>>> import ast
>>> eval('{1:11,2:22}')
{1: 11, 2: 22}
>>> ast.literal_eval('{1:11,2:22}')
{1: 11, 2: 22}

>>> eval('{1,2}')
{1, 2}
>>> ast.literal_eval('{1,2}')
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    ast.literal_eval('{1,2}')
  File "C:\Python31\lib\ast.py", line 80, in literal_eval
    return _convert(node_or_string)
  File "C:\Python31\lib\ast.py", line 79, in _convert
    raise ValueError('malformed string')
ValueError: malformed string

>>> 

I haven't tested for 2.7 or 3.2 but this bug probably applies also there.