Compiler.ast helper function "literal_eval" in python 2.4

[Gabriel Genellina]

En Fri, 04 Sep 2009 18:43:28 -0300, Sean Talts <xitrium at gmail.com>  
escribió:

> I'm trying to parse some python with the compiler module, select a
> subset of the AST returned, and then evaluate that subset, all in
> python 2.4.  It seems like in python 2.6 the compiler.ast.literal_eval
> function may be what I'm looking for, but unfortunately for my project
> we are restricted to using 2.4.  I was wondering if there was some way
> programmers performed this way back when?

Better late than never :)

This is a backport of 2.6 literal_eval using the compiler package as of  
2.4; I had to add UnarySub (else -3 would be invalid) and change some  
attribute names, but basically it's the same code. Untested except as  
shown.

 from compiler import parse
 from compiler.ast import *

def literal_eval(node_or_string):
     """
     Safely evaluate an expression node or a string containing a Python
     expression.  The string or node provided may only consist of the  
following
     Python literal structures: strings, numbers, tuples, lists, dicts,  
booleans,
     and None.
     """
     _safe_names = {'None': None, 'True': True, 'False': False}
     if isinstance(node_or_string, basestring):
         node_or_string = parse(node_or_string, mode='eval')
     if isinstance(node_or_string, Expression):
         node_or_string = node_or_string.node
     def _convert(node):
         if isinstance(node, Const) and isinstance(node.value,
                 (basestring, int, float, long, complex)):
              return node.value
         elif isinstance(node, Tuple):
             return tuple(map(_convert, node.nodes))
         elif isinstance(node, List):
             return list(map(_convert, node.nodes))
         elif isinstance(node, Dict):
             return dict((_convert(k), _convert(v)) for k, v
                         in node.items)
         elif isinstance(node, Name):
             if node.name in _safe_names:
                 return _safe_names[node.name]
         elif isinstance(node, UnarySub):
             return -_convert(node.expr)
         raise ValueError('malformed string')
     return _convert(node_or_string)

py> print literal_eval("(1, [-2.,'3',4j], {-5:None, u'6':True})")
(1, [-2.0, '3', 4j], {-5: None, u'6': True})

-- 
Gabriel Genellina