:mod:`ast` --- Abstract Syntax Trees
.. module:: ast :synopsis: Abstract Syntax Tree classes and manipulation.
.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
.. sectionauthor:: Georg Brandl <georg@python.org>
.. testsetup::
import ast
Source code: :source:`Lib/ast.py`
The :mod:`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 :data:`ast.PyCF_ONLY_AST` as a flag to the :func:`compile` built-in function, or using the :func:`parse` helper provided in this module. The result will be a tree of objects whose classes all inherit from :class:`ast.AST`. An abstract syntax tree can be compiled into a Python code object using the built-in :func:`compile` function.
The abstract grammar is currently defined as follows:
.. literalinclude:: ../../Parser/Python.asdl :language: asdl
This is the base of all AST node classes. The actual node classes are derived from the :file:`Parser/Python.asdl` file, which is reproduced :ref:`above <abstract-grammar>`. They are defined in the :mod:`_ast` C module and re-exported in :mod:`ast`.
There is one class defined for each left-hand side symbol in the abstract grammar (for example, :class:`ast.stmt` or :class:`ast.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, :class:`ast.BinOp` inherits from :class:`ast.expr`. For production rules with alternatives (aka "sums"), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.
.. index:: single: ? (question mark); in AST grammar
.. index:: single: * (asterisk); in AST grammar
.. attribute:: _fields Each concrete class has an attribute :attr:`_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, :class:`ast.BinOp` instances have an attribute :attr:`left` of type :class:`ast.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 with :func:`compile`.
.. attribute:: lineno
col_offset
end_lineno
end_col_offset
Instances of :class:`ast.expr` and :class:`ast.stmt` subclasses have
:attr:`lineno`, :attr:`col_offset`, :attr:`end_lineno`, and
:attr:`end_col_offset` attributes. The :attr:`lineno` and :attr:`end_lineno`
are the first and last line numbers of source text span (1-indexed so the
first line is line 1) and the :attr:`col_offset` and :attr:`end_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 :class:`ast.T` parses its arguments as follows:
- If there are positional arguments, there must be as many as there are items in :attr:`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 :class:`ast.UnaryOp` node, you could use
node = ast.UnaryOp() node.op = ast.USub() node.operand = ast.Constant() node.operand.value = 5 node.operand.lineno = 0 node.operand.col_offset = 0 node.lineno = 0 node.col_offset = 0
or the more compact
node = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0),
lineno=0, col_offset=0)
.. versionchanged:: 3.8 Class :class:`ast.Constant` is now used for all constants.
.. versionchanged:: 3.9 Simple indices are represented by their value, extended slices are represented as tuples.
.. deprecated:: 3.8 Old classes :class:`ast.Num`, :class:`ast.Str`, :class:`ast.Bytes`, :class:`ast.NameConstant` and :class:`ast.Ellipsis` 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:: 3.9 Old classes :class:`ast.Index` and :class:`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.
Note
The descriptions of the specific node classes displayed here were initially adapted from the fantastic Green Tree Snakes project and all its contributors.
A constant value. The value attribute of the Constant literal contains the
Python object it represents. The values represented can be simple types
such as a number, string or None, but also immutable container types
(tuples and frozensets) if all of their elements are constant.
>>> print(ast.dump(ast.parse('123', mode='eval'), indent=4))
Expression(
body=Constant(value=123))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 :class:`JoinedStr`.
valueis any expression node (such as a literal, a variable, or a function call).conversionis an integer:- -1: no formatting
- 115:
!sstring formatting - 114:
!rrepr formatting - 97:
!aascii formatting
format_specis a :class:`JoinedStr` node representing the formatting of the value, orNoneif no format was specified. Bothconversionandformat_speccan be set at the same time.
An f-string, comprising a series of :class:`FormattedValue` and :class:`Constant` 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())],
keywords=[]),
conversion=-1,
format_spec=JoinedStr(
values=[
Constant(value='.3')]))]))A list or tuple. elts holds a list of nodes representing the elements.
ctx is :class:`Store` if the container is an assignment target (i.e.
(x,y)=something), and :class:`Load` 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()))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)]))A dictionary. keys and values hold lists of nodes representing the
keys and the values respectively, in matching order (what would be returned
when calling dictionary.keys() and dictionary.values()).
When doing dictionary unpacking using dictionary literals the expression to be
expanded goes in the values list, with a None at the corresponding
position in keys.
>>> 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())]))A variable name. id holds the name as a string, and ctx is one of
the following types.
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()))],
type_ignores=[])
>>> print(ast.dump(ast.parse('a = 1'), indent=4))
Module(
body=[
Assign(
targets=[
Name(id='a', ctx=Store())],
value=Constant(value=1))],
type_ignores=[])
>>> print(ast.dump(ast.parse('del a'), indent=4))
Module(
body=[
Delete(
targets=[
Name(id='a', ctx=Del())])],
type_ignores=[])A *var variable reference. value holds the variable, typically a
:class:`Name` node. This type must be used when building a :class:`Call`
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()))],
type_ignores=[])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, a :class:`Constant`, a
:class:`Name`, a :class:`Lambda`, a :class:`Yield` or :class:`YieldFrom` node.
>>> print(ast.dump(ast.parse('-a'), indent=4))
Module(
body=[
Expr(
value=UnaryOp(
op=USub(),
operand=Name(id='a', ctx=Load())))],
type_ignores=[])A unary operation. op is the operator, and operand any expression
node.
Unary operator tokens. :class:`Not` is the not keyword, :class:`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())))A binary operation (like addition or division). op is the operator, and
left and right 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())))Binary operator tokens.
A boolean operation, 'or' or 'and'. op is :class:`Or` or :class:`And`.
values are the values involved. Consecutive operations with the same
operator, such as a or b or c, are collapsed into one node with several
values.
This doesn't include not, which is a :class:`UnaryOp`.
>>> 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())]))Boolean operator tokens.
A comparison of two or more values. left is the first value in the
comparison, ops the list of operators, and comparators 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)]))Comparison operator tokens.
A function call. func is the function, which will often be a
:class:`Name` or :class:`Attribute` object. Of the arguments:
argsholds a list of the arguments passed by position.keywordsholds a list of :class:`keyword` objects representing arguments passed by keyword.
When creating a Call node, args and keywords are required, but
they can be 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()))]))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.
An expression such as a if b else c. Each field holds a single node, so
in the following example, all three are :class:`Name` 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())))Attribute access, e.g. d.keys. value is a node, typically a
:class:`Name`. attr is a bare string giving the name of the attribute,
and ctx is :class:`Load`, :class:`Store` or :class:`Del` 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()))A named expression. This AST node is produced by the assignment expressions operator (also known as the walrus operator). As opposed to the :class:`Assign` node in which the first argument can be multiple nodes, in this case bothtargetandvaluemust 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)))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 a :class:`Tuple` and contain a :class:`Slice`.
ctx is :class:`Load`, :class:`Store` or :class:`Del`
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()))Regular slicing (on the form lower:upper or lower:upper:step).
Can occur only inside the slice field of :class:`Subscript`, either
directly or as an element of :class:`Tuple`.
>>> 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()))List and set comprehensions, generator expressions, and dictionary
comprehensions. elt (or key and value) is a single node
representing the part that will be evaluated for each item.
generators is a list of :class:`comprehension` 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()),
ifs=[],
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()),
ifs=[],
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()),
ifs=[],
is_async=0)]))One for clause in a comprehension. target is the reference to use for
each element - typically a :class:`Name` or :class:`Tuple` node. iter
is the object to iterate over. ifs is a list of test expressions: each
for clause can have multiple ifs.
is_async indicates a comprehension is asynchronous (using an
async for instead of for). 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())],
keywords=[]),
generators=[
comprehension(
target=Name(id='line', ctx=Store()),
iter=Name(id='file', ctx=Load()),
ifs=[],
is_async=0),
comprehension(
target=Name(id='c', ctx=Store()),
iter=Name(id='line', ctx=Load()),
ifs=[],
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()),
ifs=[],
is_async=1)]))An assignment. targets is a list of nodes, and value is a single node.
Multiple nodes in targets represents assigning the same value to each.
Unpacking is represented by putting a :class:`Tuple` or :class:`List`
within targets.
.. attribute:: 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))],
type_ignores=[])
>>> 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()))],
type_ignores=[])An assignment with a type annotation. target is a single node and can
be a :class:`Name`, a :class:`Attribute` or a :class:`Subscript`.
annotation is the annotation, such as a :class:`Constant` or :class:`Name`
node. value is a single optional node. simple is a boolean integer
set to True for a :class:`Name` node in target that do not appear in
between parenthesis and are hence pure names and not expressions.
>>> 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)],
type_ignores=[])
>>> 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)],
type_ignores=[])
>>> 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)],
type_ignores=[])
>>> 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)],
type_ignores=[])Augmented assignment, such as a += 1. In the following example,
target is a :class:`Name` node for x (with the :class:`Store`
context), op is :class:`Add`, and value is a :class:`Constant` with
value for 1.
The target attribute cannot be of class :class:`Tuple` or :class:`List`,
unlike the targets of :class:`Assign`.
>>> print(ast.dump(ast.parse('x += 2'), indent=4))
Module(
body=[
AugAssign(
target=Name(id='x', ctx=Store()),
op=Add(),
value=Constant(value=2))],
type_ignores=[])A raise statement. exc is the exception object to be raised, normally a
:class:`Call` or :class:`Name`, or None for a standalone raise.
cause is the optional part for y in raise 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()))],
type_ignores=[])An assertion. test holds the condition, such as a :class:`Compare` 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()))],
type_ignores=[])Represents a del statement. targets is a list of nodes, such as
:class:`Name`, :class:`Attribute` or :class:`Subscript` 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())])],
type_ignores=[])A pass statement.
>>> print(ast.dump(ast.parse('pass'), indent=4))
Module(
body=[
Pass()],
type_ignores=[])A :ref:`type alias <type-aliases>` created through the :keyword:`type`
statement. name is the name of the alias, type_params is a list of
:ref:`type parameters <ast-type-params>`, and value 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()),
type_params=[],
value=Name(id='int', ctx=Load()))],
type_ignores=[])Other statements which are only applicable inside functions or loops are described in other sections.
An import statement. names is a list of :class:`alias` 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')])],
type_ignores=[])Represents from x import y. module is a raw string of the 'from' name,
without any leading dots, or None for statements such as from . 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)],
type_ignores=[])Both parameters are raw strings of the names. asname can be None 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)],
type_ignores=[])Note
Optional clauses such as else are stored as an empty list if they're
not present.
An if statement. test holds a single node, such as a :class:`Compare`
node. body and orelse each hold a list of nodes.
elif clauses don't have a special representation in the AST, but rather
appear as extra :class:`If` nodes within the orelse 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))])])],
type_ignores=[])A for loop. target holds the variable(s) the loop assigns to, as a
single :class:`Name`, :class:`Tuple`, :class:`List`, :class:`Attribute` or
:class:`Subscript` node. iter holds the item to be looped over, again
as a single node. body and orelse contain lists of nodes to execute.
Those in orelse are executed if the loop finishes normally, rather than
via a break statement.
.. attribute:: 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))])],
type_ignores=[])A while loop. test holds the condition, such as a :class:`Compare`
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))])],
type_ignores=[])The break and continue 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()])],
orelse=[])],
type_ignores=[])try blocks. All attributes are list of nodes to execute, except for
handlers, which is a list of :class:`ExceptHandler` 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))])],
type_ignores=[])try blocks which are followed by except* clauses. The attributes are the
same as for :class:`Try` but the :class:`ExceptHandler` nodes in handlers
are interpreted as except* blocks rather then except.
>>> 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))])],
orelse=[],
finalbody=[])],
type_ignores=[])A single except clause. type is the exception type it will match,
typically a :class:`Name` node (or None for a catch-all except: clause).
name is a raw string for the name to hold the exception, or None if
the clause doesn't have as 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()])],
orelse=[],
finalbody=[])],
type_ignores=[])A with block. items is a list of :class:`withitem` nodes representing
the context managers, and body is the indented block inside the context.
.. attribute:: type_comment
``type_comment`` is an optional string with the type annotation as a comment.
A single context manager in a with block. context_expr is the context
manager, often a :class:`Call` node. optional_vars is a :class:`Name`,
:class:`Tuple` or :class:`List` for the as foo part, or None 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())],
keywords=[]))])],
type_ignores=[])A match statement. subject holds the subject of the match (the object
that is being matched against the cases) and cases contains an iterable of
:class:`match_case` nodes with the different cases.
A single case pattern in a match statement. pattern contains the
match pattern that the subject will be matched against. Note that the
:class:`AST` 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()),
patterns=[],
kwd_attrs=[],
kwd_patterns=[]),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])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))])])],
type_ignores=[])A match literal pattern that compares by identity. value is the
singleton to be compared against: None, True, or False. 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))])])],
type_ignores=[])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 a MatchStar 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))])])],
type_ignores=[])Matches the rest of the sequence in a variable length match sequence pattern.
If name is not None, 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))])])],
type_ignores=[])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 not None, 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(keys=[], patterns=[], rest='rest'),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])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))],
kwd_attrs=[],
kwd_patterns=[]),
body=[
Expr(
value=Constant(value=Ellipsis))]),
match_case(
pattern=MatchClass(
cls=Name(id='Point3D', ctx=Load()),
patterns=[],
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))])])],
type_ignores=[])A match "as-pattern", capture pattern or wildcard pattern. pattern
contains the match pattern that the subject will be matched against.
If the pattern is None, 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. If name is None, pattern must also be None
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))])])],
type_ignores=[])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))])])],
type_ignores=[]):ref:`Type parameters <type-params>` can exist on classes, functions, and type aliases.
A :class:`typing.TypeVar`. name is the name of the type variable.
bound is the bound or constraints, if any. If bound is a :class:`Tuple`,
it represents constraints; otherwise it represents the bound.
>>> print(ast.dump(ast.parse("type Alias[T: int] = list[T]"), indent=4))
Module(
body=[
TypeAlias(
name=Name(id='Alias', ctx=Store()),
type_params=[
TypeVar(
name='T',
bound=Name(id='int', ctx=Load()))],
value=Subscript(
value=Name(id='list', ctx=Load()),
slice=Name(id='T', ctx=Load()),
ctx=Load()))],
type_ignores=[])A :class:`typing.ParamSpec`. name is the name of the parameter specification.
>>> print(ast.dump(ast.parse("type Alias[**P] = Callable[P, int]"), indent=4))
Module(
body=[
TypeAlias(
name=Name(id='Alias', ctx=Store()),
type_params=[
ParamSpec(name='P')],
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()))],
type_ignores=[])A :class:`typing.TypeVarTuple`. name is the name of the type variable tuple.
>>> 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')],
value=Subscript(
value=Name(id='tuple', ctx=Load()),
slice=Tuple(
elts=[
Starred(
value=Name(id='Ts', ctx=Load()),
ctx=Load())],
ctx=Load()),
ctx=Load()))],
type_ignores=[])A function definition.
nameis a raw string of the function name.type_paramsis a list of :ref:`type parameters <ast-type-params>`.argsis an :class:`arguments` node.bodyis the list of nodes inside the function.decorator_listis the list of decorators to be applied, stored outermost first (i.e. the first in the list will be applied last).returnsis the return annotation.
.. attribute:: type_comment
``type_comment`` is an optional string with the type annotation as a comment.
lambda is a minimal function definition that can be used inside an
expression. Unlike :class:`FunctionDef`, body holds a single node.
>>> print(ast.dump(ast.parse('lambda x,y: ...'), indent=4))
Module(
body=[
Expr(
value=Lambda(
args=arguments(
posonlyargs=[],
args=[
arg(arg='x'),
arg(arg='y')],
kwonlyargs=[],
kw_defaults=[],
defaults=[]),
body=Constant(value=Ellipsis)))],
type_ignores=[])The arguments for a function.
posonlyargs,argsandkwonlyargsare lists of :class:`arg` nodes.varargandkwargare single :class:`arg` nodes, referring to the*args, **kwargsparameters.kw_defaultsis a list of default values for keyword-only arguments. If one isNone, the corresponding argument is required.defaultsis a list of default values for arguments that can be passed positionally. If there are fewer defaults, they correspond to the last n arguments.
A single argument in a list. arg is a raw string of the argument
name, annotation is its annotation, such as a :class:`Str` or
:class:`Name` node.
.. attribute:: 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(
posonlyargs=[],
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'),
type_params=[])],
type_ignores=[])A return statement.
>>> print(ast.dump(ast.parse('return 4'), indent=4))
Module(
body=[
Return(
value=Constant(value=4))],
type_ignores=[])A yield or yield from expression. Because these are expressions, they
must be wrapped in a :class:`Expr` 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())))],
type_ignores=[])
>>> print(ast.dump(ast.parse('yield from x'), indent=4))
Module(
body=[
Expr(
value=YieldFrom(
value=Name(id='x', ctx=Load())))],
type_ignores=[])global and nonlocal 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'])],
type_ignores=[])
>>> print(ast.dump(ast.parse('nonlocal x,y,z'), indent=4))
Module(
body=[
Nonlocal(
names=[
'x',
'y',
'z'])],
type_ignores=[])A class definition.
nameis a raw string for the class nametype_paramsis a list of :ref:`type parameters <ast-type-params>`.basesis a list of nodes for explicitly specified base classes.keywordsis a list of :class:`keyword` nodes, principally for 'metaclass'. Other keywords will be passed to the metaclass, as per PEP-3115.bodyis a list of nodes representing the code within the class definition.decorator_listis a list of nodes, as in :class:`FunctionDef`.
>>> 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())],
type_params=[])],
type_ignores=[])An async def function definition. Has the same fields as
:class:`FunctionDef`.
An await expression. value is what it waits for.
Only valid in the body of an :class:`AsyncFunctionDef`.
>>> print(ast.dump(ast.parse("""\
... async def f():
... await other_func()
... """), indent=4))
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=[])async for loops and async with context managers. They have the same
fields as :class:`For` and :class:`With`, respectively. Only valid in the
body of an :class:`AsyncFunctionDef`.
Note
When a string is parsed by :func:`ast.parse`, operator nodes (subclasses of :class:`ast.operator`, :class:`ast.unaryop`, :class:`ast.cmpop`, :class:`ast.boolop` and :class:`ast.expr_context`) on the returned tree will be singletons. Changes to one will be reflected in all other occurrences of the same value (e.g. :class:`ast.Add`).
:mod:`ast` Helpers
Apart from the node classes, the :mod:`ast` module defines these utility functions and classes for traversing abstract syntax trees:
.. function:: parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)
Parse the source into an AST node. Equivalent to ``compile(source,
filename, mode, ast.PyCF_ONLY_AST)``.
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 adding :data:`ast.PyCF_TYPE_COMMENTS` to the
flags passed to :func:`compile()`. This will report syntax errors
for misplaced type comments. Without this flag, type comments will
be ignored, and the ``type_comment`` field on selected AST nodes
will always be ``None``. In addition, the locations of ``# type:
ignore`` comments will be returned as the ``type_ignores``
attribute of :class:`Module` (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]``.
Also, setting ``feature_version`` to a tuple ``(major, minor)``
will attempt to parse using that Python version's grammar.
Currently ``major`` must equal to ``3``. For example, setting
``feature_version=(3, 4)`` will allow the use of ``async`` and
``await`` as variable names. The lowest supported version is
``(3, 4)``; the highest is ``sys.version_info[0:2]``.
If source contains a null character ('\0'), :exc:`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 :exc:`SyntaxError`
exceptions. For instance, the source ``return 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, :func:`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.
.. versionchanged:: 3.8
Added ``type_comments``, ``mode='func_type'`` and ``feature_version``.
.. function:: unparse(ast_obj)
Unparse an :class:`ast.AST` object and generate a string with code
that would produce an equivalent :class:`ast.AST` object if parsed
back with :func:`ast.parse`.
.. warning::
The produced code string will not necessarily be equal to the original
code that generated the :class:`ast.AST` object (without any compiler
optimizations, such as constant tuples/frozensets).
.. warning::
Trying to unparse a highly complex expression would result with
:exc:`RecursionError`.
.. versionadded:: 3.9
.. function:: 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`` and ``Ellipsis``.
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 :func:`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 :exc:`ValueError`, :exc:`TypeError`, :exc:`SyntaxError`,
:exc:`MemoryError` and :exc:`RecursionError` depending on the malformed
input.
.. versionchanged:: 3.2
Now allows bytes and set literals.
.. versionchanged:: 3.9
Now supports creating empty sets with ``'set()'``.
.. versionchanged:: 3.10
For string inputs, leading spaces and tabs are now stripped.
.. function:: get_docstring(node, clean=True)
Return the docstring of the given *node* (which must be a
:class:`FunctionDef`, :class:`AsyncFunctionDef`, :class:`ClassDef`,
or :class:`Module` node), or ``None`` if it has no docstring.
If *clean* is true, clean up the docstring's indentation with
:func:`inspect.cleandoc`.
.. versionchanged:: 3.5
:class:`AsyncFunctionDef` is now supported.
.. function:: get_source_segment(source, node, *, padded=False) Get source code segment of the *source* that generated *node*. If some location information (:attr:`lineno`, :attr:`end_lineno`, :attr:`col_offset`, or :attr:`end_col_offset`) is missing, return ``None``. If *padded* is ``True``, the first line of a multi-line statement will be padded with spaces to match its original position. .. versionadded:: 3.8
.. function:: fix_missing_locations(node) When you compile a node tree with :func:`compile`, the compiler expects :attr:`lineno` and :attr:`col_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*.
.. function:: 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.
.. function:: copy_location(new_node, old_node) Copy source location (:attr:`lineno`, :attr:`col_offset`, :attr:`end_lineno`, and :attr:`end_col_offset`) from *old_node* to *new_node* if possible, and return *new_node*.
.. function:: iter_fields(node) Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields`` that is present on *node*.
.. function:: 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.
.. function:: 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.
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 :meth:`visit` method.
This class is meant to be subclassed, with the subclass adding visitor methods.
.. method:: visit(node)
Visit a node. The default implementation calls the method called
:samp:`self.visit_{classname}` where *classname* is the name of the node
class, or :meth:`generic_visit` if that method doesn't exist.
.. method:: generic_visit(node) This visitor calls :meth:`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 :meth:`generic_visit` or visits them itself.
Don't use the :class:`NodeVisitor` if you want to apply changes to nodes during traversal. For this a special visitor exists (:class:`NodeTransformer`) that allows modifications.
.. deprecated:: 3.8 Methods :meth:`visit_Num`, :meth:`visit_Str`, :meth:`visit_Bytes`, :meth:`visit_NameConstant` and :meth:`visit_Ellipsis` are deprecated now and will not be called in future Python versions. Add the :meth:`visit_Constant` method to handle all constant nodes.
A :class:`NodeVisitor` subclass that walks the abstract syntax tree and allows modification of nodes.
The :class:`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 is None, 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) to data['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 :meth:`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 :class:`NodeTransformer` introduces new nodes (that weren't part of original tree) without giving them location information (such as :attr:`lineno`), :func:`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)
.. function:: dump(node, annotate_fields=True, include_attributes=False, *, indent=None)
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.
.. versionchanged:: 3.9
Added the *indent* option.
The following flags may be passed to :func:`compile` in order to change effects on the compilation of a program:
.. data:: PyCF_ALLOW_TOP_LEVEL_AWAIT Enables support for top-level ``await``, ``async for``, ``async with`` and async comprehensions. .. versionadded:: 3.8
.. data:: PyCF_ONLY_AST Generates and returns an abstract syntax tree instead of returning a compiled code object.
.. data:: PyCF_TYPE_COMMENTS Enables support for :pep:`484` and :pep:`526` style type comments (``# type: <type>``, ``# type: ignore <stuff>``). .. versionadded:: 3.8
.. versionadded:: 3.9
The :mod:`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:
.. program:: ast
.. cmdoption:: -h, --help Show the help message and exit.
.. cmdoption:: -m <mode>
--mode <mode>
Specify what kind of code must be compiled, like the *mode* argument
in :func:`parse`.
.. cmdoption:: --no-type-comments Don't parse type comments.
.. cmdoption:: -a, --include-attributes Include attributes such as line numbers and column offsets.
.. cmdoption:: -i <indent>
--indent <indent>
Indentation of nodes in AST (number of spaces).
If :file:`infile` is specified its contents are parsed to AST and dumped to stdout. Otherwise, the content is read from stdin.
.. seealso::
`Green Tree Snakes <https://greentreesnakes.readthedocs.io/>`_, an external
documentation resource, has good details on working with Python ASTs.
`ASTTokens <https://asttokens.readthedocs.io/en/latest/user-guide.html>`_
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 <https://leoeditor.com/appendices.html#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 <https://libcst.readthedocs.io/>`_ 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 <https://parso.readthedocs.io>`_ 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.