"ast" --- Abstract Syntax Trees
*******************************

**Source code:** Lib/ast.py

======================================================================

The "ast" module helps Python applications to process trees of the
Python abstract syntax grammar.  The abstract syntax itself might
change with each Python release; this module helps to find out
programmatically what the current grammar looks like.

An abstract syntax tree can be generated by passing
"ast.PyCF_ONLY_AST" as a flag to the "compile()" built-in function, or
using the "parse()" helper provided in this module.  The result will
be a tree of objects whose classes all inherit from "ast.AST".  An
abstract syntax tree can be compiled into a Python code object using
the built-in "compile()" function.


Abstract Grammar
================

The abstract grammar is currently defined as follows:

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

   module Python
   {
       mod = Module(stmt* body, type_ignore* type_ignores)
           | Interactive(stmt* body)
           | Expression(expr body)
           | FunctionType(expr* argtypes, expr returns)

       stmt = FunctionDef(identifier name, arguments args,
                          stmt* body, expr* decorator_list, expr? returns,
                          string? type_comment)
             | AsyncFunctionDef(identifier name, arguments args,
                                stmt* body, expr* decorator_list, expr? returns,
                                string? type_comment)

             | ClassDef(identifier name,
                expr* bases,
                keyword* keywords,
                stmt* body,
                expr* decorator_list)
             | Return(expr? value)

             | Delete(expr* targets)
             | Assign(expr* targets, expr value, string? type_comment)
             | AugAssign(expr target, operator op, expr value)
             -- 'simple' indicates that we annotate simple name without parens
             | AnnAssign(expr target, expr annotation, expr? value, int simple)

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

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

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

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

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

             -- BoolOp() can use left & right?
       expr = BoolOp(boolop op, expr* values)
            | NamedExpr(expr target, expr value)
            | BinOp(expr left, operator op, expr right)
            | UnaryOp(unaryop op, expr operand)
            | Lambda(arguments args, expr body)
            | IfExp(expr test, expr body, expr orelse)
            | Dict(expr* keys, expr* values)
            | Set(expr* elts)
            | ListComp(expr elt, comprehension* generators)
            | SetComp(expr elt, comprehension* generators)
            | DictComp(expr key, expr value, comprehension* generators)
            | GeneratorExp(expr elt, comprehension* generators)
            -- the grammar constrains where yield expressions can occur
            | Await(expr value)
            | Yield(expr? value)
            | YieldFrom(expr value)
            -- need sequences for compare to distinguish between
            -- x < 4 < 3 and (x < 4) < 3
            | Compare(expr left, cmpop* ops, expr* comparators)
            | Call(expr func, expr* args, keyword* keywords)
            | FormattedValue(expr value, int? conversion, expr? format_spec)
            | JoinedStr(expr* values)
            | Constant(constant value, string? kind)

            -- the following expression can appear in assignment context
            | Attribute(expr value, identifier attr, expr_context ctx)
            | Subscript(expr value, expr slice, expr_context ctx)
            | Starred(expr value, expr_context ctx)
            | Name(identifier id, expr_context ctx)
            | List(expr* elts, expr_context ctx)
            | Tuple(expr* elts, expr_context ctx)

            -- can appear only in Subscript
            | Slice(expr? lower, expr? upper, expr? step)

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

       expr_context = Load | Store | Del

       boolop = And | Or

       operator = Add | Sub | Mult | MatMult | Div | Mod | Pow | LShift
                    | RShift | BitOr | BitXor | BitAnd | FloorDiv

       unaryop = Invert | Not | UAdd | USub

       cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn

       comprehension = (expr target, expr iter, expr* ifs, int is_async)

       excepthandler = ExceptHandler(expr? type, identifier? name, stmt* body)
                       attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)

       arguments = (arg* posonlyargs, arg* args, arg? vararg, arg* kwonlyargs,
                    expr* kw_defaults, arg? kwarg, expr* defaults)

       arg = (identifier arg, expr? annotation, string? type_comment)
              attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)

       -- keyword arguments supplied to call (NULL identifier for **kwargs)
       keyword = (identifier? arg, expr value)
                  attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)

       -- import name with optional 'as' alias.
       alias = (identifier name, identifier? asname)

       withitem = (expr context_expr, expr? optional_vars)

       type_ignore = TypeIgnore(int lineno, string tag)
   }


Node classes
============

class ast.AST

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

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

   _fields

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

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

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

   lineno
   col_offset
   end_lineno
   end_col_offset

      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]".

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

   * If there are positional arguments, there must be as many as there
     are items in "T._fields"; they will be assigned as attributes of
     these names.

   * If there are keyword arguments, they will set the attributes of
     the same names to the given values.

   For example, to create and populate an "ast.UnaryOp" node, you
   could use

      node = ast.UnaryOp()
      node.op = ast.USub()
      node.operand = ast.Constant()
      node.operand.value = 5
      node.operand.lineno = 0
      node.operand.col_offset = 0
      node.lineno = 0
      node.col_offset = 0

   or the more compact

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

Changed in version 3.8: Class "ast.Constant" is now used for all
constants.

Changed in version 3.9: Simple indices are represented by their value,
extended slices are represented as tuples.

Deprecated since version 3.8: Old classes "ast.Num", "ast.Str",
"ast.Bytes", "ast.NameConstant" and "ast.Ellipsis" are still
available, but they will be removed in future Python releases.  In the
meantime, instantiating them will return an instance of a different
class.

Deprecated since version 3.9: Old classes "ast.Index" and
"ast.ExtSlice" are still available, but they will be removed in future
Python releases. In the meantime, instantiating them will return an
instance of a different class.

Note:

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


Literals
--------

class ast.Constant(value)

   A constant value. The "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=[])


Expressions
-----------

class ast.Expr(value)

   When an expression, such as a function call, appears as a statement
   by itself with its return value not used or stored, it is wrapped
   in this container. "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
------------

Note:

  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".

Note:

  When a string is parsed by "ast.parse()", operator nodes (subclasses
  of "ast.operator", "ast.unaryop", "ast.cmpop", "ast.boolop" and
  "ast.expr_context") on the returned tree will be singletons. Changes
  to one will be reflected in all other occurrences of the same value
  (e.g. "ast.Add").


"ast" Helpers
=============

Apart from the node classes, the "ast" module defines these utility
functions and classes for traversing abstract syntax trees:

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

   Parse the source into an AST node.  Equivalent to "compile(source,
   filename, mode, ast.PyCF_ONLY_AST)".

   If "type_comments=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]".

   Warning:

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

   Changed in version 3.8: Added "type_comments", "mode='func_type'"
   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()".

   Warning:

     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).

   Warning:

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

   New in version 3.9.

ast.literal_eval(node_or_string)

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

   This can be used for 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.

   Warning:

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

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

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

ast.get_docstring(node, clean=True)

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

   Changed in version 3.5: "AsyncFunctionDef" is now supported.

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

   Get source code segment of the *source* that generated *node*. If
   some location information ("lineno", "end_lineno", "col_offset", 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.

   New in version 3.8.

ast.fix_missing_locations(node)

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

ast.increment_lineno(node, n=1)

   Increment the line number and end line number of each node in the
   tree starting at *node* by *n*. This is useful to "move code" to a
   different location in a file.

ast.copy_location(new_node, old_node)

   Copy source location ("lineno", "col_offset", "end_lineno", 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.

   Deprecated since version 3.8: 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

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

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

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

      class RewriteName(NodeTransformer):

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

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

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

   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))

   Usually you use the transformer like this:

      node = YourTransformer().visit(node)

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

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

   If *indent* is a non-negative integer or string, then the tree will
   be pretty-printed with that indent level.  An indent level of 0,
   negative, or """" will only insert newlines.  "None" (the default)
   selects the single line representation. Using a positive integer
   indent indents that many spaces per level.  If *indent* is a string
   (such as ""\t""), that string is used to indent each level.

   Changed in version 3.9: Added the *indent* option.


Compiler Flags
==============

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

ast.PyCF_ALLOW_TOP_LEVEL_AWAIT

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

   New in version 3.8.

ast.PyCF_ONLY_AST

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

ast.PyCF_TYPE_COMMENTS

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

   New in version 3.8.


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

New in version 3.9.

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

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

The following options are accepted:

-h, --help

   Show the help message and exit.

-m <mode>
--mode <mode>

   Specify what kind of code must be compiled, like the *mode*
   argument in "parse()".

--no-type-comments

   Don't parse type comments.

-a, --include-attributes

   Include attributes such as line numbers and column offsets.

-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.

See also:

  Green Tree Snakes, an external documentation resource, has good
  details on working with Python ASTs.

  ASTTokens annotates Python ASTs with the positions of tokens and
  text in the source code that generated them. This is helpful for
  tools that make source code transformations.

  leoAst.py unifies the token-based and parse-tree-based views of
  python programs by inserting two-way links between tokens and ast
  nodes.

  LibCST parses code as a Concrete Syntax Tree that looks like an ast
  tree and keeps all formatting details. It's useful for building
  automated refactoring (codemod) applications and linters.

  Parso is a Python parser that supports error recovery and round-trip
  parsing for different Python versions (in multiple Python versions).
  Parso is also able to list multiple syntax errors in your python
  file.
