ast
— Abstract Syntax TreesSource 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.
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)
| AsyncFunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? type_comment)
| ClassDef(identifier name,
expr* bases,
keyword* keywords,
stmt* body,
expr* decorator_list)
| Return(expr? value)
| Delete(expr* targets)
| Assign(expr* targets, expr value, string? type_comment)
| 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)
| Raise(expr? exc, expr? cause)
| Try(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)
| JoinedStr(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)
withitem = (expr context_expr, expr? optional_vars)
type_ignore = TypeIgnore(int lineno, string tag)
}
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
below. They are defined in the _ast
C
module and re-exported in ast
.
There is one class defined for each left-hand side symbol in the abstract
grammar (for example, ast.stmt
or 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,
ast.BinOp
inherits from 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.
_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 attribute left
of type 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 compile()
.
lineno
col_offset
end_lineno
end_col_offset
Instances of ast.expr
and ast.stmt
subclasses have
lineno
, col_offset
, lineno
, and col_offset
attributes. The lineno
and end_lineno
are the first and
last line numbers of source text span (1-indexed so the first line is line 1)
and the col_offset
and 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 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
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)
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.
Deprecated since version 3.8: Old classes ast.Num
, ast.Str
, ast.Bytes
,
ast.NameConstant
and 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 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.
ast.
Constant
(value)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))
ast.
FormattedValue
(value, conversion, format_spec)JoinedStr
.value
is any expression node (such as a literal, a variable, or a function call).conversion
is an integer:!s
string formatting!r
repr formatting!a
ascii formattingformat_spec
is a JoinedStr
node representing the formatting of the value, or None
if no format was specified. Both conversion
and format_spec
can be set at the same time.ast.
JoinedStr
(values)An f-string, comprising a series of FormattedValue
and 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')]))]))
ast.
List
(elts, ctx)ast.
Tuple
(elts, ctx)A list or tuple. elts
holds a list of nodes representing the elements.
ctx
is Store
if the container is an assignment target (i.e.
(x,y)=something
), and 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()))
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)]))
ast.
Dict
(keys, values)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())]))
ast.
Name
(id, ctx)id
holds the name as a string, and ctx
is one of the following types.ast.
Load
ast.
Store
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()))],
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=[])
ast.
Starred
(value, ctx)A *var
variable reference. value
holds the variable, typically a
Name
node. This type must be used when building a 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=[])
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, a Constant
, a
Name
, a Lambda
, a Yield
or 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=[])
ast.
UnaryOp
(op, operand)op
is the operator, and operand
any expression node.ast.
UAdd
ast.
USub
ast.
Not
ast.
Invert
Unary operator tokens. Not
is the not
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())))
ast.
BinOp
(left, op, right)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())))
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
ast.
BoolOp
(op, values)A boolean operation, ‘or’ or ‘and’. op
is Or
or 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 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())]))
ast.
And
class ast.
Or
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, 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)]))
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
ast.
Call
(func, args, keywords, starargs, kwargs)A function call. func
is the function, which will often be a
Name
or Attribute
object. Of the arguments:
args
holds a list of the arguments passed by position.
keywords
holds a list of keyword
objects representing
arguments passed by keyword.
When creating a Call
node, args
and keywords
are required, but
they can be empty lists. starargs
and kwargs
are optional.
>>> 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()))]))
ast.
keyword
(arg, value)arg
is a raw string of the parameter name, value
is a node to pass in.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 are 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())))
ast.
Attribute
(value, attr, ctx)Attribute access, e.g. d.keys
. value
is a node, typically a
Name
. attr
is a bare string giving the name of the attribute,
and ctx
is Load
, Store
or 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()))
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)))
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 a Tuple
and contain a Slice
.
ctx
is Load
, Store
or 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()))
ast.
Slice
(lower, upper, step)Regular slicing (on the form lower:upper
or lower:upper:step
).
Can occur only inside the slice field of Subscript
, either
directly or as an element of 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()))
ast.
ListComp
(elt, generators)ast.
SetComp
(elt, generators)ast.
GeneratorExp
(elt, generators)ast.
DictComp
(key, value, generators)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 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)]))
ast.
comprehension
(target, iter, ifs, is_async)One for
clause in a comprehension. target
is the reference to use for
each element - typically a Name
or 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)]))
ast.
Assign
(targets, value, type_comment)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 Tuple
or List
within targets
.
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=[])
ast.
AnnAssign
(target, annotation, value, simple)An assignment with a type annotation. target
is a single node and can
be a Name
, a Attribute
or a Subscript
.
annotation
is the annotation, such as a Constant
or Name
node. value
is a single optional node. simple
is a boolean integer
set to True for a 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=[])
ast.
AugAssign
(target, op, value)Augmented assignment, such as a += 1
. In the following example,
target
is a Name
node for x
(with the Store
context), op
is Add
, and value
is a Constant
with
value for 1.
The target
attribute connot be of class Tuple
or List
,
unlike the targets of 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=[])
ast.
Raise
(exc, cause)A raise
statement. exc
is the exception object to be raised, normally a
Call
or 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=[])
ast.
Assert
(test, msg)An assertion. test
holds the condition, such as a 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=[])
ast.
Delete
(targets)Represents a del
statement. targets
is a list of nodes, such as
Name
, Attribute
or 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=[])
ast.
Pass
A pass
statement.
>>> print(ast.dump(ast.parse('pass'), indent=4))
Module(
body=[
Pass()],
type_ignores=[])
Other statements which are only applicable inside functions or loops are described in other sections.
ast.
Import
(names)An import statement. names
is a list of 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=[])
ast.
ImportFrom
(module, names, level)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=[])
ast.
alias
(name, asname)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.
ast.
If
(test, body, orelse)An if
statement. test
holds a single node, such as a 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 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=[])
ast.
For
(target, iter, body, orelse, type_comment)A for
loop. target
holds the variable(s) the loop assigns to, as a
single Name
, Tuple
or List
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.
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=[])
ast.
While
(test, body, orelse)A while
loop. test
holds the condition, such as a 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=[])
ast.
Break
ast.
Continue
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=[])
ast.
Try
(body, handlers, orelse, finalbody)try
blocks. All attributes are list of nodes to execute, except for
handlers
, which is a list of 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=[])
ast.
ExceptHandler
(type, name, body)A single except
clause. type
is the exception type it will match,
typically a 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=[])
ast.
With
(items, body, type_comment)with
block. items
is a list of withitem
nodes representing the context managers, and body
is the indented block inside the context.type_comment
type_comment
is an optional string with the type annotation as a comment.ast.
withitem
(context_expr, optional_vars)A single context manager in a with
block. context_expr
is the context
manager, often a Call
node. optional_vars
is a Name
,
Tuple
or 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=[])
ast.
FunctionDef
(name, args, body, decorator_list, returns, type_comment)A function definition.
name
is a raw string of the function name.
args
is a arguments
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_comment
type_comment
is an optional string with the type annotation as a comment.
ast.
Lambda
(args, body)lambda
is a minimal function definition that can be used inside an
expression. Unlike 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=[])
ast.
arguments
(posonlyargs, args, vararg, kwonlyargs, kw_defaults, kwarg, defaults)posonlyargs
, args
and kwonlyargs
are lists of arg
nodes.vararg
and kwarg
are single arg
nodes, referring to the *args, **kwargs
parameters.kw_defaults
is a list of default values for keyword-only arguments. If one is None
, 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.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 a Str
or
Name
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(
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_ignores=[])
ast.
Return
(value)A return
statement.
>>> print(ast.dump(ast.parse('return 4'), indent=4))
Module(
body=[
Return(
value=Constant(value=4))],
type_ignores=[])
ast.
Yield
(value)ast.
YieldFrom
(value)A yield
or yield from
expression. Because these are expressions, they
must be wrapped in a 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=[])
ast.
Global
(names)ast.
Nonlocal
(names)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=[])
ast.
ClassDef
(name, bases, keywords, starargs, kwargs, body, decorator_list)A class definition.
name
is a raw string for the class name
bases
is a list of nodes for explicitly specified base classes.
keywords
is a list of keyword
nodes, principally for ‘metaclass’.
Other keywords will be passed to the metaclass, as per PEP-3115.
starargs
and kwargs
are each a single node, as in a function call.
starargs will be expanded to join the list of base classes, and kwargs will
be passed to the metaclass.
body
is a list of nodes representing the code within the class
definition.
decorator_list
is a list of nodes, as in 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_ignores=[])
ast.
AsyncFunctionDef
(name, args, body, decorator_list, returns, type_comment)async def
function definition. Has the same fields as FunctionDef
.ast.
Await
(value)await
expression. value
is what it waits for. Only valid in the body of an 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_ignores=[])
ast.
AsyncFor
(target, iter, body, orelse, type_comment)class ast.
AsyncWith
(items, body, type_comment)
async for
loops and async with
context managers. They have the same fields as For
and With
, respectively. Only valid in the body of an AsyncFunctionDef
.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 (e.g. ast.Add
).
ast
HelpersApart 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)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 ast.PyCF_TYPE_COMMENTS
to the
flags passed to 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 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]
.
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'
and feature_version
.
ast.
unparse
(ast_obj)Unparse an ast.AST
object and generate a string with code
that would produce an equivalent ast.AST
object if parsed
back with ast.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
.
New in version 3.9.
ast.
literal_eval
(node_or_string)Safely evaluate an expression node or a string containing 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, and None
.
This can be used for safely evaluating strings containing Python values from untrusted sources without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.
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 3.2: Now allows bytes and set literals.
Changed in version 3.9: Now supports creating empty sets with 'set()'
.
ast.
get_docstring
(node, clean=True)Return the docstring of the given node (which must be a
FunctionDef
, AsyncFunctionDef
, ClassDef
,
or Module
node), or None
if it has no docstring.
If clean is true, clean up the docstring’s indentation with
inspect.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
, or 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.
New in version 3.8.
ast.
fix_missing_locations
(node)compile()
, the compiler expects lineno
and 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.ast.
increment_lineno
(node, n=1)ast.
copy_location
(new_node, old_node)lineno
, col_offset
, end_lineno
, and end_col_offset
) from old_node to new_node if possible, and return new_node.ast.
iter_fields
(node)(fieldname, value)
for each field in node._fields
that is present on node.ast.
iter_child_nodes
(node)ast.
walk
(node)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, or generic_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.
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: Methods visit_Num()
, visit_Str()
, visit_Bytes()
,
visit_NameConstant()
and visit_Ellipsis()
are deprecated
now and will not be called in future Python versions. Add the
visit_Constant()
method to handle all constant nodes.
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 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 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 as
lineno
), 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)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.
Changed in version 3.9: Added the indent option.
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.
New in version 3.8.
ast.
PyCF_ONLY_AST
ast.
PyCF_TYPE_COMMENTS
Enables support for PEP 484 and PEP 526 style type comments
(# type: <type>
, # type: ignore <stuff>
).
New in version 3.8.
New 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
-m
<mode>
--mode
<mode>
parse()
.--no-type-comments
-a
,
--include-attributes
-i
<indent>
--indent
<indent>
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.