I have a boolean expression in a string. eg. 20 < 30. Is there a simple way to parse and evaluate this string so it will return True (in this case).
ast.literal_eval("20 < 30") does not work.
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3 Answers
Is this a user-defined string, or one you're defining?
If it's a string you're creating, you could use eval (eval("20 < 30")), but if the string is given by the user, you might want to sanitize it first...
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RickyA
I create it. No worries :) Eval is indeed a very nice and very BIG security hole.
>>> eval("20<30")
True
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ast.literal_eval shouldn't work since (from the docs) "The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and None.". The expression 20<30 requires some kind of evaluation, before it returns a bool.
A safer suggestion would be to split the string on the operator and literal_eval each side before passing to eval, ie.
import ast
expr = "20 < 30"
operator = "<"
lhs,rhs = map(ast.literal_eval, map(str.strip, expr.split(operator)))
eval("%s %s %s"%(lhs,operator,rhs))
Wrapping the thing in a try, except clause will catch some input errors when evaluating lhs,rhs.
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