Question

I have been working to build a complex data structure which would return a dictionary. Currently that class return string object of the form

{
   cset : x,    
   b1   : y,
   b2   : z,    
   dep  : {    
              cset : x1,
              b1   : y1,    
              b2   : z1,    
              dep  : {    
                         cset : x2,     
                         b1   : y2,    
                         b2   : z2,    
                         dep  : <same as above.it  recurses few more levels>                         
                         ...  
                     }
        }
    }

I want to convert this whole string object into dictionary.
I read on one of the articles to use pickle module, but I don’t want to serialize it into some file and use it.

Ref : http://bytes.com/topic/python/answers/36059-convert-dictionary-string-vice-versa

I am looking for some other neater ways of doing it, if possible.

Would be great to know any such ways.

Answer 1

Don’t use eval. If you are sure that the string will always contain a valid Python dict, use ast.literal_eval. This works pretty much like eval, but it only evaluates if the expression is a valid dict,list, etc. and throws an exceptions if it isn’t. This is way safer than trying to evaluate strings that may contain arbitrary code at runtime.

From the docs:

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.

This can be used for safely evaluating
strings containing Python expressions
from untrusted sources without the
need to parse the values oneself.

Code Example:

>>> import ast
>>> ast.literal_eval("1+1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/ast.py", line 68, in literal_eval
    return _convert(node_or_string)
  File "/usr/lib/python2.6/ast.py", line 67, in _convert
    raise ValueError('malformed string')
ValueError: malformed string
>>> ast.literal_eval("\"1+1\"")
'1+1'
>>> ast.literal_eval("{'a': 2, 'b': 3, 3:'xyz'}")
{'a': 2, 3: 'xyz', 'b': 3}