"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", "lineno", and "col_offset" attributes.  The
      "lineno" and "end_lineno" are the first and last line numbers of
      source text span (1-indexed so the first line is line 1) and the
      "col_offset" and "end_col_offset" are the corresponding UTF-8
      byte offsets of the first and last tokens that generated the
      node. The UTF-8 offset is recorded because the parser uses UTF-8
      internally.

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

   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.


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 a "arguments" node.

   * "body" is the list of nodes inside the function.

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

   * "returns" is the return annotation.

   type_comment

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

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".


"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]".

   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.


Command-Line Usage
==================

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]

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).

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.
