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use of ast.literal_eval in python

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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')
b = "x in ('abc', 'xyz')"
from ast import literal_eval
ValueError: malformed node or string: 

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

posted May 20, 2013 by anonymous

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2 Answers

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It seems to me you can't use ast.literal_eval() [ ] to evaluate that kind of expression because it's just for literals [ ].
Why don't you use eval()?

answer May 20, 2013 by anonymous
0 votes

An SQL constraint has a whole lot that Python can't do conveniently, and vice versa, so I think you do yourself a disservice by trying to tie yourself to that.

The problem here is that 'in' is an operator, so this is not a literal. One possible solution would be to separate out your operator (and have just a handful permitted), and then use ast.literal_eval for
just the second operand - something like this:

import ast
import operator
ops = {
 'in':lambda x,y: x in y, # operator.contains has the args backwards
 '==':operator.eq, # or use '=' for more SQL-like syntax

op, value = 'in', "('abc', 'xyz')"
x = 'abc'

if ops[op](x,ast.literal_eval(value)):
 print("Constraint passed")
 print("Ignore this one")
answer May 20, 2013 by anonymous
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