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ast — 抽象句法树

`ast` — 抽象句法树 ¶

源代码： Lib/ast.py

The ast 模块帮助 Python 应用程序处理 Python 抽象句法语法树。抽象句法本身可以随着每 Python 发行而改变；此模块以编程方式帮助找出当前语法看起来像什么。

可以生成抽象句法树通过传递 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() 函数。

抽象语法 ¶

The abstract grammar is currently defined as follows:

-- ASDL's 4 builtin types are:
-- identifier, int, string, constant
module Python
{
    mod = Module(stmt* body, type_ignore* type_ignores)
        | Interactive(stmt* body)
        | Expression(expr body)
        | FunctionType(expr* argtypes, expr returns)
    stmt = FunctionDef(identifier name, arguments args,
                       stmt* body, expr* decorator_list, expr? returns,
                       string? type_comment, type_param* type_params)
          | AsyncFunctionDef(identifier name, arguments args,
                             stmt* body, expr* decorator_list, expr? returns,
                             string? type_comment, type_param* type_params)
          | ClassDef(identifier name,
             expr* bases,
             keyword* keywords,
             stmt* body,
             expr* decorator_list,
             type_param* type_params)
          | Return(expr? value)
          | Delete(expr* targets)
          | Assign(expr* targets, expr value, string? type_comment)
          | TypeAlias(expr name, type_param* type_params, expr value)
          | AugAssign(expr target, operator op, expr value)
          -- 'simple' indicates that we annotate simple name without parens
          | AnnAssign(expr target, expr annotation, expr? value, int simple)
          -- use 'orelse' because else is a keyword in target languages
          | For(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
          | AsyncFor(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
          | While(expr test, stmt* body, stmt* orelse)
          | If(expr test, stmt* body, stmt* orelse)
          | With(withitem* items, stmt* body, string? type_comment)
          | AsyncWith(withitem* items, stmt* body, string? type_comment)
          | Match(expr subject, match_case* cases)
          | Raise(expr? exc, expr? cause)
          | Try(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
          | TryStar(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
          | Assert(expr test, expr? msg)
          | Import(alias* names)
          | ImportFrom(identifier? module, alias* names, int? level)
          | Global(identifier* names)
          | Nonlocal(identifier* names)
          | Expr(expr value)
          | Pass | Break | Continue
          -- col_offset is the byte offset in the utf8 string the parser uses
          attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
          -- BoolOp() can use left & right?
    expr = BoolOp(boolop op, expr* values)
         | NamedExpr(expr target, expr value)
         | BinOp(expr left, operator op, expr right)
         | UnaryOp(unaryop op, expr operand)
         | Lambda(arguments args, expr body)
         | IfExp(expr test, expr body, expr orelse)
         | Dict(expr* keys, expr* values)
         | Set(expr* elts)
         | ListComp(expr elt, comprehension* generators)
         | SetComp(expr elt, comprehension* generators)
         | DictComp(expr key, expr value, comprehension* generators)
         | GeneratorExp(expr elt, comprehension* generators)
         -- the grammar constrains where yield expressions can occur
         | Await(expr value)
         | Yield(expr? value)
         | YieldFrom(expr value)
         -- need sequences for compare to distinguish between
         -- x < 4 < 3 and (x < 4) < 3
         | Compare(expr left, cmpop* ops, expr* comparators)
         | Call(expr func, expr* args, keyword* keywords)
         | FormattedValue(expr value, int conversion, expr? format_spec)
         | 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)
             attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
    withitem = (expr context_expr, expr? optional_vars)
    match_case = (pattern pattern, expr? guard, stmt* body)
    pattern = MatchValue(expr value)
            | MatchSingleton(constant value)
            | MatchSequence(pattern* patterns)
            | MatchMapping(expr* keys, pattern* patterns, identifier? rest)
            | MatchClass(expr cls, pattern* patterns, identifier* kwd_attrs, pattern* kwd_patterns)
            | MatchStar(identifier? name)
            -- The optional "rest" MatchMapping parameter handles capturing extra mapping keys
            | MatchAs(pattern? pattern, identifier? name)
            | MatchOr(pattern* patterns)
             attributes (int lineno, int col_offset, int end_lineno, int end_col_offset)
    type_ignore = TypeIgnore(int lineno, string tag)
    type_param = TypeVar(identifier name, expr? bound)
               | ParamSpec(identifier name)
               | TypeVarTuple(identifier name)
               attributes (int lineno, int col_offset, int end_lineno, int end_col_offset)
}

节点类 ¶

class ast. AST ¶

This is the base of all AST node classes. The actual node classes are derived from the Parser/Python.asdl file, which is reproduced above . They are defined in the _ast C module and re-exported in ast .

There is one class defined for each left-hand side symbol in the abstract grammar (for example, ast.stmt or ast.expr ). In addition, there is one class defined for each constructor on the right-hand side; these classes inherit from the classes for the left-hand side trees. For example, ast.BinOp 继承自 ast.expr . For production rules with alternatives (aka “sums”), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.

_fields ¶

Each concrete class has an attribute _fields which gives the names of all child nodes.

Each instance of a concrete class has one attribute for each child node, of the type as defined in the grammar. For example, ast.BinOp instances have an attribute left 类型 ast.expr .

If these attributes are marked as optional in the grammar (using a question mark), the value might be None . If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST with compile() .

lineno ¶

col_offset ¶

end_lineno ¶

end_col_offset ¶

实例化的 ast.expr and ast.stmt subclasses have lineno , col_offset , end_lineno ，和 end_col_offset 属性。 lineno and end_lineno are the first and last line numbers of source text span (1-indexed so the first line is line 1) and the col_offset and end_col_offset are the corresponding UTF-8 byte offsets of the first and last tokens that generated the node. The UTF-8 offset is recorded because the parser uses UTF-8 internally.

Note that the end positions are not required by the compiler and are therefore optional. The end offset is after the last symbol, for example one can get the source segment of a one-line expression node using source_line[node.col_offset : node.end_col_offset] .

The constructor of a class ast.T parses its arguments as follows:

If there are positional arguments, there must be as many as there are items in T._fields ; they will be assigned as attributes of these names.
If there are keyword arguments, they will set the attributes of the same names to the given values.

例如，要创建和填充 ast.UnaryOp 节点，可以使用

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)

3.8 版改变：类 ast.Constant is now used for all constants.

3.9 版改变： Simple indices are represented by their value, extended slices are represented as tuples.

从 3.8 版起弃用：旧类 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.

从 3.9 版起弃用：旧类 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.

注意

The descriptions of the specific node classes displayed here were initially adapted from the fantastic Green Tree Snakes project and all its contributors.

Root nodes ¶

class ast. 模块 ( body , type_ignores ) ¶

A Python module, as with file input . Node type generated by ast.parse() in the default "exec" mode .

body 是 list of the module’s 语句 .

type_ignores 是 list of the module’s type ignore comments; see ast.parse() 了解更多细节。

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

class ast. 表达式 ( body ) ¶

A single Python expression input . Node type generated by ast.parse() 当 mode is "eval" .

body is a single node, one of the expression types .

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

class ast. Interactive ( body ) ¶

A single interactive input , like in 交互模式 . Node type generated by ast.parse() 当 mode is "single" .

body 是 list of statement nodes .

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

class ast. FunctionType ( argtypes , 返回 ) ¶

A representation of an old-style type comments for functions, as Python versions prior to 3.5 didn’t support PEP 484 annotations. Node type generated by ast.parse() 当 mode is "func_type" .

Such type comments would look like this:

def sum_two_number(a, b):
    # type: (int, int) -> int
    return a + b

argtypes 是 list of expression nodes .

返回 is a single expression node .

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

Added in version 3.8.

文字 ¶

class ast. 常量 ( value ) ¶

常量值。 value 属性在 Constant literal contains the Python object it represents. The values represented can be simple types such as a number, string or None , but also immutable container types (tuples and frozensets) if all of their elements are constant.

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

class ast. FormattedValue ( value , conversion , format_spec ) ¶

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

value is any expression node (such as a literal, a variable, or a function call).
conversion 是整数：
- -1: no formatting
- 115: !s string formatting
- 114: !r repr formatting
- 97: !a ascii formatting
format_spec 是 JoinedStr node representing the formatting of the value, or None if no format was specified. Both conversion and format_spec can be set at the same time.

class ast. JoinedStr ( values ) ¶

An f-string, comprising a series of FormattedValue and Constant 节点。

>>> 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 ) ¶

列表或元组。 elts holds a list of nodes representing the elements. ctx is Store if the container is an assignment target (i.e. (x,y)=something )，和 Load 否则。

>>> print(ast.dump(ast.parse('[1, 2, 3]', mode='eval'), indent=4))
Expression(
    body=List(
        elts=[
            Constant(value=1),
            Constant(value=2),
            Constant(value=3)],
        ctx=Load()))
>>> print(ast.dump(ast.parse('(1, 2, 3)', mode='eval'), indent=4))
Expression(
    body=Tuple(
        elts=[
            Constant(value=1),
            Constant(value=2),
            Constant(value=3)],
        ctx=Load()))

class ast. Set ( elts ) ¶

A set. elts holds a list of nodes representing the set’s elements.

>>> print(ast.dump(ast.parse('{1, 2, 3}', mode='eval'), indent=4))
Expression(
    body=Set(
        elts=[
            Constant(value=1),
            Constant(value=2),
            Constant(value=3)]))

class ast. Dict ( keys , values ) ¶

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

When doing dictionary unpacking using dictionary literals the expression to be expanded goes in the values list, with a None at the corresponding position in keys .

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

变量 ¶

class ast. 名称 ( id , ctx ) ¶: A variable name. id holds the name as a string, and ctx is one of the following types.

class ast. Load ¶

class ast. Store ¶

class ast. Del ¶

Variable references can be used to load the value of a variable, to assign a new value to it, or to delete it. Variable references are given a context to distinguish these cases.

>>> print(ast.dump(ast.parse('a'), indent=4))
Module(
    body=[
        Expr(
            value=Name(id='a', ctx=Load()))],
    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 *var variable reference. value holds the variable, typically a Name node. This type must be used when building a Call node with *args .

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

表达式 ¶

class ast. Expr ( value ) ¶

When an expression, such as a function call, appears as a statement by itself with its return value not used or stored, it is wrapped in this container. value holds one of the other nodes in this section, a Constant ， Name ， Lambda ， Yield or YieldFrom 节点。

>>> 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. op is the operator, and operand any expression node.

class ast. UAdd ¶

class ast. USub ¶

class ast. Not ¶

class ast. Invert ¶

Unary operator tokens. Not 是 not keyword, Invert 是 ~ 运算符。

>>> print(ast.dump(ast.parse('not x', mode='eval'), indent=4))
Expression(
    body=UnaryOp(
        op=Not(),
        operand=Name(id='x', ctx=Load())))

class ast. BinOp ( left , op , right ) ¶

A binary operation (like addition or division). op is the operator, and left and right are any expression nodes.

>>> print(ast.dump(ast.parse('x + y', mode='eval'), indent=4))
Expression(
    body=BinOp(
        left=Name(id='x', ctx=Load()),
        op=Add(),
        right=Name(id='y', ctx=Load())))

class ast. 添加 ¶
class ast. Sub ¶
class ast. Mult ¶
class ast. Div ¶
class ast. FloorDiv ¶
class ast. Mod ¶
class ast. Pow ¶
class ast. LShift ¶
class ast. RShift ¶
class ast. BitOr ¶
class ast. BitXor ¶
class ast. BitAnd ¶
class ast. MatMult ¶: Binary operator tokens.

class ast. BoolOp ( op , values ) ¶

A boolean operation, ‘or’ or ‘and’. op is Or or And . values are the values involved. Consecutive operations with the same operator, such as a or b or c , are collapsed into one node with several values.

This doesn’t include not , which is a UnaryOp .

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

class ast. And ¶
class ast. Or ¶: Boolean operator tokens.

class ast. 比较 ( left , ops , comparators ) ¶

A comparison of two or more values. left is the first value in the comparison, ops the list of operators, and comparators the list of values after the first element in the comparison.

>>> print(ast.dump(ast.parse('1 <= a < 10', mode='eval'), indent=4))
Expression(
    body=Compare(
        left=Constant(value=1),
        ops=[
            LtE(),
            Lt()],
        comparators=[
            Name(id='a', ctx=Load()),
            Constant(value=10)]))

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. 在 ¶
class ast. NotIn ¶: Comparison operator tokens.

class ast. 调用 ( func , args , keywords ) ¶

A function call. func is the function, which will often be a Name or Attribute object. Of the arguments:

args holds a list of the arguments passed by position.
keywords holds a list of keyword objects representing arguments passed by keyword.

当创建 Call node, args and keywords are required, but they can be empty lists.

>>> print(ast.dump(ast.parse('func(a, b=c, *d, **e)', mode='eval'), indent=4))
Expression(
    body=Call(
        func=Name(id='func', ctx=Load()),
        args=[
            Name(id='a', ctx=Load()),
            Starred(
                value=Name(id='d', ctx=Load()),
                ctx=Load())],
        keywords=[
            keyword(
                arg='b',
                value=Name(id='c', ctx=Load())),
            keyword(
                value=Name(id='e', ctx=Load()))]))

class ast. keyword ( arg , value ) ¶: A keyword argument to a function call or class definition. arg is a raw string of the parameter name, value is a node to pass in.

class ast. IfExp ( test , body , orelse ) ¶

An expression such as a if b else c . Each field holds a single node, so in the following example, all three are Name 节点。

>>> print(ast.dump(ast.parse('a if b else c', mode='eval'), indent=4))
Expression(
    body=IfExp(
        test=Name(id='b', ctx=Load()),
        body=Name(id='a', ctx=Load()),
        orelse=Name(id='c', ctx=Load())))

class ast. Attribute ( value , attr , ctx ) ¶

Attribute access, e.g. d.keys . value is a node, typically a Name . attr is a bare string giving the name of the attribute, and ctx is Load , Store or Del according to how the attribute is acted on.

>>> print(ast.dump(ast.parse('snake.colour', mode='eval'), indent=4))
Expression(
    body=Attribute(
        value=Name(id='snake', ctx=Load()),
        attr='colour',
        ctx=Load()))

class ast. NamedExpr ( target , value ) ¶

A named expression. This AST node is produced by the assignment expressions operator (also known as the walrus operator). As opposed to the Assign node in which the first argument can be multiple nodes, in this case both target and value must be single nodes.

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

Added in version 3.8.

下标 ¶

class ast. Subscript ( value , slice , ctx ) ¶

A subscript, such as l[1] . value is the subscripted object (usually sequence or mapping). slice is an index, slice or key. It can be a Tuple and contain a Slice . ctx is Load , Store or Del according to the action performed with the subscript.

>>> print(ast.dump(ast.parse('l[1:2, 3]', mode='eval'), indent=4))
Expression(
    body=Subscript(
        value=Name(id='l', ctx=Load()),
        slice=Tuple(
            elts=[
                Slice(
                    lower=Constant(value=1),
                    upper=Constant(value=2)),
                Constant(value=3)],
            ctx=Load()),
        ctx=Load()))

class ast. 切片 ( lower , upper , step ) ¶

Regular slicing (on the form lower:upper or lower:upper:step ). Can occur only inside the slice 字段对于 Subscript , either directly or as an element of Tuple .

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

推导 ¶

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 (或 key and value ) is a single node representing the part that will be evaluated for each item.

generators 是列表化的 comprehension 节点。

>>> 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 for clause in a comprehension. target is the reference to use for each element - typically a Name or Tuple 节点。 iter is the object to iterate over. ifs is a list of test expressions: each for clause can have multiple ifs .

is_async indicates a comprehension is asynchronous (using an async for 而不是 for ). The value is an integer (0 or 1).

>>> print(ast.dump(ast.parse('[ord(c) for line in file for c in line]', mode='eval'),
...                indent=4)) # Multiple comprehensions in one.
Expression(
    body=ListComp(
        elt=Call(
            func=Name(id='ord', ctx=Load()),
            args=[
                Name(id='c', ctx=Load())],
            keywords=[]),
        generators=[
            comprehension(
                target=Name(id='line', ctx=Store()),
                iter=Name(id='file', ctx=Load()),
                ifs=[],
                is_async=0),
            comprehension(
                target=Name(id='c', ctx=Store()),
                iter=Name(id='line', ctx=Load()),
                ifs=[],
                is_async=0)]))
>>> print(ast.dump(ast.parse('(n**2 for n in it if n>5 if n<10)', mode='eval'),
...                indent=4)) # generator comprehension
Expression(
    body=GeneratorExp(
        elt=BinOp(
            left=Name(id='n', ctx=Load()),
            op=Pow(),
            right=Constant(value=2)),
        generators=[
            comprehension(
                target=Name(id='n', ctx=Store()),
                iter=Name(id='it', ctx=Load()),
                ifs=[
                    Compare(
                        left=Name(id='n', ctx=Load()),
                        ops=[
                            Gt()],
                        comparators=[
                            Constant(value=5)]),
                    Compare(
                        left=Name(id='n', ctx=Load()),
                        ops=[
                            Lt()],
                        comparators=[
                            Constant(value=10)])],
                is_async=0)]))
>>> print(ast.dump(ast.parse('[i async for i in soc]', mode='eval'),
...                indent=4)) # Async comprehension
Expression(
    body=ListComp(
        elt=Name(id='i', ctx=Load()),
        generators=[
            comprehension(
                target=Name(id='i', ctx=Store()),
                iter=Name(id='soc', ctx=Load()),
                ifs=[],
                is_async=1)]))

语句 ¶

class ast. 赋值 ( 目标 , value , type_comment ) ¶

An assignment. targets is a list of nodes, and value is a single node.

Multiple nodes in targets represents assigning the same value to each. Unpacking is represented by putting a Tuple or List 在 targets .

type_comment ¶: type_comment is an optional string with the type annotation as a comment.

>>> print(ast.dump(ast.parse('a = b = 1'), indent=4)) # Multiple assignment
Module(
    body=[
        Assign(
            targets=[
                Name(id='a', ctx=Store()),
                Name(id='b', ctx=Store())],
            value=Constant(value=1))],
    type_ignores=[])
>>> print(ast.dump(ast.parse('a,b = c'), indent=4)) # Unpacking
Module(
    body=[
        Assign(
            targets=[
                Tuple(
                    elts=[
                        Name(id='a', ctx=Store()),
                        Name(id='b', ctx=Store())],
                    ctx=Store())],
            value=Name(id='c', ctx=Load()))],
    type_ignores=[])

class ast. AnnAssign ( target , annotation , value , simple ) ¶

An assignment with a type annotation. target is a single node and can be a Name ， Attribute 或 Subscript . annotation is the annotation, such as a Constant or Name 节点。 value is a single optional node. simple is a boolean integer set to True for a Name node in target that do not appear in between parenthesis and are hence pure names and not expressions.

>>> print(ast.dump(ast.parse('c: int'), indent=4))
Module(
    body=[
        AnnAssign(
            target=Name(id='c', ctx=Store()),
            annotation=Name(id='int', ctx=Load()),
            simple=1)],
    type_ignores=[])
>>> print(ast.dump(ast.parse('(a): int = 1'), indent=4)) # Annotation with parenthesis
Module(
    body=[
        AnnAssign(
            target=Name(id='a', ctx=Store()),
            annotation=Name(id='int', ctx=Load()),
            value=Constant(value=1),
            simple=0)],
    type_ignores=[])
>>> print(ast.dump(ast.parse('a.b: int'), indent=4)) # Attribute annotation
Module(
    body=[
        AnnAssign(
            target=Attribute(
                value=Name(id='a', ctx=Load()),
                attr='b',
                ctx=Store()),
            annotation=Name(id='int', ctx=Load()),
            simple=0)],
    type_ignores=[])
>>> print(ast.dump(ast.parse('a[1]: int'), indent=4)) # Subscript annotation
Module(
    body=[
        AnnAssign(
            target=Subscript(
                value=Name(id='a', ctx=Load()),
                slice=Constant(value=1),
                ctx=Store()),
            annotation=Name(id='int', ctx=Load()),
            simple=0)],
    type_ignores=[])

class ast. AugAssign ( target , op , value ) ¶

Augmented assignment, such as a += 1 . In the following example, target 是 Name node for x (with the Store context), op is Add ，和 value 是 Constant with value for 1.

The target attribute cannot be of class Tuple or List , unlike the targets of Assign .

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

class ast. 引发 ( exc , cause ) ¶

A raise 语句。 exc is the exception object to be raised, normally a Call or Name ，或 None for a standalone raise . cause is the optional part for y in raise x from y .

>>> print(ast.dump(ast.parse('raise x from y'), indent=4))
Module(
    body=[
        Raise(
            exc=Name(id='x', ctx=Load()),
            cause=Name(id='y', ctx=Load()))],
    type_ignores=[])

class ast. Assert ( test , msg ) ¶

断言。 test holds the condition, such as a Compare 节点。 msg holds the failure message.

>>> print(ast.dump(ast.parse('assert x,y'), indent=4))
Module(
    body=[
        Assert(
            test=Name(id='x', ctx=Load()),
            msg=Name(id='y', ctx=Load()))],
    type_ignores=[])

class ast. 删除 ( 目标 ) ¶

Represents a del 语句。 targets is a list of nodes, such as Name , Attribute or Subscript 节点。

>>> 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 pass 语句。

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

class ast. TypeAlias ( name , type_params , value ) ¶

A 类型别名 created through the type 语句。 name is the name of the alias, type_params 是列表化的 type parameters ，和 value is the value of the type alias.

>>> print(ast.dump(ast.parse('type Alias = int'), indent=4))
Module(
    body=[
        TypeAlias(
            name=Name(id='Alias', ctx=Store()),
            type_params=[],
            value=Name(id='int', ctx=Load()))],
    type_ignores=[])

3.12 版添加。

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

导入 ¶

class ast. Import ( 名称 ) ¶

An import statement. names 是列表化的 alias 节点。

>>> 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 ( 模块 , 名称 , level ) ¶

Represents from x import y . module is a raw string of the ‘from’ name, without any leading dots, or None for statements such as from . import foo . level is an integer holding the level of the relative import (0 means absolute import).

>>> print(ast.dump(ast.parse('from y import x,y,z'), indent=4))
Module(
    body=[
        ImportFrom(
            module='y',
            names=[
                alias(name='x'),
                alias(name='y'),
                alias(name='z')],
            level=0)],
    type_ignores=[])

class ast. alias ( name , asname ) ¶

Both parameters are raw strings of the names. asname 可以是 None if the regular name is to be used.

>>> print(ast.dump(ast.parse('from ..foo.bar import a as b, c'), indent=4))
Module(
    body=[
        ImportFrom(
            module='foo.bar',
            names=[
                alias(name='a', asname='b'),
                alias(name='c')],
            level=2)],
    type_ignores=[])

控制流 ¶

注意

可选子句譬如 else are stored as an empty list if they’re not present.

class ast. 若 ( test , body , orelse ) ¶

An if 语句。 test holds a single node, such as a Compare 节点。 body and orelse each hold a list of nodes.

elif clauses don’t have a special representation in the AST, but rather appear as extra If nodes within the orelse section of the previous one.

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

class ast. For ( target , iter , body , orelse , type_comment ) ¶

A for 循环。 target holds the variable(s) the loop assigns to, as a single Name , Tuple , List , Attribute or Subscript 节点。 iter holds the item to be looped over, again as a single node. body and orelse contain lists of nodes to execute. Those in orelse are executed if the loop finishes normally, rather than via a break 语句。

type_comment ¶: type_comment is an optional string with the type annotation as a comment.

>>> print(ast.dump(ast.parse("""
... for x in y:
...     ...
... else:
...     ...
... """), indent=4))
Module(
    body=[
        For(
            target=Name(id='x', ctx=Store()),
            iter=Name(id='y', ctx=Load()),
            body=[
                Expr(
                    value=Constant(value=Ellipsis))],
            orelse=[
                Expr(
                    value=Constant(value=Ellipsis))])],
    type_ignores=[])

class ast. While ( test , body , orelse ) ¶

A while 循环。 test holds the condition, such as a Compare 节点。

>> 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 break and continue 语句。

>>> 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 ) ¶

try blocks. All attributes are list of nodes to execute, except for handlers , which is a list of ExceptHandler 节点。

>>> 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. TryStar ( body , handlers , orelse , finalbody ) ¶

try blocks which are followed by except* clauses. The attributes are the same as for Try but the ExceptHandler nodes in handlers are interpreted as except* blocks rather then except .

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

Added in version 3.11.

class ast. ExceptHandler ( type , name , body ) ¶

A single except 子句。 type is the exception type it will match, typically a Name node (or None for a catch-all except: clause). name is a raw string for the name to hold the exception, or None if the clause doesn’t have as foo . body is a list of nodes.

>>> print(ast.dump(ast.parse("""\
... try:
...     a + 1
... except TypeError:
...     pass
... """), indent=4))
Module(
    body=[
        Try(
            body=[
                Expr(
                    value=BinOp(
                        left=Name(id='a', ctx=Load()),
                        op=Add(),
                        right=Constant(value=1)))],
            handlers=[
                ExceptHandler(
                    type=Name(id='TypeError', ctx=Load()),
                    body=[
                        Pass()])],
            orelse=[],
            finalbody=[])],
    type_ignores=[])

class ast. 采用 ( items , body , type_comment ) ¶

A with 块。 items 是列表化的 withitem nodes representing the context managers, and body is the indented block inside the context.

type_comment ¶: type_comment is an optional string with the type annotation as a comment.

class ast. withitem ( context_expr , optional_vars ) ¶

A single context manager in a with 块。 context_expr is the context manager, often a Call 节点。 optional_vars 是 Name , Tuple or List 为 as foo part, or None if that isn’t used.

>>> print(ast.dump(ast.parse("""\
... with a as b, c as d:
...    something(b, d)
... """), indent=4))
Module(
    body=[
        With(
            items=[
                withitem(
                    context_expr=Name(id='a', ctx=Load()),
                    optional_vars=Name(id='b', ctx=Store())),
                withitem(
                    context_expr=Name(id='c', ctx=Load()),
                    optional_vars=Name(id='d', ctx=Store()))],
            body=[
                Expr(
                    value=Call(
                        func=Name(id='something', ctx=Load()),
                        args=[
                            Name(id='b', ctx=Load()),
                            Name(id='d', ctx=Load())],
                        keywords=[]))])],
    type_ignores=[])

Pattern matching ¶

class ast. Match ( subject , cases ) ¶

A match 语句。 subject holds the subject of the match (the object that is being matched against the cases) and cases contains an iterable of match_case nodes with the different cases.

Added in version 3.10.

class ast. match_case ( pattern , guard , body ) ¶

A single case pattern in a match 语句。 pattern contains the match pattern that the subject will be matched against. Note that the AST nodes produced for patterns differ from those produced for expressions, even when they share the same syntax.

The guard attribute contains an expression that will be evaluated if the pattern matches the subject.

body contains a list of nodes to execute if the pattern matches and the result of evaluating the guard expression is true.

>>> print(ast.dump(ast.parse("""
... match x:
...     case [x] if x>0:
...         ...
...     case tuple():
...         ...
... """), indent=4))
Module(
    body=[
        Match(
            subject=Name(id='x', ctx=Load()),
            cases=[
                match_case(
                    pattern=MatchSequence(
                        patterns=[
                            MatchAs(name='x')]),
                    guard=Compare(
                        left=Name(id='x', ctx=Load()),
                        ops=[
                            Gt()],
                        comparators=[
                            Constant(value=0)]),
                    body=[
                        Expr(
                            value=Constant(value=Ellipsis))]),
                match_case(
                    pattern=MatchClass(
                        cls=Name(id='tuple', ctx=Load()),
                        patterns=[],
                        kwd_attrs=[],
                        kwd_patterns=[]),
                    body=[
                        Expr(
                            value=Constant(value=Ellipsis))])])],
    type_ignores=[])

Added in version 3.10.

class ast. MatchValue ( value ) ¶

A match literal or value pattern that compares by equality. value is an expression node. Permitted value nodes are restricted as described in the match statement documentation. This pattern succeeds if the match subject is equal to the evaluated value.

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

Added in version 3.10.

class ast. MatchSingleton ( value ) ¶

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

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

Added in version 3.10.

class ast. MatchSequence ( patterns ) ¶

A match sequence pattern. patterns contains the patterns to be matched against the subject elements if the subject is a sequence. Matches a variable length sequence if one of the subpatterns is a MatchStar node, otherwise matches a fixed length sequence.

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

Added in version 3.10.

class ast. MatchStar ( name ) ¶

Matches the rest of the sequence in a variable length match sequence pattern. If name 不是 None , a list containing the remaining sequence elements is bound to that name if the overall sequence pattern is successful.

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

Added in version 3.10.

class ast. MatchMapping ( keys , patterns , rest ) ¶

A match mapping pattern. keys is a sequence of expression nodes. patterns is a corresponding sequence of pattern nodes. rest is an optional name that can be specified to capture the remaining mapping elements. Permitted key expressions are restricted as described in the match statement documentation.

This pattern succeeds if the subject is a mapping, all evaluated key expressions are present in the mapping, and the value corresponding to each key matches the corresponding subpattern. If rest 不是 None , a dict containing the remaining mapping elements is bound to that name if the overall mapping pattern is successful.

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

Added in version 3.10.

class ast. MatchClass ( cls , patterns , kwd_attrs , kwd_patterns ) ¶

A match class pattern. cls is an expression giving the nominal class to be matched. patterns is a sequence of pattern nodes to be matched against the class defined sequence of pattern matching attributes. kwd_attrs is a sequence of additional attributes to be matched (specified as keyword arguments in the class pattern), kwd_patterns are the corresponding patterns (specified as keyword values in the class pattern).

This pattern succeeds if the subject is an instance of the nominated class, all positional patterns match the corresponding class-defined attributes, and any specified keyword attributes match their corresponding pattern.

Note: classes may define a property that returns self in order to match a pattern node against the instance being matched. Several builtin types are also matched that way, as described in the match statement documentation.

>>> print(ast.dump(ast.parse("""
... match x:
...     case Point2D(0, 0):
...         ...
...     case Point3D(x=0, y=0, z=0):
...         ...
... """), indent=4))
Module(
    body=[
        Match(
            subject=Name(id='x', ctx=Load()),
            cases=[
                match_case(
                    pattern=MatchClass(
                        cls=Name(id='Point2D', ctx=Load()),
                        patterns=[
                            MatchValue(
                                value=Constant(value=0)),
                            MatchValue(
                                value=Constant(value=0))],
                        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.

class ast. MatchAs ( pattern , name ) ¶

A match “as-pattern”, capture pattern or wildcard pattern. pattern contains the match pattern that the subject will be matched against. If the pattern is None , the node represents a capture pattern (i.e a bare name) and will always succeed.

The name attribute contains the name that will be bound if the pattern is successful. If name is None , pattern must also be None and the node represents the wildcard pattern.

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

Added in version 3.10.

class ast. MatchOr ( patterns ) ¶

A match “or-pattern”. An or-pattern matches each of its subpatterns in turn to the subject, until one succeeds. The or-pattern is then deemed to succeed. If none of the subpatterns succeed the or-pattern fails. The patterns attribute contains a list of match pattern nodes that will be matched against the subject.

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

Added in version 3.10.

Type parameters ¶

Type parameters can exist on classes, functions, and type aliases.

class ast. TypeVar ( name , bound ) ¶

A typing.TypeVar . name is the name of the type variable. bound is the bound or constraints, if any. If bound 是 Tuple , it represents constraints; otherwise it represents the bound.

>>> print(ast.dump(ast.parse("type Alias[T: int] = list[T]"), indent=4))
Module(
    body=[
        TypeAlias(
            name=Name(id='Alias', ctx=Store()),
            type_params=[
                TypeVar(
                    name='T',
                    bound=Name(id='int', ctx=Load()))],
            value=Subscript(
                value=Name(id='list', ctx=Load()),
                slice=Name(id='T', ctx=Load()),
                ctx=Load()))],
    type_ignores=[])

3.12 版添加。

class ast. ParamSpec ( name ) ¶

A typing.ParamSpec . name is the name of the parameter specification.

>>> print(ast.dump(ast.parse("type Alias[**P] = Callable[P, int]"), indent=4))
Module(
    body=[
        TypeAlias(
            name=Name(id='Alias', ctx=Store()),
            type_params=[
                ParamSpec(name='P')],
            value=Subscript(
                value=Name(id='Callable', ctx=Load()),
                slice=Tuple(
                    elts=[
                        Name(id='P', ctx=Load()),
                        Name(id='int', ctx=Load())],
                    ctx=Load()),
                ctx=Load()))],
    type_ignores=[])

3.12 版添加。

class ast. TypeVarTuple ( name ) ¶

A typing.TypeVarTuple . name is the name of the type variable tuple.

>>> print(ast.dump(ast.parse("type Alias[*Ts] = tuple[*Ts]"), indent=4))
Module(
    body=[
        TypeAlias(
            name=Name(id='Alias', ctx=Store()),
            type_params=[
                TypeVarTuple(name='Ts')],
            value=Subscript(
                value=Name(id='tuple', ctx=Load()),
                slice=Tuple(
                    elts=[
                        Starred(
                            value=Name(id='Ts', ctx=Load()),
                            ctx=Load())],
                    ctx=Load()),
                ctx=Load()))],
    type_ignores=[])

3.12 版添加。

函数和类定义 ¶

class ast. FunctionDef ( name , args , body , decorator_list , 返回 , type_comment , type_params ) ¶

A function definition.

name is a raw string of the function name.
args 是 arguments 节点。
body is the list of nodes inside the function.
decorator_list is the list of decorators to be applied, stored outermost first (i.e. the first in the list will be applied last).
returns is the return annotation.
type_params 是列表化的 type parameters .

type_comment ¶: type_comment is an optional string with the type annotation as a comment.

Changed in version 3.12: 添加 type_params .

class ast. Lambda ( args , body ) ¶

lambda is a minimal function definition that can be used inside an expression. Unlike FunctionDef , body holds a single node.

>>> print(ast.dump(ast.parse('lambda x,y: ...'), indent=4))
Module(
    body=[
        Expr(
            value=Lambda(
                args=arguments(
                    posonlyargs=[],
                    args=[
                        arg(arg='x'),
                        arg(arg='y')],
                    kwonlyargs=[],
                    kw_defaults=[],
                    defaults=[]),
                body=Constant(value=Ellipsis)))],
    type_ignores=[])

class ast. arguments ( posonlyargs , args , vararg , kwonlyargs , kw_defaults , kwarg , defaults ) ¶

The arguments for a function.

posonlyargs , args and kwonlyargs are lists of arg 节点。
vararg and kwarg are single arg nodes, referring to the *args, **kwargs 参数。
kw_defaults is a list of default values for keyword-only arguments. If one is None , the corresponding argument is required.
defaults is a list of default values for arguments that can be passed positionally. If there are fewer defaults, they correspond to the last n arguments.

class ast. arg ( arg , annotation , type_comment ) ¶

A single argument in a list. arg is a raw string of the argument name; annotation is its annotation, such as a Name 节点。

type_comment ¶: type_comment is an optional string with the type annotation as a comment

>>> print(ast.dump(ast.parse("""\
... @decorator1
... @decorator2
... def f(a: 'annotation', b=1, c=2, *d, e, f=3, **g) -> 'return annotation':
...     pass
... """), indent=4))
Module(
    body=[
        FunctionDef(
            name='f',
            args=arguments(
                posonlyargs=[],
                args=[
                    arg(
                        arg='a',
                        annotation=Constant(value='annotation')),
                    arg(arg='b'),
                    arg(arg='c')],
                vararg=arg(arg='d'),
                kwonlyargs=[
                    arg(arg='e'),
                    arg(arg='f')],
                kw_defaults=[
                    None,
                    Constant(value=3)],
                kwarg=arg(arg='g'),
                defaults=[
                    Constant(value=1),
                    Constant(value=2)]),
            body=[
                Pass()],
            decorator_list=[
                Name(id='decorator1', ctx=Load()),
                Name(id='decorator2', ctx=Load())],
            returns=Constant(value='return annotation'),
            type_params=[])],
    type_ignores=[])

class ast. 返回 ( value ) ¶

A return 语句。

>>> 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 yield or yield from expression. Because these are expressions, they must be wrapped in a Expr node if the value sent back is not used.

>>> print(ast.dump(ast.parse('yield x'), indent=4))
Module(
    body=[
        Expr(
            value=Yield(
                value=Name(id='x', ctx=Load())))],
    type_ignores=[])
>>> print(ast.dump(ast.parse('yield from x'), indent=4))
Module(
    body=[
        Expr(
            value=YieldFrom(
                value=Name(id='x', ctx=Load())))],
    type_ignores=[])

class ast. Global ( 名称 ) ¶

class ast. Nonlocal ( 名称 ) ¶

global and nonlocal 语句。 names is a list of raw strings.

>>> print(ast.dump(ast.parse('global x,y,z'), indent=4))
Module(
    body=[
        Global(
            names=[
                'x',
                'y',
                'z'])],
    type_ignores=[])
>>> print(ast.dump(ast.parse('nonlocal x,y,z'), indent=4))
Module(
    body=[
        Nonlocal(
            names=[
                'x',
                'y',
                'z'])],
    type_ignores=[])

class ast. ClassDef ( name , bases , keywords , body , decorator_list , type_params ) ¶

类定义。

name is a raw string for the class name
bases is a list of nodes for explicitly specified base classes.
keywords 是列表化的 keyword nodes, principally for ‘metaclass’. Other keywords will be passed to the metaclass, as per PEP-3115 .
body is a list of nodes representing the code within the class definition.
decorator_list is a list of nodes, as in FunctionDef .
type_params 是列表化的 type parameters .

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

Changed in version 3.12: 添加 type_params .

async 和 await ¶

class ast. AsyncFunctionDef ( name , args , body , decorator_list , 返回 , type_comment , type_params ) ¶

An async def function definition. Has the same fields as FunctionDef .

Changed in version 3.12: 添加 type_params .

class ast. Await ( value ) ¶: An await 表达式。 value is what it waits for. Only valid in the body of an AsyncFunctionDef .

>>> print(ast.dump(ast.parse("""\
... async def f():
...     await other_func()
... """), indent=4))
Module(
    body=[
        AsyncFunctionDef(
            name='f',
            args=arguments(
                posonlyargs=[],
                args=[],
                kwonlyargs=[],
                kw_defaults=[],
                defaults=[]),
            body=[
                Expr(
                    value=Await(
                        value=Call(
                            func=Name(id='other_func', ctx=Load()),
                            args=[],
                            keywords=[])))],
            decorator_list=[],
            type_params=[])],
    type_ignores=[])

class ast. AsyncFor ( target , iter , body , orelse , type_comment ) ¶
class ast. AsyncWith ( items , body , type_comment ) ¶: async for 循环和 async with 上下文管理器。它们拥有相同的字段如 For and With , respectively. Only valid in the body of an AsyncFunctionDef .

注意

当剖析字符串通过 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` 帮手 ¶

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) .

若 type_comments=True is given, the parser is modified to check and return type comments as specified by PEP 484 and PEP 526 . This is equivalent to adding ast.PyCF_TYPE_COMMENTS to the flags passed to compile() . This will report syntax errors for misplaced type comments. Without this flag, type comments will be ignored, and the type_comment field on selected AST nodes will always be None . In addition, the locations of # type: ignore comments will be returned as the type_ignores attribute of Module (otherwise it is always an empty list).

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

设置 feature_version to a tuple (major, minor) will result in a “best-effort” attempt to parse using that Python version’s grammar. For example, setting feature_version=(3, 9) will attempt to disallow parsing of match statements. Currently major must equal to 3 . The lowest supported version is (3, 4) (and this may increase in future Python versions); the highest is sys.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 to feature_version .

If source contains a null character ( \0 ), ValueError 被引发。

警告

Note that successfully parsing source code into an AST object doesn’t guarantee that the source code provided is valid Python code that can be executed as the compilation step can raise further SyntaxError exceptions. For instance, the source return 42 generates a valid AST node for a return statement, but it cannot be compiled alone (it needs to be inside a function node).

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

警告

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

3.8 版改变：添加 type_comments , mode='func_type' and feature_version .

ast. unparse ( ast_obj ) ¶

Unparse an ast.AST object and generate a string with code that would produce an equivalent ast.AST object if parsed back with ast.parse() .

警告

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

警告

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

Added in version 3.9.

ast. literal_eval ( node_or_string ) ¶

Evaluate an expression node or a string containing only a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans, None and Ellipsis .

This can be used for evaluating strings containing Python values without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.

This function had been documented as “safe” in the past without defining what that meant. That was misleading. This is specifically designed not to execute Python code, unlike the more general 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.

警告

It is possible to crash the Python interpreter due to stack depth limitations in Python’s AST compiler.

It can raise ValueError , TypeError , SyntaxError , MemoryError and RecursionError depending on the malformed input.

3.2 版改变：现在允许字节和集文字。

3.9 版改变：现在支持创建空集采用 'set()' .

3.10 版改变： For string inputs, leading spaces and tabs are now stripped.

ast. get_docstring ( node , clean = True ) ¶

Return the docstring of the given node (which must be a FunctionDef , AsyncFunctionDef , ClassDef ，或 Module node), or None if it has no docstring. If clean is true, clean up the docstring’s indentation with inspect.cleandoc() .

3.5 版改变： AsyncFunctionDef 现在支持。

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 ，或 end_col_offset ) is missing, return None .

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

Added in version 3.8.

ast. fix_missing_locations ( node ) ¶: When you compile a node tree with compile() , the compiler expects lineno and col_offset attributes for every node that supports them. This is rather tedious to fill in for generated nodes, so this helper adds these attributes recursively where not already set, by setting them to the values of the parent node. It works recursively starting at node .

ast. increment_lineno ( node , n = 1 ) ¶: 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 ，和 end_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 in node._fields that is present on node .

ast. iter_child_nodes ( node ) ¶: Yield all direct child nodes of node , that is, all fields that are nodes and all items of fields that are lists of nodes.

ast. walk ( node ) ¶: Recursively yield all descendant nodes in the tree starting at node (including node itself), in no specified order. This is useful if you only want to modify nodes in place and don’t care about the context.

class ast. NodeVisitor ¶

A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the visit() 方法。

This class is meant to be subclassed, with the subclass adding visitor methods.

visit ( node ) ¶: Visit a node. The default implementation calls the method called self.visit_classname where classname is the name of the node class, or generic_visit() if that method doesn’t exist.

generic_visit ( node ) ¶

This visitor calls visit() on all children of the node.

Note that child nodes of nodes that have a custom visitor method won’t be visited unless the visitor calls generic_visit() or visits them itself.

visit_Constant ( node ) ¶: Handles all constant nodes.

不使用 NodeVisitor if you want to apply changes to nodes during traversal. For this a special visitor exists ( NodeTransformer ) that allows modifications.

从 3.8 版起弃用：方法 visit_Num() , visit_Str() , visit_Bytes() , visit_NameConstant() and visit_Ellipsis() are deprecated now and will not be called in future Python versions. Add the visit_Constant() method to handle all constant nodes.

class ast. NodeTransformer ¶

A NodeVisitor subclass that walks the abstract syntax tree and allows modification of nodes.

The NodeTransformer will walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method is None , the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.

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

class RewriteName(NodeTransformer):
    def visit_Name(self, node):
        return Subscript(
            value=Name(id='data', ctx=Load()),
            slice=Constant(value=node.id),
            ctx=node.ctx
        )

Keep in mind that if the node you’re operating on has child nodes you must either transform the child nodes yourself or call the generic_visit() method for the node first.

For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node.

若 NodeTransformer introduces new nodes (that weren’t part of original tree) without giving them location information (such as lineno ), fix_missing_locations() should be called with the new sub-tree to recalculate the location information:

tree = ast.parse('foo', mode='eval')
new_tree = fix_missing_locations(RewriteName().visit(tree))

Usually you use the transformer like this:

node = YourTransformer().visit(node)

ast. dump ( node , annotate_fields = True , include_attributes = False , * , indent = None ) ¶

Return a formatted dump of the tree in node . This is mainly useful for debugging purposes. If annotate_fields is true (by default), the returned string will show the names and the values for fields. If annotate_fields is false, the result string will be more compact by omitting unambiguous field names. Attributes such as line numbers and column offsets are not dumped by default. If this is wanted, include_attributes can be set to true.

若 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 "" 将仅插入换行符。 None (the default) selects the single line representation. Using a positive integer indent indents that many spaces per level. If indent 是字符串 (譬如 "\t" )，使用该字符串缩进每个级别。

3.9 版改变：添加 indent 选项。

编译器标志 ¶

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

ast. PyCF_ALLOW_TOP_LEVEL_AWAIT ¶

Enables support for top-level await , async for , async with and async comprehensions.

Added in version 3.8.

ast. PyCF_ONLY_AST ¶: Generates and returns an abstract syntax tree instead of returning a compiled code object.

ast. PyCF_TYPE_COMMENTS ¶

Enables support for PEP 484 and PEP 526 style type comments ( # type: <type> , # type: ignore <stuff> ).

Added in version 3.8.

命令行用法 ¶

Added in version 3.9.

The ast module can be executed as a script from the command line. It is as simple as:

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

The following options are accepted:

-h , --help ¶: Show the help message and exit.

-m <mode> ¶
--mode <mode> ¶: Specify what kind of code must be compiled, like the mode argument in parse() .

--no-type-comments ¶: Don’t parse type comments.

-a , --include-attributes ¶: Include attributes such as line numbers and column offsets.

-i <indent> ¶
--indent <indent> ¶: Indentation of nodes in AST (number of spaces).

若 infile is specified its contents are parsed to AST and dumped to stdout. Otherwise, the content is read from stdin.

另请参阅

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.

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