"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.


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", "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]".

   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)

3.8 版後已棄用: Class "ast.Constant" is now used for all constants.
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.


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

The abstract grammar is currently defined as follows:

   -- ASDL's 5 builtin types are:
   -- identifier, int, string, object, constant

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

           -- not really an actual node but useful in Jython's typesystem.
           | Suite(stmt* body)

       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

             -- XXX Jython will be different
             -- 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, slice 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)

             -- 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 | AugLoad | AugStore | Param

       slice = Slice(expr? lower, expr? upper, expr? step)
             | ExtSlice(slice* dims)
             | Index(expr value)

       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)

       -- 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)
   }


"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]".

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

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

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.

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

   3.2 版更變: Now allows bytes and set literals.

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: "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.

   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

   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 copy_location(Subscript(
                  value=Name(id='data', ctx=Load()),
                  slice=Index(value=Constant(value=node.id)),
                  ctx=node.ctx
              ), node)

   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.

   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.

   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()".

-a, --include-attributes

   Include attributes such as line numbers and column offsets.

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

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