ast— Abstract Syntax Trees

Source code:Lib/ast.py

---

Theastmodule 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 passingast.PyCF_ONLY_ASTas a flag to thecompile()built-in function, or using theparse() helper provided in this module. The result will be a tree of objects whose classes all inherit fromast.AST.An abstract syntax tree can be compiled into a Python code object using the built-incompile()function.

Abstract Grammar

The abstract grammar is currently defined as follows:

-- ASDL's 4 builtin types are:
-- identifier, int, string, constant

modulePython
{
mod=Module(stmt*body,type_ignore*type_ignores)
|Interactive(stmt*body)
|Expression(exprbody)
|FunctionType(expr*argtypes,exprreturns)

stmt=FunctionDef(identifiername,argumentsargs,
stmt*body,expr*decorator_list,expr?returns,
string?type_comment,type_param*type_params)
|AsyncFunctionDef(identifiername,argumentsargs,
stmt*body,expr*decorator_list,expr?returns,
string?type_comment,type_param*type_params)

|ClassDef(identifiername,
expr*bases,
keyword*keywords,
stmt*body,
expr*decorator_list,
type_param*type_params)
|Return(expr?value)

|Delete(expr*targets)
|Assign(expr*targets,exprvalue,string?type_comment)
|TypeAlias(exprname,type_param*type_params,exprvalue)
|AugAssign(exprtarget,operatorop,exprvalue)
-- 'simple' indicates that we annotate simple name without parens
|AnnAssign(exprtarget,exprannotation,expr?value,intsimple)

-- use 'orelse' because else is a keyword in target languages
|For(exprtarget,expriter,stmt*body,stmt*orelse,string?type_comment)
|AsyncFor(exprtarget,expriter,stmt*body,stmt*orelse,string?type_comment)
|While(exprtest,stmt*body,stmt*orelse)
|If(exprtest,stmt*body,stmt*orelse)
|With(withitem*items,stmt*body,string?type_comment)
|AsyncWith(withitem*items,stmt*body,string?type_comment)

|Match(exprsubject,match_case*cases)

|Raise(expr?exc,expr?cause)
|Try(stmt*body,excepthandler*handlers,stmt*orelse,stmt*finalbody)
|TryStar(stmt*body,excepthandler*handlers,stmt*orelse,stmt*finalbody)
|Assert(exprtest,expr?msg)

|Import(alias*names)
|ImportFrom(identifier?module,alias*names,int?level)

|Global(identifier*names)
|Nonlocal(identifier*names)
|Expr(exprvalue)
|Pass|Break|Continue

-- col_offset is the byte offset in the utf8 string the parser uses
attributes(intlineno,intcol_offset,int?end_lineno,int?end_col_offset)

-- BoolOp() can use left & right?
expr=BoolOp(boolopop,expr*values)
|NamedExpr(exprtarget,exprvalue)
|BinOp(exprleft,operatorop,exprright)
|UnaryOp(unaryopop,exproperand)
|Lambda(argumentsargs,exprbody)
|IfExp(exprtest,exprbody,exprorelse)
|Dict(expr*keys,expr*values)
|Set(expr*elts)
|ListComp(exprelt,comprehension*generators)
|SetComp(exprelt,comprehension*generators)
|DictComp(exprkey,exprvalue,comprehension*generators)
|GeneratorExp(exprelt,comprehension*generators)
-- the grammar constrains where yield expressions can occur
|Await(exprvalue)
|Yield(expr?value)
|YieldFrom(exprvalue)
-- need sequences for compare to distinguish between
-- x < 4 < 3 and (x < 4) < 3
|Compare(exprleft,cmpop*ops,expr*comparators)
|Call(exprfunc,expr*args,keyword*keywords)
|FormattedValue(exprvalue,intconversion,expr?format_spec)
|JoinedStr(expr*values)
|Constant(constantvalue,string?kind)

-- the following expression can appear in assignment context
|Attribute(exprvalue,identifierattr,expr_contextctx)
|Subscript(exprvalue,exprslice,expr_contextctx)
|Starred(exprvalue,expr_contextctx)
|Name(identifierid,expr_contextctx)
|List(expr*elts,expr_contextctx)
|Tuple(expr*elts,expr_contextctx)

-- 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(intlineno,intcol_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=(exprtarget,expriter,expr*ifs,intis_async)

excepthandler=ExceptHandler(expr?type,identifier?name,stmt*body)
attributes(intlineno,intcol_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=(identifierarg,expr?annotation,string?type_comment)
attributes(intlineno,intcol_offset,int?end_lineno,int?end_col_offset)

-- keyword arguments supplied to call (NULL identifier for **kwargs)
keyword=(identifier?arg,exprvalue)
attributes(intlineno,intcol_offset,int?end_lineno,int?end_col_offset)

-- import name with optional 'as' alias.
alias=(identifiername,identifier?asname)
attributes(intlineno,intcol_offset,int?end_lineno,int?end_col_offset)

withitem=(exprcontext_expr,expr?optional_vars)

match_case=(patternpattern,expr?guard,stmt*body)

pattern=MatchValue(exprvalue)
|MatchSingleton(constantvalue)
|MatchSequence(pattern*patterns)
|MatchMapping(expr*keys,pattern*patterns,identifier?rest)
|MatchClass(exprcls,pattern*patterns,identifier*kwd_attrs,pattern*kwd_patterns)

|MatchStar(identifier?name)
-- The optional "rest" MatchMapping parameter handles capturing extra mapping keys

|MatchAs(pattern?pattern,identifier?name)
|MatchOr(pattern*patterns)

attributes(intlineno,intcol_offset,intend_lineno,intend_col_offset)

type_ignore=TypeIgnore(intlineno,stringtag)

type_param=TypeVar(identifiername,expr?bound)
|ParamSpec(identifiername)
|TypeVarTuple(identifiername)
attributes(intlineno,intcol_offset,intend_lineno,intend_col_offset)
}

Node classes

classast.AST

This is the base of all AST node classes. The actual node classes are derived from theParser/Python.asdlfile, which is reproduced above.They are defined in the_astC module and re-exported inast.

There is one class defined for each left-hand side symbol in the abstract grammar (for example,ast.stmtorast.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.BinOpinherits fromast.expr.For production rules with alternatives (aka “sums” ), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.

_fields

Each concrete class has an attribute_fieldswhich 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 attributeleftof typeast.expr.

If these attributes are marked as optional in the grammar (using a question mark), the value might beNone.If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST withcompile().

lineno

col_offset

end_lineno

end_col_offset

Instances ofast.exprandast.stmtsubclasses have lineno,col_offset,end_lineno,and end_col_offsetattributes. Thelinenoandend_lineno are the first and last line numbers of source text span (1-indexed so the first line is line 1) and thecol_offsetandend_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 isafterthe 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 classast.Tparses its arguments as follows:

• If there are positional arguments, there must be as many as there are items inT._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 anast.UnaryOpnode, 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:Classast.Constantis 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 classesast.Num,ast.Str,ast.Bytes, ast.NameConstantandast.Ellipsisare 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 classesast.Indexandast.ExtSliceare 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 fantasticGreen Tree Snakesproject and all its contributors.

Root nodes

classast.Module(body,type_ignores)

A Python module, as withfile input. Node type generated byast.parse()in the default"exec"mode.

bodyis alistof the module’sStatements.

type_ignoresis alistof the module’s type ignore comments; seeast.parse()for more details.

>>>print(ast.dump(ast.parse('x = 1'),indent=4))
Module(
body=[
Assign(
targets=[
Name(id='x', ctx=Store())],
value=Constant(value=1))],
type_ignores=[])

classast.Expression(body)

A single Pythonexpression input. Node type generated byast.parse()whenmodeis"eval".

bodyis a single node, one of theexpression types.

>>>print(ast.dump(ast.parse('123',mode='eval'),indent=4))
Expression(
body=Constant(value=123))

classast.Interactive(body)

A singleinteractive input,like inInteractive Mode. Node type generated byast.parse()whenmodeis"single".

bodyis alistofstatement nodes.

>>>print(ast.dump(ast.parse('x = 1; y = 2',mode='single'),indent=4))
Interactive(
body=[
Assign(
targets=[
Name(id='x', ctx=Store())],
value=Constant(value=1)),
Assign(
targets=[
Name(id='y', ctx=Store())],
value=Constant(value=2))])

classast.FunctionType(argtypes,returns)

A representation of an old-style type comments for functions, as Python versions prior to 3.5 didn’t supportPEP 484annotations. Node type generated byast.parse()whenmodeis"func_type".

Such type comments would look like this:

defsum_two_number(a,b):
# type: (int, int) -> int
returna+b

argtypesis alistofexpression nodes.

returnsis a singleexpression node.

>>>print(ast.dump(ast.parse('(int, str) -> List[int]',mode='func_type'),indent=4))
FunctionType(
argtypes=[
Name(id='int', ctx=Load()),
Name(id='str', ctx=Load())],
returns=Subscript(
value=Name(id='List', ctx=Load()),
slice=Name(id='int', ctx=Load()),
ctx=Load()))

Added in version 3.8.

Literals

classast.Constant(value)

A constant value. Thevalueattribute of theConstantliteral contains the Python object it represents. The values represented can be simple types such as a number, string orNone,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))

classast.FormattedValue(value,conversion,format_spec)

Node representing a single formatting field in an f-string. If the string contains a single formatting field and nothing else the node can be isolated otherwise it appears inJoinedStr.

• 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 aJoinedStrnode representing the formatting of the value, orNoneif no format was specified. Both conversionandformat_speccan be set at the same time.

classast.JoinedStr(values)

An f-string, comprising a series ofFormattedValueandConstant 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')]))]))

classast.List(elts,ctx)

classast.Tuple(elts,ctx)

A list or tuple.eltsholds a list of nodes representing the elements. ctxisStoreif the container is an assignment target (i.e. (x,y)=something), andLoadotherwise.

>>>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()))

classast.Set(elts)

A set.eltsholds 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)]))

classast.Dict(keys,values)

A dictionary.keysandvalueshold lists of nodes representing the keys and the values respectively, in matching order (what would be returned when callingdictionary.keys()anddictionary.values()).

When doing dictionary unpacking using dictionary literals the expression to be expanded goes in thevalueslist, with aNoneat the corresponding position inkeys.

>>>print(ast.dump(ast.parse('{ "a":1, **d}',mode='eval'),indent=4))
Expression(
body=Dict(
keys=[
Constant(value='a'),
None],
values=[
Constant(value=1),
Name(id='d', ctx=Load())]))

Variables

classast.Name(id,ctx)

A variable name.idholds the name as a string, andctxis one of the following types.

classast.Load

classast.Store

classast.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=[])

classast.Starred(value,ctx)

A*varvariable reference.valueholds the variable, typically a Namenode. This type must be used when building aCall node with*args.

>>>print(ast.dump(ast.parse('a, *b = it'),indent=4))
Module(
body=[
Assign(
targets=[
Tuple(
elts=[
Name(id='a', ctx=Store()),
Starred(
value=Name(id='b', ctx=Store()),
ctx=Store())],
ctx=Store())],
value=Name(id='it', ctx=Load()))],
type_ignores=[])

Expressions

classast.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. valueholds one of the other nodes in this section, aConstant,a Name,aLambda,aYieldorYieldFromnode.

>>>print(ast.dump(ast.parse('-a'),indent=4))
Module(
body=[
Expr(
value=UnaryOp(
op=USub(),
operand=Name(id='a', ctx=Load())))],
type_ignores=[])

classast.UnaryOp(op,operand)

A unary operation.opis the operator, andoperandany expression node.

classast.UAdd

classast.USub

classast.Not

classast.Invert

Unary operator tokens.Notis thenotkeyword,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())))

classast.BinOp(left,op,right)

A binary operation (like addition or division).opis the operator, and leftandrightare 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())))

classast.Add

classast.Sub

classast.Mult

classast.Div

classast.FloorDiv

classast.Mod

classast.Pow

classast.LShift

classast.RShift

classast.BitOr

classast.BitXor

classast.BitAnd

classast.MatMult

Binary operator tokens.

classast.BoolOp(op,values)

A boolean operation, ‘or’ or ‘and’.opisOrorAnd. valuesare the values involved. Consecutive operations with the same operator, such asaorborc,are collapsed into one node with several values.

This doesn’t includenot,which is aUnaryOp.

>>>print(ast.dump(ast.parse('x or y',mode='eval'),indent=4))
Expression(
body=BoolOp(
op=Or(),
values=[
Name(id='x', ctx=Load()),
Name(id='y', ctx=Load())]))

classast.And

classast.Or

Boolean operator tokens.

classast.Compare(left,ops,comparators)

A comparison of two or more values.leftis the first value in the comparison,opsthe list of operators, andcomparatorsthe 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)]))

classast.Eq

classast.NotEq

classast.Lt

classast.LtE

classast.Gt

classast.GtE

classast.Is

classast.IsNot

classast.In

classast.NotIn

Comparison operator tokens.

classast.Call(func,args,keywords)

A function call.funcis the function, which will often be a NameorAttributeobject. Of the arguments:

• argsholds a list of the arguments passed by position.

• keywordsholds a list ofkeywordobjects representing arguments passed by keyword.

When creating aCallnode,argsandkeywordsare 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()))]))

classast.keyword(arg,value)

A keyword argument to a function call or class definition.argis a raw string of the parameter name,valueis a node to pass in.

classast.IfExp(test,body,orelse)

An expression such asaifbelsec.Each field holds a single node, so in the following example, all three areNamenodes.

>>>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())))

classast.Attribute(value,attr,ctx)

Attribute access, e.g.d.keys.valueis a node, typically a Name.attris a bare string giving the name of the attribute, andctxisLoad,StoreorDelaccording 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()))

classast.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 theAssign node in which the first argument can be multiple nodes, in this case both targetandvaluemust be single nodes.

>>>print(ast.dump(ast.parse('(x:= 4)',mode='eval'),indent=4))
Expression(
body=NamedExpr(
target=Name(id='x', ctx=Store()),
value=Constant(value=4)))

Added in version 3.8.

Subscripting

classast.Subscript(value,slice,ctx)

A subscript, such asl[1].valueis the subscripted object (usually sequence or mapping).sliceis an index, slice or key. It can be aTupleand contain aSlice. ctxisLoad,StoreorDel 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()))

classast.Slice(lower,upper,step)

Regular slicing (on the formlower:upperorlower:upper:step). Can occur only inside theslicefield ofSubscript,either directly or as an element ofTuple.

>>>print(ast.dump(ast.parse('l[1:2]',mode='eval'),indent=4))
Expression(
body=Subscript(
value=Name(id='l', ctx=Load()),
slice=Slice(
lower=Constant(value=1),
upper=Constant(value=2)),
ctx=Load()))

Comprehensions

classast.ListComp(elt,generators)

classast.SetComp(elt,generators)

classast.GeneratorExp(elt,generators)

classast.DictComp(key,value,generators)

List and set comprehensions, generator expressions, and dictionary comprehensions.elt(orkeyandvalue) is a single node representing the part that will be evaluated for each item.

generatorsis a list ofcomprehensionnodes.

>>>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)]))

classast.comprehension(target,iter,ifs,is_async)

Oneforclause in a comprehension.targetis the reference to use for each element - typically aNameorTuplenode.iter is the object to iterate over.ifsis a list of test expressions: each forclause can have multipleifs.

is_asyncindicates a comprehension is asynchronous (using an asyncforinstead offor). The value is an integer (0 or 1).

>>>print(ast.dump(ast.parse('[ord(c) for line in file for c in line]',mode='eval'),
...indent=4))# Multiple comprehensions in one.
Expression(
body=ListComp(
elt=Call(
func=Name(id='ord', ctx=Load()),
args=[
Name(id='c', ctx=Load())],
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)]))

Statements

classast.Assign(targets,value,type_comment)

An assignment.targetsis a list of nodes, andvalueis a single node.

Multiple nodes intargetsrepresents assigning the same value to each. Unpacking is represented by putting aTupleorList withintargets.

type_comment

type_commentis 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=[])

classast.AnnAssign(target,annotation,value,simple)

An assignment with a type annotation.targetis a single node and can be aName,anAttributeor aSubscript. annotationis the annotation, such as aConstantorName node.valueis a single optional node.

simpleis always either 0 (indicating a “complex” target) or 1 (indicating a “simple” target). A “simple” target consists solely of a Namenode that does not appear between parentheses; all other targets are considered complex. Only simple targets appear in the__annotations__dictionary of modules and classes.

>>>print(ast.dump(ast.parse('c: int'),indent=4))
Module(
body=[
AnnAssign(
target=Name(id='c', ctx=Store()),
annotation=Name(id='int', ctx=Load()),
simple=1)],
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=[])

classast.AugAssign(target,op,value)

Augmented assignment, such asa+=1.In the following example, targetis aNamenode forx(with theStore context),opisAdd,andvalueis aConstantwith value for 1.

Thetargetattribute cannot be of classTupleorList, unlike the targets ofAssign.

>>>print(ast.dump(ast.parse('x += 2'),indent=4))
Module(
body=[
AugAssign(
target=Name(id='x', ctx=Store()),
op=Add(),
value=Constant(value=2))],
type_ignores=[])

classast.Raise(exc,cause)

Araisestatement.excis the exception object to be raised, normally a CallorName,orNonefor a standaloneraise. causeis the optional part foryinraisexfromy.

>>>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=[])

classast.Assert(test,msg)

An assertion.testholds the condition, such as aComparenode. msgholds 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=[])

classast.Delete(targets)

Represents adelstatement.targetsis a list of nodes, such as Name,AttributeorSubscriptnodes.

>>>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=[])

classast.Pass

Apassstatement.

>>>print(ast.dump(ast.parse('pass'),indent=4))
Module(
body=[
Pass()],
type_ignores=[])

classast.TypeAlias(name,type_params,value)

Atype aliascreated through thetype statement.nameis the name of the alias,type_paramsis a list of type parameters,andvalueis 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=[])

Added in version 3.12.

Other statements which are only applicable inside functions or loops are described in other sections.

Imports

classast.Import(names)

An import statement.namesis a list ofaliasnodes.

>>>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=[])

classast.ImportFrom(module,names,level)

Representsfromximporty.moduleis a raw string of the ‘from’ name, without any leading dots, orNonefor statements such asfrom.importfoo. levelis 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=[])

classast.alias(name,asname)

Both parameters are raw strings of the names.asnamecan beNoneif 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=[])

Control flow

Note

Optional clauses such aselseare stored as an empty list if they’re not present.

classast.If(test,body,orelse)

Anifstatement.testholds a single node, such as aCompare node.bodyandorelseeach hold a list of nodes.

elifclauses don’t have a special representation in the AST, but rather appear as extraIfnodes within theorelsesection 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=[])

classast.For(target,iter,body,orelse,type_comment)

Aforloop.targetholds the variable(s) the loop assigns to, as a singleName,Tuple,List,Attributeor Subscriptnode.iterholds the item to be looped over, again as a single node.bodyandorelsecontain lists of nodes to execute. Those inorelseare executed if the loop finishes normally, rather than via abreakstatement.

type_comment

type_commentis 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=[])

classast.While(test,body,orelse)

Awhileloop.testholds the condition, such as aCompare node.

>> print(ast.dump(ast.parse( "" "
... while x:
......
... else:
......
... "" "), indent=4))
Module(
body=[
While(
test=Name(id='x', ctx=Load()),
body=[
Expr(
value=Constant(value=Ellipsis))],
orelse=[
Expr(
value=Constant(value=Ellipsis))])],
type_ignores=[])

classast.Break

classast.Continue

Thebreakandcontinuestatements.

>>>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=[])

classast.Try(body,handlers,orelse,finalbody)

tryblocks. All attributes are list of nodes to execute, except for handlers,which is a list ofExceptHandlernodes.

>>>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=[])

classast.TryStar(body,handlers,orelse,finalbody)

tryblocks which are followed byexcept*clauses. The attributes are the same as forTrybut theExceptHandlernodes inhandlers are interpreted asexcept*blocks rather thenexcept.

>>>print(ast.dump(ast.parse("""
...try:
......
...except* Exception:
......
..."""),indent=4))
Module(
body=[
TryStar(
body=[
Expr(
value=Constant(value=Ellipsis))],
handlers=[
ExceptHandler(
type=Name(id='Exception', ctx=Load()),
body=[
Expr(
value=Constant(value=Ellipsis))])],
orelse=[],
finalbody=[])],
type_ignores=[])

Added in version 3.11.

classast.ExceptHandler(type,name,body)

A singleexceptclause.typeis the exception type it will match, typically aNamenode (orNonefor a catch-allexcept:clause). nameis a raw string for the name to hold the exception, orNoneif the clause doesn’t haveasfoo.bodyis 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=[])

classast.With(items,body,type_comment)

Awithblock.itemsis a list ofwithitemnodes representing the context managers, andbodyis the indented block inside the context.

type_comment

type_commentis an optional string with the type annotation as a comment.

classast.withitem(context_expr,optional_vars)

A single context manager in awithblock.context_expris the context manager, often aCallnode.optional_varsis aName, TupleorListfor theasfoopart, orNoneif 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=[])

Pattern matching

classast.Match(subject,cases)

Amatchstatement.subjectholds the subject of the match (the object that is being matched against the cases) andcasescontains an iterable of match_casenodes with the different cases.

Added in version 3.10.

classast.match_case(pattern,guard,body)

A single case pattern in amatchstatement.patterncontains the match pattern that the subject will be matched against. Note that the ASTnodes produced for patterns differ from those produced for expressions, even when they share the same syntax.

Theguardattribute contains an expression that will be evaluated if the pattern matches the subject.

bodycontains 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=[])

Added in version 3.10.

classast.MatchValue(value)

A match literal or value pattern that compares by equality.valueis 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=[])

Added in version 3.10.

classast.MatchSingleton(value)

A match literal pattern that compares by identity.valueis the singleton to be compared against:None,True,orFalse.This pattern succeeds if the match subject is the given constant.

>>>print(ast.dump(ast.parse("""
...match x:
...case None:
......
..."""),indent=4))
Module(
body=[
Match(
subject=Name(id='x', ctx=Load()),
cases=[
match_case(
pattern=MatchSingleton(value=None),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])

Added in version 3.10.

classast.MatchSequence(patterns)

A match sequence pattern.patternscontains 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 aMatchStarnode, 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=[])

Added in version 3.10.

classast.MatchStar(name)

Matches the rest of the sequence in a variable length match sequence pattern. Ifnameis notNone,a list containing the remaining sequence elements is bound to that name if the overall sequence pattern is successful.

>>>print(ast.dump(ast.parse("""
...match x:
...case [1, 2, *rest]:
......
...case [*_]:
......
..."""),indent=4))
Module(
body=[
Match(
subject=Name(id='x', ctx=Load()),
cases=[
match_case(
pattern=MatchSequence(
patterns=[
MatchValue(
value=Constant(value=1)),
MatchValue(
value=Constant(value=2)),
MatchStar(name='rest')]),
body=[
Expr(
value=Constant(value=Ellipsis))]),
match_case(
pattern=MatchSequence(
patterns=[
MatchStar()]),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])

Added in version 3.10.

classast.MatchMapping(keys,patterns,rest)

A match mapping pattern.keysis a sequence of expression nodes. patternsis a corresponding sequence of pattern nodes.restis 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. Ifrestis notNone,a dict containing the remaining mapping elements is bound to that name if the overall mapping pattern is successful.

>>>print(ast.dump(ast.parse("""
...match x:
...case {1: _, 2: _}:
......
...case {**rest}:
......
..."""),indent=4))
Module(
body=[
Match(
subject=Name(id='x', ctx=Load()),
cases=[
match_case(
pattern=MatchMapping(
keys=[
Constant(value=1),
Constant(value=2)],
patterns=[
MatchAs(),
MatchAs()]),
body=[
Expr(
value=Constant(value=Ellipsis))]),
match_case(
pattern=MatchMapping(keys=[], patterns=[], rest='rest'),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])

Added in version 3.10.

classast.MatchClass(cls,patterns,kwd_attrs,kwd_patterns)

A match class pattern.clsis an expression giving the nominal class to be matched.patternsis a sequence of pattern nodes to be matched against the class defined sequence of pattern matching attributes.kwd_attrsis a sequence of additional attributes to be matched (specified as keyword arguments in the class pattern),kwd_patternsare 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=[])

Added in version 3.10.

classast.MatchAs(pattern,name)

A match “as-pattern”, capture pattern or wildcard pattern.pattern contains the match pattern that the subject will be matched against. If the pattern isNone,the node represents a capture pattern (i.e a bare name) and will always succeed.

Thenameattribute contains the name that will be bound if the pattern is successful. IfnameisNone,patternmust also beNone and the node represents the wildcard pattern.

>>>print(ast.dump(ast.parse("""
...match x:
...case [x] as y:
......
...case _:
......
..."""),indent=4))
Module(
body=[
Match(
subject=Name(id='x', ctx=Load()),
cases=[
match_case(
pattern=MatchAs(
pattern=MatchSequence(
patterns=[
MatchAs(name='x')]),
name='y'),
body=[
Expr(
value=Constant(value=Ellipsis))]),
match_case(
pattern=MatchAs(),
body=[
Expr(
value=Constant(value=Ellipsis))])])],
type_ignores=[])

Added in version 3.10.

classast.MatchOr(patterns)

A match “or-pattern”. An or-pattern matches each of its subpatterns in turn to the subject, until one succeeds. The or-pattern is then deemed to succeed. If none of the subpatterns succeed the or-pattern fails. The patternsattribute 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=[])

Added in version 3.10.

Type parameters

Type parameterscan exist on classes, functions, and type aliases.

classast.TypeVar(name,bound)

Atyping.TypeVar.nameis the name of the type variable. boundis the bound or constraints, if any. Ifboundis aTuple, 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=[])

Added in version 3.12.

classast.ParamSpec(name)

Atyping.ParamSpec.nameis 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=[])

Added in version 3.12.

classast.TypeVarTuple(name)

Atyping.TypeVarTuple.nameis 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=[])

Added in version 3.12.

Function and class definitions

classast.FunctionDef(name,args,body,decorator_list,returns,type_comment,type_params)

A function definition.

• nameis a raw string of the function name.

• argsis anargumentsnode.

• 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.

• type_paramsis a list oftype parameters.

type_comment

type_commentis an optional string with the type annotation as a comment.

Changed in version 3.12:Addedtype_params.

classast.Lambda(args,body)

lambdais a minimal function definition that can be used inside an expression. UnlikeFunctionDef,bodyholds 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=[])

classast.arguments(posonlyargs,args,vararg,kwonlyargs,kw_defaults,kwarg,defaults)

The arguments for a function.

• posonlyargs,argsandkwonlyargsare lists ofargnodes.

• varargandkwargare singleargnodes, 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.

classast.arg(arg,annotation,type_comment)

A single argument in a list.argis a raw string of the argument name;annotationis its annotation, such as aNamenode.

type_comment

type_commentis 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=[])

classast.Return(value)

Areturnstatement.

>>>print(ast.dump(ast.parse('return 4'),indent=4))
Module(
body=[
Return(
value=Constant(value=4))],
type_ignores=[])

classast.Yield(value)

classast.YieldFrom(value)

Ayieldoryieldfromexpression. Because these are expressions, they must be wrapped in anExprnode 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=[])

classast.Global(names)

classast.Nonlocal(names)

globalandnonlocalstatements.namesis 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=[])

classast.ClassDef(name,bases,keywords,body,decorator_list,type_params)

A class definition.

• nameis a raw string for the class name

• basesis a list of nodes for explicitly specified base classes.

• keywordsis a list ofkeywordnodes, principally for ‘metaclass’. Other keywords will be passed to the metaclass, as perPEP-3115.

• bodyis a list of nodes representing the code within the class definition.

• decorator_listis a list of nodes, as inFunctionDef.

• type_paramsis a list oftype parameters.

>>>print(ast.dump(ast.parse("""\
...@decorator1
...@decorator2
...class Foo(base1, base2, metaclass=meta):
...pass
..."""),indent=4))
Module(
body=[
ClassDef(
name='Foo',
bases=[
Name(id='base1', ctx=Load()),
Name(id='base2', ctx=Load())],
keywords=[
keyword(
arg='metaclass',
value=Name(id='meta', ctx=Load()))],
body=[
Pass()],
decorator_list=[
Name(id='decorator1', ctx=Load()),
Name(id='decorator2', ctx=Load())],
type_params=[])],
type_ignores=[])

Changed in version 3.12:Addedtype_params.

Async and await

classast.AsyncFunctionDef(name,args,body,decorator_list,returns,type_comment,type_params)

Anasyncdeffunction definition. Has the same fields as FunctionDef.

Changed in version 3.12:Addedtype_params.

classast.Await(value)

Anawaitexpression.valueis what it waits for. Only valid in the body of anAsyncFunctionDef.

>>>print(ast.dump(ast.parse("""\
...async def f():
...await other_func()
..."""),indent=4))
Module(
body=[
AsyncFunctionDef(
name='f',
args=arguments(
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=[])

classast.AsyncFor(target,iter,body,orelse,type_comment)

classast.AsyncWith(items,body,type_comment)

asyncforloops andasyncwithcontext managers. They have the same fields asForandWith,respectively. Only valid in the body of anAsyncFunctionDef.

Note

When a string is parsed byast.parse(),operator nodes (subclasses ofast.operator,ast.unaryop,ast.cmpop, ast.boolopandast.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).

astHelpers

Apart from the node classes, theastmodule 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 tocompile(source, filename,mode,ast.PyCF_ONLY_AST).

Iftype_comments=Trueis given, the parser is modified to check and return type comments as specified byPEP 484andPEP 526. This is equivalent to addingast.PyCF_TYPE_COMMENTSto the flags passed tocompile().This will report syntax errors for misplaced type comments. Without this flag, type comments will be ignored, and thetype_commentfield on selected AST nodes will always beNone.In addition, the locations of#type: ignorecomments will be returned as thetype_ignores attribute ofModule(otherwise it is always an empty list).

In addition, ifmodeis'func_type',the input syntax is modified to correspond toPEP 484“signature type comments”, e.g.(str,int)->List[str].

Settingfeature_versionto a tuple(major,minor)will result in a “best-effort” attempt to parse using that Python version’s grammar. For example, settingfeature_version=(3,9)will attempt to disallow parsing ofmatchstatements. Currentlymajormust equal to3.The lowest supported version is (3,4)(and this may increase in future Python versions); the highest issys.version_info[0:2].“Best-effort” attempt means there is no guarantee that the parse (or success of the parse) is the same as when run on the Python version corresponding tofeature_version.

If source contains a null character (\0),ValueErroris 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 furtherSyntaxError exceptions. For instance, the sourcereturn42generates a valid AST node for a return statement, but it cannot be compiled alone (it needs to be inside a function node).

In particular,ast.parse()won’t do any scoping checks, which the compilation step does.

Warning

It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.

Changed in version 3.8:Addedtype_comments,mode='func_type'andfeature_version.

ast.unparse(ast_obj)

Unparse anast.ASTobject and generate a string with code that would produce an equivalentast.ASTobject if parsed back withast.parse().

Warning

The produced code string will not necessarily be equal to the original code that generated theast.ASTobject (without any compiler optimizations, such as constant tuples/frozensets).

Warning

Trying to unparse a highly complex expression would result with RecursionError.

Added in version 3.9.

ast.literal_eval(node_or_string)

Evaluate an expression node or a string containing only a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans,NoneandEllipsis.

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 generaleval().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 raiseValueError,TypeError,SyntaxError, MemoryErrorandRecursionErrordepending on the malformed input.

Changed in version 3.2:Now allows bytes and set literals.

Changed in version 3.9:Now supports creating empty sets with'set()'.

Changed in version 3.10:For string inputs, leading spaces and tabs are now stripped.

ast.get_docstring(node,clean=True)

Return the docstring of the givennode(which must be a FunctionDef,AsyncFunctionDef,ClassDef, orModulenode), orNoneif it has no docstring. Ifcleanis true, clean up the docstring’s indentation with inspect.cleandoc().

Changed in version 3.5:AsyncFunctionDefis now supported.

ast.get_source_segment(source,node,*,padded=False)

Get source code segment of thesourcethat generatednode. If some location information (lineno,end_lineno, col_offset,orend_col_offset) is missing, returnNone.

IfpaddedisTrue,the first line of a multi-line statement will be padded with spaces to match its original position.

Added in version 3.8.

ast.fix_missing_locations(node)

When you compile a node tree withcompile(),the compiler expects linenoandcol_offsetattributes 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 atnode.

ast.increment_lineno(node,n=1)

Increment the line number and end line number of each node in the tree starting atnodebyn.This is useful to “move code” to a different location in a file.

ast.copy_location(new_node,old_node)

Copy source location (lineno,col_offset,end_lineno, andend_col_offset) fromold_nodetonew_nodeif possible, and returnnew_node.

ast.iter_fields(node)

Yield a tuple of(fieldname,value)for each field innode._fields that is present onnode.

ast.iter_child_nodes(node)

Yield all direct child nodes ofnode,that is, all fields that are nodes and all items of fields that are lists of nodes.

ast.walk(node)

Recursively yield all descendant nodes in the tree starting atnode (includingnodeitself), in no specified order. This is useful if you only want to modify nodes in place and don’t care about the context.

classast.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 thevisit()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_classnamewhereclassnameis the name of the node class, orgeneric_visit()if that method doesn’t exist.

generic_visit(node)

This visitor callsvisit()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 callsgeneric_visit()or visits them itself.

visit_Constant(node)

Handles all constant nodes.

Don’t use theNodeVisitorif you want to apply changes to nodes during traversal. For this a special visitor exists (NodeTransformer) that allows modifications.

Deprecated since version 3.8:Methodsvisit_Num(),visit_Str(),visit_Bytes(), visit_NameConstant()andvisit_Ellipsis()are deprecated now and will not be called in future Python versions. Add the visit_Constant()method to handle all constant nodes.

classast.NodeTransformer

ANodeVisitorsubclass that walks the abstract syntax tree and allows modification of nodes.

TheNodeTransformerwill walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method isNone,the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.

Here is an example transformer that rewrites all occurrences of name lookups (foo) todata['foo']:

classRewriteName(NodeTransformer):

defvisit_Name(self,node):
returnSubscript(
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 thegeneric_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.

IfNodeTransformerintroduces 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 innode.This is mainly useful for debugging purposes. Ifannotate_fieldsis true (by default), the returned string will show the names and the values for fields. Ifannotate_fieldsis 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_attributescan be set to true.

Ifindentis 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. Ifindentis a string (such as"\t"), that string is used to indent each level.

Changed in version 3.9:Added theindentoption.

Compiler Flags

The following flags may be passed tocompile()in order to change effects on the compilation of a program:

ast.PyCF_ALLOW_TOP_LEVEL_AWAIT

Enables support for top-levelawait,asyncfor,asyncwith and async comprehensions.

Added in version 3.8.

ast.PyCF_ONLY_AST

Generates and returns an abstract syntax tree instead of returning a compiled code object.

ast.PyCF_TYPE_COMMENTS

Enables support forPEP 484andPEP 526style type comments (#type:<type>,#type:ignore<stuff>).

Added in version 3.8.

Command-Line Usage

Added in version 3.9.

Theastmodule can be executed as a script from the command line. It is as simple as:

python-mast[-m<mode>][-a][infile]

The following options are accepted:

-h,--help

Show the help message and exit.

-m<mode>

--mode<mode>

Specify what kind of code must be compiled, like themodeargument inparse().

--no-type-comments

Don’t parse type comments.

-a,--include-attributes

Include attributes such as line numbers and column offsets.

-i<indent>

--indent<indent>

Indentation of nodes in AST (number of spaces).

Ifinfileis 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.

LibCSTparses 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.

Parsois 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.