you must either transform the child nodes yourself or call the
generic_visit()
method for the node first.For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node.
If
NodeTransformer
introduces new nodes (that weren’t part of original tree) without giving them location information (such aslineno
),fix_missing_locations()
should be called with the new sub-tree to recalculate the location information:tree = ast.parse('foo', mode='eval') new_tree = fix_missing_locations(RewriteName().visit(tree))
Usually you use the transformer like this:
node = YourTransformer().visit(node)
- ast.dump(node, annotate_fields=True, include_attributes=False, *, indent=None, show_empty=False)¶
Return a formatted dump of the tree in node. This is mainly useful for debugging purposes. If annotate_fields is true (by default), the returned string will show the names and the values for fields. If annotate_fields is false, the result string will be more compact by omitting unambiguous field names. Attributes such as line numbers and column offsets are not dumped by default. If this is wanted, include_attributes can be set to true.
If indent is a non-negative integer or string, then the tree will be pretty-printed with that indent level. An indent level of 0, negative, or
""
will only insert newlines.None
(the default) selects the single line representation. Using a positive integer indent indents that many spaces per level. If indent is a string (such as"\t"
), that string is used to indent each level.If show_empty is false (the default), optional empty lists will be omitted from the output. Optional
None
values are always omitted.Changed in version 3.9: Added the indent option.
Changed in version 3.13: Added the show_empty option.
>>> print(ast.dump(ast.parse("""\ ... async def f(): ... await other_func() ... """), indent=4, show_empty=True)) Module( body=[ AsyncFunctionDef( name='f', args=arguments( posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[]), body=[ Expr( value=Await( value=Call( func=Name(id='other_func', ctx=Load()), args=[], keywords=[])))], decorator_list=[], type_params=[])], type_ignores=[])
Compiler flags¶
The following flags may be passed to compile()
in order to change
effects on the compilation of a program:
- ast.PyCF_ALLOW_TOP_LEVEL_AWAIT¶
Enables support for top-level
await
,async for
,async with
and async comprehensions.Added in version 3.8.
- ast.PyCF_ONLY_AST¶
Generates and returns an abstract syntax tree instead of returning a compiled code object.
- ast.PyCF_OPTIMIZED_AST¶
The returned AST is optimized according to the optimize argument in
compile()
orast.parse()
.Added in version 3.13.
- ast.PyCF_TYPE_COMMENTS¶
Enables support for PEP 484 and PEP 526 style type comments (
# type: <type>
,# type: ignore <stuff>
).Added in version 3.8.
- ast.compare(a, b, /, *, compare_attributes=False)¶
Recursively compares two ASTs.
compare_attributes affects whether AST attributes are considered in the comparison. If compare_attributes is
False
(default), then attributes are ignored. Otherwise they must all be equal. This option is useful to check whether the ASTs are structurally equal but differ in whitespace or similar details. Attributes include line numbers and column offsets.Added in version 3.14.
Command-line usage¶
Added in version 3.9.
The ast
module can be executed as a script from the command line.
It is as simple as:
python -m ast [-m <mode>] [-a] [infile]
The following options are accepted:
- -h, --help¶
Show the help message and exit.
- -m <mode>¶
- --mode <mode>¶
Specify what kind of code must be compiled, like the mode argument in
parse()
.
- --no-type-comments¶
Don’t parse type comments.
- -a, --include-attributes¶
Include attributes such as line numbers and column offsets.
- --feature-version <version>¶
Python version in the format 3.x (for example, 3.10). Defaults to the current version of the interpreter.
Added in version 3.14.
- -O <level>¶
- --optimize <level>¶
Optimization level for parser. Defaults to no optimization.
Added in version 3.14.
- --show-empty¶
Show empty lists and fields that are
None
. Defaults to not showing empty objects.Added in version 3.14.
If infile
is specified its contents are parsed to AST and dumped
to stdout. Otherwise, the content is read from stdin.
See also
Green Tree Snakes, an external documentation resource, has good details on working with Python ASTs.
ASTTokens annotates Python ASTs with the positions of tokens and text in the source code that generated them. This is helpful for tools that make source code transformations.
leoAst.py unifies the token-based and parse-tree-based views of python programs by inserting two-way links between tokens and ast nodes.
LibCST parses code as a Concrete Syntax Tree that looks like an ast tree and keeps all formatting details. It’s useful for building automated refactoring (codemod) applications and linters.
Parso is a Python parser that supports error recovery and round-trip parsing for different Python versions (in multiple Python versions). Parso is also able to list multiple syntax errors in your Python file.