# `ast` --- 抽象语法树

源代码： Lib/ast.py

`ast` 模块帮助 Python 程序处理 Python 语法的抽象语法树。抽象语法或许会随着 Python 的更新发布而改变；该模块能够帮助理解当前语法在编程层面的样貌。

抽象语法树可通过将 `ast.PyCF_ONLY_AST` 作为旗标传递给 `compile()` 内置函数来生成，或是使用此模块中提供的 `parse()` 辅助函数。返回结果将是一个对象树，，其中的类都继承自 `ast.AST`。抽象语法树可被内置的 `compile()` 函数编译为一个 Python 代码对象。

## 抽象文法

抽象文法目前定义如下

    -- 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)
    }

## 节点类

class `ast.``AST`

  这是所有 AST 节点类的基类。实际上，这些节点类派生自 `Parser/Python.asdl` 文件，其中定义的语法树示例 如下。它们在 C 语言模块 `_ast` 中定义，并被导出至 `ast` 模块。

  抽象语法定义的每个左侧符号(比方说， `ast.stmt` 或者 `ast.expr`)定义了一个类。另外，在抽象语法定义的右侧，对每一个构造器也定义了一个类；这些类继承自树左侧的类。比如，`ast.BinOp` 继承自 `ast.expr`。对于多分支产生式(也就是"和规则")，树右侧的类是抽象的；只有特定构造器结点的实例能被构造。

  `_fields`

    每个具体类都有个属性 `_fields`, 用来给出所有子节点的名字。

    每个具体类的实例对它每个子节点都有一个属性，对应类型如文法中所定义。比如，`ast.BinOp` 的实例有个属性 `left`，类型是 `ast.expr`.

    如果这些属性在文法中标记为可选（使用问号），对应值可能会是 `None`。如果这些属性有零或多个（用星号标记），对应值会用Python的列表来表示。所有可能的属性必须在用 `compile()` 编译得到AST时给出，且是有效的值。

  `lineno`
  `col_offset`
  `end_lineno`
  `end_col_offset`

    Instances of `ast.expr` and `ast.stmt` subclasses have `lineno`, `col_offset`, `end_lineno`, and `end_col_offset` attributes. The `lineno` and `end_lineno` are the first and last line numbers of the 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]`.

  一个类的构造器 `ast.T` 像下面这样parse它的参数。

  - 如果有位置参数，它们必须和 `T._fields` 中的元素一样多；他们会像这些名字的属性一样被赋值。

  - 如果有关键字参数，它们必须被设为和给定值同名的属性。

  比方说，要创建和填充节点 `ast.UnaryOp`，你得用

      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

  或者更紧凑点

      node = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0),
                         lineno=0, col_offset=0)

3.8 版更變: Class `ast.Constant` is now used for all constants.

3.9 版更變: Simple indices are represented by their value, extended slices are represented as tuples.

3.8 版後已棄用: 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.

3.9 版後已棄用: 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.

備註

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

### 字面值

class `ast.``Constant`(value)

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

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

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` is an integer:

    - -1: no formatting

    - 115: `!s` string formatting

    - 114: `!r` repr formatting

    - 97: `!a` ascii formatting

  - `format_spec` is a `JoinedStr` node representing the formatting of the value, or `None` if no format was specified. Both `conversion` and `format_spec` can be set at the same time.

class `ast.``JoinedStr`(values)

  An f-string, comprising a series of `FormattedValue` and `Constant` nodes.

      >>> print(ast.dump(ast.parse('f"sin({a}) is {sin(a):.3}"', mode='eval'), indent=4))
      Expression(
          body=JoinedStr(
              values=[
                  Constant(value='sin('),
                  FormattedValue(
                      value=Name(id='a', ctx=Load()),
                      conversion=-1),
                  Constant(value=') is '),
                  FormattedValue(
                      value=Call(
                          func=Name(id='sin', ctx=Load()),
                          args=[
                              Name(id='a', ctx=Load())],
                          keywords=[]),
                      conversion=-1,
                      format_spec=JoinedStr(
                          values=[
                              Constant(value='.3')]))]))

class `ast.``List`(elts, ctx)
class `ast.``Tuple`(elts, ctx)

  A list or tuple. `elts` holds a list of nodes representing the elements. `ctx` is `Store` if the container is an assignment target (i.e. `(x,y)=something`), and `Load` otherwise.

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

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())]))

### Variables

class `ast.``Name`(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`, a `Name`, a `Lambda`, a `Yield` or `YieldFrom` node.

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

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` is the `not` keyword, `Invert` is the `~` operator.

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

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.``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'. `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.``Compare`(left, ops, comparators)

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

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

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

  - `args` holds a list of the arguments passed by position.

  - `keywords` holds a list of `keyword` objects representing arguments passed by keyword.

  When creating a `Call` node, `args` and `keywords` are required, but they can be empty lists. `starargs` and `kwargs` are optional.

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

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` nodes.

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

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)))

#### Subscripting

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.``Slice`(lower, upper, step)

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

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

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

  `generators` is a list of `comprehension` nodes.

      >>> print(ast.dump(ast.parse('[x for x in numbers]', mode='eval'), indent=4))
      Expression(
          body=ListComp(
              elt=Name(id='x', ctx=Load()),
              generators=[
                  comprehension(
                      target=Name(id='x', ctx=Store()),
                      iter=Name(id='numbers', ctx=Load()),
                      ifs=[],
                      is_async=0)]))
      >>> print(ast.dump(ast.parse('{x: x**2 for x in numbers}', mode='eval'), indent=4))
      Expression(
          body=DictComp(
              key=Name(id='x', ctx=Load()),
              value=BinOp(
                  left=Name(id='x', ctx=Load()),
                  op=Pow(),
                  right=Constant(value=2)),
              generators=[
                  comprehension(
                      target=Name(id='x', ctx=Store()),
                      iter=Name(id='numbers', ctx=Load()),
                      ifs=[],
                      is_async=0)]))
      >>> print(ast.dump(ast.parse('{x for x in numbers}', mode='eval'), indent=4))
      Expression(
          body=SetComp(
              elt=Name(id='x', ctx=Load()),
              generators=[
                  comprehension(
                      target=Name(id='x', ctx=Store()),
                      iter=Name(id='numbers', ctx=Load()),
                      ifs=[],
                      is_async=0)]))

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` node. `iter` is the object to iterate over. `ifs` is a list of test expressions: each `for` clause can have multiple `ifs`.

  `is_async` indicates a comprehension is asynchronous (using an `async for` instead of `for`). The value is an integer (0 or 1).

      >>> print(ast.dump(ast.parse('[ord(c) for line in file for c in line]', mode='eval'),
      ...                indent=4)) # Multiple comprehensions in one.
      Expression(
          body=ListComp(
              elt=Call(
                  func=Name(id='ord', ctx=Load()),
                  args=[
                      Name(id='c', ctx=Load())],
                  keywords=[]),
              generators=[
                  comprehension(
                      target=Name(id='line', ctx=Store()),
                      iter=Name(id='file', ctx=Load()),
                      ifs=[],
                      is_async=0),
                  comprehension(
                      target=Name(id='c', ctx=Store()),
                      iter=Name(id='line', ctx=Load()),
                      ifs=[],
                      is_async=0)]))

      >>> print(ast.dump(ast.parse('(n**2 for n in it if n>5 if n<10)', mode='eval'),
      ...                indent=4)) # generator comprehension
      Expression(
          body=GeneratorExp(
              elt=BinOp(
                  left=Name(id='n', ctx=Load()),
                  op=Pow(),
                  right=Constant(value=2)),
              generators=[
                  comprehension(
                      target=Name(id='n', ctx=Store()),
                      iter=Name(id='it', ctx=Load()),
                      ifs=[
                          Compare(
                              left=Name(id='n', ctx=Load()),
                              ops=[
                                  Gt()],
                              comparators=[
                                  Constant(value=5)]),
                          Compare(
                              left=Name(id='n', ctx=Load()),
                              ops=[
                                  Lt()],
                              comparators=[
                                  Constant(value=10)])],
                      is_async=0)]))

      >>> print(ast.dump(ast.parse('[i async for i in soc]', mode='eval'),
      ...                indent=4)) # Async comprehension
      Expression(
          body=ListComp(
              elt=Name(id='i', ctx=Load()),
              generators=[
                  comprehension(
                      target=Name(id='i', ctx=Store()),
                      iter=Name(id='soc', ctx=Load()),
                      ifs=[],
                      is_async=1)]))

### Statements

class `ast.``Assign`(targets, value, type_comment)

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

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

  `type_comment`

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

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

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

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

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

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

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

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

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

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

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

  The `target` attribute connot be of class `Tuple` or `List`, unlike the targets of `Assign`.

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

class `ast.``Raise`(exc, cause)

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

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

class `ast.``Assert`(test, msg)

  An assertion. `test` holds the condition, such as a `Compare` node. `msg` holds the failure message.

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

class `ast.``Delete`(targets)

  Represents a `del` statement. `targets` is a list of nodes, such as `Name`, `Attribute` or `Subscript` nodes.

      >>> print(ast.dump(ast.parse('del x,y,z'), indent=4))
      Module(
          body=[
              Delete(
                  targets=[
                      Name(id='x', ctx=Del()),
                      Name(id='y', ctx=Del()),
                      Name(id='z', ctx=Del())])],
          type_ignores=[])

class `ast.``Pass`

  A `pass` statement.

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

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

#### Imports

class `ast.``Import`(names)

  An import statement. `names` is a list of `alias` nodes.

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

class `ast.``ImportFrom`(module, names, level)

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

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

class `ast.``alias`(name, asname)

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

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

### Control flow

備註

Optional clauses such as `else` are stored as an empty list if they're not present.

class `ast.``If`(test, body, orelse)

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

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

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

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

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

  `type_comment`

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

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

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

  A `while` loop. `test` holds the condition, such as a `Compare` node.

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

class `ast.``Break`
class `ast.``Continue`

  The `break` and `continue` statements.

      >>> print(ast.dump(ast.parse("""\
      ... for a in b:
      ...     if a > 5:
      ...         break
      ...     else:
      ...         continue
      ...
      ... """), indent=4))
      Module(
          body=[
              For(
                  target=Name(id='a', ctx=Store()),
                  iter=Name(id='b', ctx=Load()),
                  body=[
                      If(
                          test=Compare(
                              left=Name(id='a', ctx=Load()),
                              ops=[
                                  Gt()],
                              comparators=[
                                  Constant(value=5)]),
                          body=[
                              Break()],
                          orelse=[
                              Continue()])],
                  orelse=[])],
          type_ignores=[])

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` nodes.

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

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

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

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

class `ast.``With`(items, body, type_comment)

  A `with` block. `items` is a list of `withitem` nodes representing the context managers, and `body` is the indented block inside the context.

  `type_comment`

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

class `ast.``withitem`(context_expr, optional_vars)

  A single context manager in a `with` block. `context_expr` is the context manager, often a `Call` node. `optional_vars` is a `Name`, `Tuple` or `List` for the `as foo` part, or `None` if that isn't used.

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

### Function and class definitions

class `ast.``FunctionDef`(name, args, body, decorator_list, returns, type_comment)

  A function definition.

  - `name` is a raw string of the function name.

  - `args` is an `arguments` node.

  - `body` is the list of nodes inside the function.

  - `decorator_list` is the list of decorators to be applied, stored outermost first (i.e. the first in the list will be applied last).

  - `returns` is the return annotation.

  `type_comment`

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

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` nodes.

  - `vararg` and `kwarg` are single `arg` nodes, referring to the `*args, **kwargs` parameters.

  - `kw_defaults` is a list of default values for keyword-only arguments. If one is `None`, the corresponding argument is required.

  - `defaults` is a list of default values for arguments that can be passed positionally. If there are fewer defaults, they correspond to the last n arguments.

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 `Str` or `Name` node.

  `type_comment`

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

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

class `ast.``Return`(value)

  A `return` statement.

      >>> 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`(names)
class `ast.``Nonlocal`(names)

  `global` and `nonlocal` statements. `names` is a list of raw strings.

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

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

class `ast.``ClassDef`(name, bases, keywords, starargs, kwargs, body, decorator_list)

  A class definition.

  - `name` is a raw string for the class name

  - `bases` is a list of nodes for explicitly specified base classes.

  - `keywords` is a list of `keyword` nodes, principally for 'metaclass'. Other keywords will be passed to the metaclass, as per PEP-3115.

  - `starargs` and `kwargs` are each a single node, as in a function call. starargs will be expanded to join the list of base classes, and kwargs will be passed to the metaclass.

  - `body` is a list of nodes representing the code within the class definition.

  - `decorator_list` is a list of nodes, as in `FunctionDef`.

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

### Async and await

class `ast.``AsyncFunctionDef`(name, args, body, decorator_list, returns, type_comment)

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

class `ast.``Await`(value)

  An `await` expression. `value` is what it waits for. Only valid in the body of an `AsyncFunctionDef`.

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

class `ast.``AsyncFor`(target, iter, body, orelse, type_comment)
class `ast.``AsyncWith`(items, body, type_comment)

  `async for` loops and `async with` context managers. They have the same fields as `For` and `With`, respectively. Only valid in the body of an `AsyncFunctionDef`.

備註

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` 中的辅助函数

除了节点类， `ast` 模块里为遍历抽象语法树定义了这些工具函数和类:

`ast.``parse`(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)

  把源码解析为AST节点。和 `compile(source, filename, mode,ast.PyCF_ONLY_AST)` 等价。

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

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

  Also, setting `feature_version` to a tuple `(major, minor)` will attempt to parse using that Python version's grammar. Currently `major` must equal to `3`. For example, setting `feature_version=(3, 4)` will allow the use of `async` and `await` as variable names. The lowest supported version is `(3, 4)`; the highest is `sys.version_info[0:2]`.

  If source contains a null character ('0'), `ValueError` is raised.

  > 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

  足够复杂或是巨大的字符串可能导致Python解释器的崩溃，因为Python的AST编译器是有栈深限制的。

  3.8 版更變: Added `type_comments`, `mode='func_type'` and `feature_version`.

`ast.``unparse`(ast_obj)

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

  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`.

  3.9 版新加入.

`ast.``literal_eval`(node_or_string)

  对表达式节点以及包含Python字面量或容器的字符串进行安全的求值。传入的字符串或者节点里可能只包含下列的Python字面量结构: 字符串，字节对象(bytes)，数值，元组，列表，字典，集合，布尔值和 `None`。

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

  警告

  足够复杂或是巨大的字符串可能导致Python解释器的崩溃，因为Python的AST编译器是有栈深限制的。

  3.2 版更變: 目前支持字节和集合。

  3.9 版更變: Now supports creating empty sets with `'set()'`.

`ast.``get_docstring`(node, clean=True)

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

  3.5 版更變: 目前支持 `AsyncFunctionDef`

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

  Get source code segment of the source that generated node. If some location information (`lineno`, `end_lineno`, `col_offset`, or `end_col_offset`) is missing, return `None`.

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

  3.8 版新加入.

`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`, and `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()` method.

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

  `visit`(node)

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

  `generic_visit`(node)

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

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

  Don't use the `NodeVisitor` if you want to apply changes to nodes during traversal. For this a special visitor exists (`NodeTransformer`) that allows modifications.

  3.8 版後已棄用: Methods `visit_Num()`, `visit_Str()`, `visit_Bytes()`, `visit_NameConstant()` and `visit_Ellipsis()` are deprecated now and will not be called in future Python versions. Add the `visit_Constant()` method to handle all constant nodes.

class `ast.``NodeTransformer`

  子类 `NodeVisitor` 用于遍历抽象语法树，并允许修改节点。

  `NodeTransformer` 将遍历抽象语法树并使用visitor方法的返回值去替换或移除旧节点。如果visitor方法的返回值为 `None` , 则该节点将从其位置移除，否则将替换为返回值。当返回值是原始节点时，无需替换。

  如下是一个转换器示例，它将所有出现的名称 (`foo`) 重写为 `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
              )

  请记住，如果您正在操作的节点具有子节点，则必须先转换其子节点或为该节点调用 `generic_visit()` 方法。

  对于属于语句集合（适用于所有语句节点）的节点，访问者还可以返回节点列表而不仅仅是单个节点。

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

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

  通常你可以像这样使用转换器:

      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.

  3.9 版更變: 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 with` and async comprehensions.

  3.8 版新加入.

`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>`).

  3.8 版新加入.

## 命令行用法

3.9 版新加入.

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]

可以接受以下选项：

`-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).

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

也參考

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.