How to check if a string contains only characters from a given set in python

Question

I have a a user-inputted polynomial and I only want to use it if it only has characters in the string 1234567890^-+x.

How can I check if it does or not without using external packages? I only want to use built-in Python 2.5 functions.

I am writing a program that runs on any Mac without needing external packages.

Answer 1

Here are some odd ;-) ways to do it:

good = set('1234567890^-+x')

if set(input_string) <= good:
    # it's good
else:
    # it's bad

or

if input_string.strip('1234567890^-+x'):
    # it's bad!
else:
    # it's good