ast — Abstract Syntax Trees¶
Source code: Lib/ast.py
The ast module helps Python applications to process trees of the Python
abstract syntax grammar. The abstract syntax itself might change with each
Python release; this module helps to find out programmatically what the current
grammar looks like.
An abstract syntax tree can be generated by passing ast.PyCF_ONLY_AST as
a flag to the compile() built-in function, or using the parse()
helper provided in this module. The result will be a tree of objects whose
classes all inherit from ast.AST. An abstract syntax tree can be
compiled into a Python code object using the built-in compile() function.
Abstract Grammar¶
The abstract grammar is currently defined as follows:
-- ASDL's 4 builtin types are:
-- identifier, int, string, constant
module Python
{
mod = Module(stmt* body, type_ignore* type_ignores)
| Interactive(stmt* body)
| Expression(expr body)
| FunctionType(expr* argtypes, expr returns)
stmt = FunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list, expr? returns,
string? type_comment)
| 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)
}
Node classes¶
-
class
ast.AST¶ This is the base of all AST node classes. The actual node classes are derived from the
Parser/Python.asdlfile, which is reproduced below. 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.BinOpinstances have an attributeleftof typeast.expr.If these attributes are marked as optional in the grammar (using a question mark), the value might be
None. If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST withcompile().
-
lineno¶ -
col_offset¶ -
end_lineno¶ -
end_col_offset¶ Instances of
ast.exprandast.stmtsubclasses havelineno,col_offset,end_lineno, andend_col_offsetattributes. Thelinenoandend_linenoare the first and last line numbers of the source text span (1-indexed so the first line is line 1), and thecol_offsetandend_col_offsetare 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.Tparses 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.UnaryOpnode, you could usenode = 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.
Note
The descriptions of the specific node classes displayed here were initially adapted from the fantastic Green Tree Snakes project and all its contributors.
Literals¶
-
class
ast.Constant(value)¶ A constant value. The
valueattribute 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))
-
class
ast.FormattedValue(value, conversion, format_spec)¶ Node representing a single formatting field in an f-string. If the string contains a single formatting field and nothing else the node can be isolated otherwise it appears in
JoinedStr.valueis any expression node (such as a literal, a variable, or a function call).conversionis an integer:-1: no formatting
115:
!sstring formatting114:
!rrepr formatting97:
!aascii formatting
format_specis aJoinedStrnode representing the formatting of the value, orNoneif no format was specified. Bothconversionandformat_speccan be set at the same time.
-
class
ast.JoinedStr(values)¶ An f-string, comprising a series of
FormattedValueandConstantnodes.>>> 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')]))]))
-
class
ast.List(elts, ctx)¶ -
class
ast.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()))
-
class
ast.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)]))
-
class
ast.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 the
valueslist, 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¶
-
class
ast.Name(id, ctx)¶ A variable name.
idholds the name as a string, andctxis one of the following types.
-
class
ast.Load¶ -
class
ast.Store¶ -
class
ast.Del¶ Variable references can be used to load the value of a variable, to assign a new value to it, or to delete it. Variable references are given a context to distinguish these cases.
>>> print(ast.dump(ast.parse('a'), indent=4)) Module( body=[ Expr( value=Name(id='a', ctx=Load()))], 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=[])
-
class
ast.Starred(value, ctx)¶ A
*varvariable reference.valueholds the variable, typically aNamenode. This type must be used when building aCallnode 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¶
-
class
ast.Expr(value)¶ When an expression, such as a function call, appears as a statement by itself with its return value not used or stored, it is wrapped in this container.
valueholds one of the other nodes in this section, aConstant, aName, 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=[])
-
class
ast.UnaryOp(op, operand)¶ A unary operation.
opis the operator, andoperandany expression node.
-
class
ast.UAdd¶ -
class
ast.USub¶ -
class
ast.Not¶ -
class
ast.Invert¶ Unary operator tokens.
Notis thenotkeyword,Invertis the~operator.>>> print(ast.dump(ast.parse('not x', mode='eval'), indent=4)) Expression( body=UnaryOp( op=Not(), operand=Name(id='x', ctx=Load())))
-
class
ast.BinOp(left, op, right)¶ A binary operation (like addition or division).
opis the operator, andleftandrightare any expression nodes.>>> print(ast.dump(ast.parse('x + y', mode='eval'), indent=4)) Expression( body=BinOp( left=Name(id='x', ctx=Load()), op=Add(), right=Name(id='y', ctx=Load())))
-
class
ast.Add¶ -
class
ast.Sub¶ -
class
ast.Mult¶ -
class
ast.Div¶ -
class
ast.FloorDiv¶ -
class
ast.Mod¶ -
class
ast.Pow¶ -
class
ast.LShift¶ -
class
ast.RShift¶ -
class
ast.BitOr¶ -
class
ast.BitXor¶ -
class
ast.BitAnd¶ -
class
ast.MatMult¶ Binary operator tokens.
-
class
ast.BoolOp(op, values)¶ A boolean operation, ‘or’ or ‘and’.
opisOrorAnd.valuesare the values involved. Consecutive operations with the same operator, such asa or b or c, are collapsed into one node with several values.This doesn’t include
not, which is aUnaryOp.>>> print(ast.dump(ast.parse('x or y', mode='eval'), indent=4)) Expression( body=BoolOp( op=Or(), values=[ Name(id='x', ctx=Load()), Name(id='y', ctx=Load())]))
-
class
ast.Compare(left, ops, comparators)¶ A comparison of two or more values.
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)]))
-
class
ast.Eq¶ -
class
ast.NotEq¶ -
class
ast.Lt¶ -
class
ast.LtE¶ -
class
ast.Gt¶ -
class
ast.GtE¶ -
class
ast.Is¶ -
class
ast.IsNot¶ -
class
ast.In¶ -
class
ast.NotIn¶ Comparison operator tokens.
-
class
ast.Call(func, args, keywords, starargs, kwargs)¶ A function call.
funcis the function, which will often be aNameorAttributeobject. Of the arguments:argsholds a list of the arguments passed by position.keywordsholds a list ofkeywordobjects representing arguments passed by keyword.
When creating a
Callnode,argsandkeywordsare required, but they can be empty lists.starargsandkwargsare 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()))]))
-
class
ast.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.
-
class
ast.IfExp(test, body, orelse)¶ An expression such as
a if b else c. Each field holds a single node, so in the following example, all three 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())))
-
class
ast.Attribute(value, attr, ctx)¶ Attribute access, e.g.
d.keys.valueis a node, typically aName.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()))
-
class
ast.NamedExpr(target, value)¶ A named expression. This AST node is produced by the assignment expressions operator (also known as the walrus operator). As opposed to the
Assignnode in which the first argument can be multiple nodes, in this case bothtargetandvaluemust be single nodes.>>> print(ast.dump(ast.parse('(x := 4)', mode='eval'), indent=4)) Expression( body=NamedExpr( target=Name(id='x', ctx=Store()), value=Constant(value=4)))
Subscripting¶
-
class
ast.Subscript(value, slice, ctx)¶ A subscript, such as
l[1].valueis the subscripted object (usually sequence or mapping).sliceis an index, slice or key. It can be aTupleand contain aSlice.ctxisLoad,StoreorDelaccording to the action performed with the subscript.>>> print(ast.dump(ast.parse('l[1:2, 3]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Tuple( elts=[ Slice( lower=Constant(value=1), upper=Constant(value=2)), Constant(value=3)], ctx=Load()), ctx=Load()))
-
class
ast.Slice(lower, upper, step)¶ Regular slicing (on the form
lower:upperorlower:upper:step). Can occur only inside the slice field ofSubscript, either directly or as an element ofTuple.>>> print(ast.dump(ast.parse('l[1:2]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Slice( lower=Constant(value=1), upper=Constant(value=2)), ctx=Load()))
Comprehensions¶
-
class
ast.ListComp(elt, generators)¶ -
class
ast.SetComp(elt, generators)¶ -
class
ast.GeneratorExp(elt, generators)¶ -
class
ast.DictComp(key, value, generators)¶ List and set comprehensions, generator expressions, and dictionary comprehensions.
elt(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)]))
-
class
ast.comprehension(target, iter, ifs, is_async)¶ One
forclause in a comprehension.targetis the reference to use for each element - typically aNameorTuplenode.iteris the object to iterate over.ifsis a list of test expressions: eachforclause can have multipleifs.is_asyncindicates a comprehension is asynchronous (using anasync forinstead 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¶
-
class
ast.Assign(targets, value, type_comment)¶ An assignment.
targetsis a list of nodes, andvalueis a single node.Multiple nodes in
targetsrepresents assigning the same value to each. Unpacking is represented by putting aTupleorListwithintargets.-
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=[])
-
-
class
ast.AnnAssign(target, annotation, value, simple)¶ An assignment with a type annotation.
targetis a single node and can be aName, aAttributeor aSubscript.annotationis the annotation, such as aConstantorNamenode.valueis a single optional node.simpleis a boolean integer set to True for aNamenode intargetthat 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=[])
-
class
ast.AugAssign(target, op, value)¶ Augmented assignment, such as
a += 1. In the following example,targetis aNamenode forx(with theStorecontext),opisAdd, andvalueis aConstantwith value for 1.The
targetattribute connot 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=[])
-
class
ast.Raise(exc, cause)¶ A
raisestatement.excis the exception object to be raised, normally aCallorName, orNonefor a standaloneraise.causeis the optional part foryinraise 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=[])
-
class
ast.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=[])
-
class
ast.Delete(targets)¶ Represents a
delstatement.targetsis a list of nodes, such asName,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=[])
-
class
ast.Pass¶ A
passstatement.>>> 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.
Imports¶
-
class
ast.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=[])
-
class
ast.ImportFrom(module, names, level)¶ Represents
from x import y.moduleis a raw string of the ‘from’ name, without any leading dots, orNonefor statements such asfrom . import foo.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=[])
-
class
ast.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 as else are stored as an empty list if they’re
not present.
-
class
ast.If(test, body, orelse)¶ An
ifstatement.testholds a single node, such as aComparenode.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=[])
-
class
ast.For(target, iter, body, orelse, type_comment)¶ A
forloop.targetholds the variable(s) the loop assigns to, as a singleName,TupleorListnode.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=[])
-
-
class
ast.While(test, body, orelse)¶ A
whileloop.testholds the condition, such as aComparenode.>> 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=[])
-
class
ast.Break¶ -
class
ast.Continue¶ The
breakandcontinuestatements.>>> 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=[])
-
class
ast.Try(body, handlers, orelse, finalbody)¶ tryblocks. All attributes are list of nodes to execute, except forhandlers, 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=[])
-
class
ast.ExceptHandler(type, name, body)¶ A single
exceptclause.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 haveas foo.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=[])
-
class
ast.With(items, body, type_comment)¶ A
withblock.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.
-
-
class
ast.withitem(context_expr, optional_vars)¶ A single context manager in a
withblock.context_expris the context manager, often aCallnode.optional_varsis aName,TupleorListfor theas foopart, 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=[])
Function and class definitions¶
-
class
ast.FunctionDef(name, args, body, decorator_list, returns, type_comment)¶ 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_comment¶ type_commentis an optional string with the type annotation as a comment.
-
class
ast.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=[])
-
class
ast.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.
-
class
ast.arg(arg, annotation, type_comment)¶ A single argument in a list.
argis a raw string of the argument name,annotationis its annotation, such as aStrorNamenode.-
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_ignores=[])
-
-
class
ast.Return(value)¶ A
returnstatement.>>> print(ast.dump(ast.parse('return 4'), indent=4)) Module( body=[ Return( value=Constant(value=4))], type_ignores=[])
-
class
ast.Yield(value)¶ -
class
ast.YieldFrom(value)¶ A
yieldoryield fromexpression. Because these are expressions, they must be wrapped in aExprnode 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=[])
-
class
ast.Global(names)¶ -
class
ast.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=[])
-
class
ast.ClassDef(name, bases, keywords, starargs, kwargs, body, decorator_list)¶ A class definition.
nameis a raw string for the class namebasesis 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 per PEP-3115.starargsandkwargsare 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.bodyis a list of nodes representing the code within the class definition.decorator_listis a list of nodes, as inFunctionDef.
>>> 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=[])
Async and await¶
-
class
ast.AsyncFunctionDef(name, args, body, decorator_list, returns, type_comment)¶ An
async deffunction definition. Has the same fields asFunctionDef.
-
class
ast.Await(value)¶ An
awaitexpression.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_ignores=[])
-
class
ast.AsyncFor(target, iter, body, orelse, type_comment)¶ -
class
ast.AsyncWith(items, body, type_comment)¶ async forloops andasync withcontext managers. They have the same fields asForandWith, respectively. Only valid in the body of anAsyncFunctionDef.
Note
When a string is parsed by ast.parse(), operator nodes (subclasses
of ast.operator, ast.unaryop, ast.cmpop,
ast.boolop and ast.expr_context) on the returned tree
will be singletons. Changes to one will be reflected in all other
occurrences of the same value (e.g. ast.Add).
ast Helpers¶
Apart from the node classes, the ast module defines these utility functions
and classes for traversing abstract syntax trees:
-
ast.parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)¶ Parse the source into an AST node. Equivalent to
compile(source, filename, mode, ast.PyCF_ONLY_AST).If
type_comments=Trueis given, the parser is modified to check and return type comments as specified by PEP 484 and PEP 526. This is equivalent to addingast.PyCF_TYPE_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_ignoresattribute ofModule(otherwise it is always an empty list).In addition, if
modeis'func_type', the input syntax is modified to correspond to PEP 484 “signature type comments”, e.g.(str, int) -> List[str].Also, setting
feature_versionto a tuple(major, minor)will attempt to parse using that Python version’s grammar. Currentlymajormust equal to3. For example, settingfeature_version=(3, 4)will allow the use ofasyncandawaitas variable names. The lowest supported version is(3, 4); the highest issys.version_info[0:2].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 further
SyntaxErrorexceptions. For instance, the sourcereturn 42generates a valid AST node for a return statement, but it cannot be compiled alone (it needs to be inside a function node).In particular,
ast.parse()won’t do any scoping checks, which the compilation step does.Warning
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.
Changed in version 3.8: Added
type_comments,mode='func_type'andfeature_version.
-
ast.unparse(ast_obj)¶ Unparse an
ast.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 the
ast.ASTobject (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)¶ 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, and
None.This can be used for evaluating strings containing Python values without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.
This function had been documented as “safe” in the past without defining what that meant. That was misleading. This is specifically designed not to execute Python code, unlike the more general
eval(). There is no namespace, no name lookups, or ability to call out. But it is not free from attack: A relatively small input can lead to memory exhaustion or to C stack exhaustion, crashing the process. There is also the possibility for excessive CPU consumption denial of service on some inputs. Calling it on untrusted data is thus not recommended.Warning
It is possible to crash the Python interpreter due to stack depth limitations in Python’s AST compiler.
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, orModulenode), orNoneif it has no docstring. If clean is true, clean up the docstring’s indentation withinspect.cleandoc().Changed in version 3.5:
AsyncFunctionDefis now supported.
-
ast.get_source_segment(source, node, *, padded=False)¶ Get source code segment of the source that generated node. If some location information (
lineno,end_lineno,col_offset, orend_col_offset) is missing, returnNone.If padded is
True, the first line of a multi-line statement will be padded with spaces to match its original position.New in version 3.8.
-
ast.fix_missing_locations(node)¶ When you compile a node tree with
compile(), the compiler expectslinenoandcol_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 at node.
-
ast.increment_lineno(node, n=1)¶ Increment the line number and end line number of each node in the tree starting at node by n. This is useful to “move code” to a different location in a file.
-
ast.copy_location(new_node, old_node)¶ Copy source location (
lineno,col_offset,end_lineno, andend_col_offset) from old_node to new_node if possible, and return new_node.
-
ast.iter_fields(node)¶ Yield a tuple of
(fieldname, value)for each field innode._fieldsthat is present on node.
-
ast.iter_child_nodes(node)¶ Yield all direct child nodes of node, that is, all fields that are nodes and all items of fields that are lists of nodes.
-
ast.walk(node)¶ Recursively yield all descendant nodes in the tree starting at node (including node itself), in no specified order. This is useful if you only want to modify nodes in place and don’t care about the context.
-
class
ast.NodeVisitor¶ A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the
visit()method.This class is meant to be subclassed, with the subclass adding visitor methods.
-
visit(node)¶ Visit a node. The default implementation calls the method called
self.visit_classnamewhere classname is the name of the node class, orgeneric_visit()if that method doesn’t exist.
-
generic_visit(node)¶ This visitor calls
visit()on all children of the node.Note that child nodes of nodes that have a custom visitor method won’t be visited unless the visitor calls
generic_visit()or visits them itself.
Don’t use the
NodeVisitorif 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()andvisit_Ellipsis()are deprecated now and will not be called in future Python versions. Add thevisit_Constant()method to handle all constant nodes.-
-
class
ast.NodeTransformer¶ A
NodeVisitorsubclass that walks the abstract syntax tree and allows modification of nodes.The
NodeTransformerwill walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method isNone, the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.Here is an example transformer that rewrites all occurrences of name lookups (
foo) todata['foo']:class RewriteName(NodeTransformer): def visit_Name(self, node): return Subscript( value=Name(id='data', ctx=Load()), slice=Constant(value=node.id), ctx=node.ctx )
Keep in mind that if the node you’re operating on has child nodes you must either transform the child nodes yourself or call the
generic_visit()method for the node first.For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node.
If
NodeTransformerintroduces new nodes (that weren’t part of original tree) without giving them location information (such aslineno),fix_missing_locations()should be called with the new sub-tree to recalculate the location information:tree = ast.parse('foo', mode='eval') new_tree = fix_missing_locations(RewriteName().visit(tree))
Usually you use the transformer like this:
node = YourTransformer().visit(node)
-
ast.dump(node, annotate_fields=True, include_attributes=False, *, indent=None)¶ 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.
Compiler Flags¶
The following flags may be passed to compile() in order to change
effects on the compilation of a program:
-
ast.PyCF_ALLOW_TOP_LEVEL_AWAIT¶ Enables support for top-level
await,async for,async withand async comprehensions.New in version 3.8.
-
ast.PyCF_ONLY_AST¶ Generates and returns an abstract syntax tree instead of returning a compiled code object.
Command-Line Usage¶
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¶ Show the help message and exit.
-
-m<mode>¶ -
--mode<mode>¶ Specify what kind of code must be compiled, like the mode argument in
parse().
-
--no-type-comments¶ Don’t parse type comments.
-
-a,--include-attributes¶ Include attributes such as line numbers and column offsets.
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.