CODEWITHSUNDEEP
pythonparsing
RickyA
RickyA

Reputation: 16029

test a boolean expression in a Python string

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.

Upvotes: 5

Views: 1733

Answers (3)

Hooked
Hooked

Reputation: 88248

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.

Upvotes: 0

chrisdotcode
chrisdotcode

Reputation: 1601

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...

Upvotes: 2

uselpa
uselpa

Reputation: 18937

>>> eval("20<30")
True

Upvotes: 1

Related Questions