Lib/ast.py

   1 # -*- coding: utf-8 -*-
   2 """
   3     ast
   4     ~~~
   5
   6     The `ast` module helps Python applications to process trees of the Python
   7     abstract syntax grammar.  The abstract syntax itself might change with
   8     each Python release; this module helps to find out programmatically what
   9     the current grammar looks like and allows modifications of it.
  10
  11     An abstract syntax tree can be generated by passing `ast.PyCF_ONLY_AST` as
  12     a flag to the `compile()` builtin function or by using the `parse()`
  13     function from this module.  The result will be a tree of objects whose
  14     classes all inherit from `ast.AST`.
  15
  16     A modified abstract syntax tree can be compiled into a Python code object
  17     using the built-in `compile()` function.
  18
  19     Additionally various helper functions are provided that make working with
  20     the trees simpler.  The main intention of the helper functions and this
  21     module in general is to provide an easy to use interface for libraries
  22     that work tightly with the python syntax (template engines for example).
  23
  24
  25     :copyright: Copyright 2008 by Armin Ronacher.
  26     :license: Python License.
  27 """
  28 from _ast import *
  29
  30
  31 def parse(expr, filename='<unknown>', mode='exec'):
  32     """
  33     Parse an expression into an AST node.
  34     Equivalent to compile(expr, filename, mode, PyCF_ONLY_AST).
  35     """
  36     return compile(expr, filename, mode, PyCF_ONLY_AST)
  37
  38
  39 def literal_eval(node_or_string):
  40     """
  41     Safely evaluate an expression node or a string containing a Python
  42     expression.  The string or node provided may only consist of the following
  43     Python literal structures: strings, numbers, tuples, lists, dicts, booleans,
  44     and None.
  45     """
  46     _safe_names = {'None': None, 'True': True, 'False': False}
  47     if isinstance(node_or_string, basestring):
  48         node_or_string = parse(node_or_string, mode='eval')
  49     if isinstance(node_or_string, Expression):
  50         node_or_string = node_or_string.body
  51     def _convert(node):
  52         if isinstance(node, Str):
  53             return node.s
  54         elif isinstance(node, Num):
  55             return node.n
  56         elif isinstance(node, Tuple):
  57             return tuple(map(_convert, node.elts))
  58         elif isinstance(node, List):
  59             return list(map(_convert, node.elts))
  60         elif isinstance(node, Dict):
  61             return dict((_convert(k), _convert(v)) for k, v
  62                         in zip(node.keys, node.values))
  63         elif isinstance(node, Name):
  64             if node.id in _safe_names:
  65                 return _safe_names[node.id]
  66         raise ValueError('malformed string')
  67     return _convert(node_or_string)
  68
  69
  70 def dump(node, annotate_fields=True, include_attributes=False):
  71     """
  72     Return a formatted dump of the tree in *node*.  This is mainly useful for
  73     debugging purposes.  The returned string will show the names and the values
  74     for fields.  This makes the code impossible to evaluate, so if evaluation is
  75     wanted *annotate_fields* must be set to False.  Attributes such as line
  76     numbers and column offsets are not dumped by default.  If this is wanted,
  77     *include_attributes* can be set to True.
  78     """
  79     def _format(node):
  80         if isinstance(node, AST):
  81             fields = [(a, _format(b)) for a, b in iter_fields(node)]
  82             rv = '%s(%s' % (node.__class__.__name__, ', '.join(
  83                 ('%s=%s' % field for field in fields)
  84                 if annotate_fields else
  85                 (b for a, b in fields)
  86             ))
  87             if include_attributes and node._attributes:
  88                 rv += fields and ', ' or ' '
  89                 rv += ', '.join('%s=%s' % (a, _format(getattr(node, a)))
  90                                 for a in node._attributes)
  91             return rv + ')'
  92         elif isinstance(node, list):
  93             return '[%s]' % ', '.join(_format(x) for x in node)
  94         return repr(node)
  95     if not isinstance(node, AST):
  96         raise TypeError('expected AST, got %r' % node.__class__.__name__)
  97     return _format(node)
  98
  99
 100 def copy_location(new_node, old_node):
 101     """
 102     Copy source location (`lineno` and `col_offset` attributes) from
 103     *old_node* to *new_node* if possible, and return *new_node*.
 104     """
 105     for attr in 'lineno', 'col_offset':
 106         if attr in old_node._attributes and attr in new_node._attributes \
 107            and hasattr(old_node, attr):
 108             setattr(new_node, attr, getattr(old_node, attr))
 109     return new_node
 110
 111
 112 def fix_missing_locations(node):
 113     """
 114     When you compile a node tree with compile(), the compiler expects lineno and
 115     col_offset attributes for every node that supports them.  This is rather
 116     tedious to fill in for generated nodes, so this helper adds these attributes
 117     recursively where not already set, by setting them to the values of the
 118     parent node.  It works recursively starting at *node*.
 119     """
 120     def _fix(node, lineno, col_offset):
 121         if 'lineno' in node._attributes:
 122             if not hasattr(node, 'lineno'):
 123                 node.lineno = lineno
 124             else:
 125                 lineno = node.lineno
 126         if 'col_offset' in node._attributes:
 127             if not hasattr(node, 'col_offset'):
 128                 node.col_offset = col_offset
 129             else:
 130                 col_offset = node.col_offset
 131         for child in iter_child_nodes(node):
 132             _fix(child, lineno, col_offset)
 133     _fix(node, 1, 0)
 134     return node
 135
 136
 137 def increment_lineno(node, n=1):
 138     """
 139     Increment the line number of each node in the tree starting at *node* by *n*.
 140     This is useful to "move code" to a different location in a file.
 141     """
 142     if 'lineno' in node._attributes:
 143         node.lineno = getattr(node, 'lineno', 0) + n
 144     for child in walk(node):
 145         if 'lineno' in child._attributes:
 146             child.lineno = getattr(child, 'lineno', 0) + n
 147     return node
 148
 149
 150 def iter_fields(node):
 151     """
 152     Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
 153     that is present on *node*.
 154     """
 155     for field in node._fields:
 156         try:
 157             yield field, getattr(node, field)
 158         except AttributeError:
 159             pass
 160
 161
 162 def iter_child_nodes(node):
 163     """
 164     Yield all direct child nodes of *node*, that is, all fields that are nodes
 165     and all items of fields that are lists of nodes.
 166     """
 167     for name, field in iter_fields(node):
 168         if isinstance(field, AST):
 169             yield field
 170         elif isinstance(field, list):
 171             for item in field:
 172                 if isinstance(item, AST):
 173                     yield item
 174
 175
 176 def get_docstring(node, clean=True):
 177     """
 178     Return the docstring for the given node or None if no docstring can
 179     be found.  If the node provided does not have docstrings a TypeError
 180     will be raised.
 181     """
 182     if not isinstance(node, (FunctionDef, ClassDef, Module)):
 183         raise TypeError("%r can't have docstrings" % node.__class__.__name__)
 184     if node.body and isinstance(node.body[0], Expr) and \
 185        isinstance(node.body[0].value, Str):
 186         if clean:
 187             import inspect
 188             return inspect.cleandoc(node.body[0].value.s)
 189         return node.body[0].value.s
 190
 191
 192 def walk(node):
 193     """
 194     Recursively yield all child nodes of *node*, in no specified order.  This is
 195     useful if you only want to modify nodes in place and don't care about the
 196     context.
 197     """
 198     from collections import deque
 199     todo = deque([node])
 200     while todo:
 201         node = todo.popleft()
 202         todo.extend(iter_child_nodes(node))
 203         yield node
 204
 205
 206 class NodeVisitor(object):
 207     """
 208     A node visitor base class that walks the abstract syntax tree and calls a
 209     visitor function for every node found.  This function may return a value
 210     which is forwarded by the `visit` method.
 211
 212     This class is meant to be subclassed, with the subclass adding visitor
 213     methods.
 214
 215     Per default the visitor functions for the nodes are ``'visit_'`` +
 216     class name of the node.  So a `TryFinally` node visit function would
 217     be `visit_TryFinally`.  This behavior can be changed by overriding
 218     the `visit` method.  If no visitor function exists for a node
 219     (return value `None`) the `generic_visit` visitor is used instead.
 220
 221     Don't use the `NodeVisitor` if you want to apply changes to nodes during
 222     traversing.  For this a special visitor exists (`NodeTransformer`) that
 223     allows modifications.
 224     """
 225
 226     def visit(self, node):
 227         """Visit a node."""
 228         method = 'visit_' + node.__class__.__name__
 229         visitor = getattr(self, method, self.generic_visit)
 230         return visitor(node)
 231
 232     def generic_visit(self, node):
 233         """Called if no explicit visitor function exists for a node."""
 234         for field, value in iter_fields(node):
 235             if isinstance(value, list):
 236                 for item in value:
 237                     if isinstance(item, AST):
 238                         self.visit(item)
 239             elif isinstance(value, AST):
 240                 self.visit(value)
 241
 242
 243 class NodeTransformer(NodeVisitor):
 244     """
 245     A :class:`NodeVisitor` subclass that walks the abstract syntax tree and
 246     allows modification of nodes.
 247
 248     The `NodeTransformer` will walk the AST and use the return value of the
 249     visitor methods to replace or remove the old node.  If the return value of
 250     the visitor method is ``None``, the node will be removed from its location,
 251     otherwise it is replaced with the return value.  The return value may be the
 252     original node in which case no replacement takes place.
 253
 254     Here is an example transformer that rewrites all occurrences of name lookups
 255     (``foo``) to ``data['foo']``::
 256
 257        class RewriteName(NodeTransformer):
 258
 259            def visit_Name(self, node):
 260                return copy_location(Subscript(
 261                    value=Name(id='data', ctx=Load()),
 262                    slice=Index(value=Str(s=node.id)),
 263                    ctx=node.ctx
 264                ), node)
 265
 266     Keep in mind that if the node you're operating on has child nodes you must
 267     either transform the child nodes yourself or call the :meth:`generic_visit`
 268     method for the node first.
 269
 270     For nodes that were part of a collection of statements (that applies to all
 271     statement nodes), the visitor may also return a list of nodes rather than
 272     just a single node.
 273
 274     Usually you use the transformer like this::
 275
 276        node = YourTransformer().visit(node)
 277     """
 278
 279     def generic_visit(self, node):
 280         for field, old_value in iter_fields(node):
 281             old_value = getattr(node, field, None)
 282             if isinstance(old_value, list):
 283                 new_values = []
 284                 for value in old_value:
 285                     if isinstance(value, AST):
 286                         value = self.visit(value)
 287                         if value is None:
 288                             continue
 289                         elif not isinstance(value, AST):
 290                             new_values.extend(value)
 291                             continue
 292                     new_values.append(value)
 293                 old_value[:] = new_values
 294             elif isinstance(old_value, AST):
 295                 new_node = self.visit(old_value)
 296                 if new_node is None:
 297                     delattr(node, field)
 298                 else:
 299                     setattr(node, field, new_node)
 300         return node