Question about ast.literal_eval

[Frank Millman]

Hi all

I am trying to emulate a SQL check constraint in Python. Quoting from 
the PostgreSQL docs, "A check constraint is the most generic constraint 
type. It allows you to specify that the value in a certain column must 
satisfy a Boolean (truth-value) expression."

The problem is that I want to store the constraint as a string, and I 
was hoping to use ast.literal_eval to evaluate it, but it does not work.

 >>> x = 'abc'
 >>> x in ('abc', xyz')
True
 >>> b = "x in ('abc', 'xyz')"
 >>> eval(b)
True
 >>> from ast import literal_eval
 >>> literal_eval(b)
ValueError: malformed node or string: <_ast.Compare object at ...>

Is there a safe way to do what I want? I am using python 3.3.

Thanks

Frank Millman