you can only allow certain characters

You can check if SOMETHINGTOCOMPARE is a subset of whitelist (set):

def check(SOMETHINGTOCOMPARE):
    whitelist = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','1','2','3','4','5','6','7','8','9','0']

    # Test whether every element in SOMETHINGTOCOMPARE is in whitelist.
    if set(SOMETHINGTOCOMPARE) <= set(whitelist):
        return True
    return False

You can test the function with:

if check('[email protected][email protected]'):
    print("You are allowed to look this username up.")
else:
    print("Your username contains special characters. Disallowed.")

if check('Anf'):
    print("You are allowed to look this username up.")
else:
    print("Your username contains special characters. Disallowed.")

if check('GGDFG94'):
    print("You are allowed to look this username up.")
else:
    print("Your username contains special characters. Disallowed.")