Check if a string contains only given characters

Convert both strings to sets and check if they are equal. If yes, your string contains a AND b AND c:

valid = set(your_string) == set('abc')...

Use issubset to check if it contains ANY of a, b, c:

valid = set(your_string) <= set('abc')

valid = set(your_string).issubset('abc')

Subtract the sets to find out invalid characters:

bad_chars = set('abcXYcba') - set('abc') # set(X,Y)

You could use any with a generator expression:

if any(c not in 'abc' for c in _str):  # Don't use str as a name.
    print('Wrong character')

Using regular expressions:

import re
if re.search('[^abc]', string):
    print('wrong character')

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