32.2. "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。


32.2.1. 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

      "ast.expr" 和 "ast.stmt" 子类的实例有 "lineno" 和 "col_offset"
      属性。"lineno" 是源代码的行数(从1开始, 所以第一行行数是1)，而
      "col_offset" 是该生成节点第一个token的UTF-8字节偏移量。记录下
      UTF-8偏移量的原因是parser内部使用UTF-8。

   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.Num()
      node.operand.n = 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.Num(5, lineno=0, col_offset=0),
                         lineno=0, col_offset=0)


32.2.2. 抽象文法
================

抽象文法目前定义如下

   -- ASDL's 7 builtin types are:
   -- identifier, int, string, bytes, object, singleton, constant
   --
   -- singleton: None, True or False
   -- constant can be None, whereas None means "no value" for object.

   module Python
   {
       mod = Module(stmt* body)
           | Interactive(stmt* body)
           | Expression(expr body)

           -- 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)
             | AsyncFunctionDef(identifier name, arguments args,
                                stmt* body, expr* decorator_list, expr? returns)

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

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

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

             | 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)

             -- BoolOp() can use left & right?
       expr = BoolOp(boolop op, expr* values)
            | 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)
            | Num(object n) -- a number as a PyObject.
            | Str(string s) -- need to specify raw, unicode, etc?
            | FormattedValue(expr value, int? conversion, expr? format_spec)
            | JoinedStr(expr* values)
            | Bytes(bytes s)
            | NameConstant(singleton value)
            | Ellipsis
            | Constant(constant value)

            -- 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)

       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)

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

       arg = (identifier arg, expr? annotation)
              attributes (int lineno, int 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)
   }



32.2.3. "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')

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

   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。

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。

   在 3.2 版更改: 目前支持字节和集合。

ast.get_docstring(node, clean=True)

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

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 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" and "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.

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=Str(s=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)

   Return a formatted dump of the tree in *node*.  This is mainly
   useful for debugging purposes.  The returned string will show the
   names and the values for fields.  This makes the code impossible to
   evaluate, so if evaluation is wanted *annotate_fields* must be set
   to "False".  Attributes such as line numbers and column offsets are
   not dumped by default.  If this is wanted, *include_attributes* can
   be set to "True".

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