Could somebody help a beginner? :)

[deanhystad]

Your logic is backward. If you want verify that all the characters from bits_string are in bits, the test should be:

result = all(i in bits for i in bits_list)

As buran mentioned there is no need to convert bits_string to bits_list, or to have a list at all. Iterators and "in" for str work very similar to how they work for list. You could write the code like this:

def valid_binary_string(binary_string):
    return all(bit in "01" for bit in binary_string)

print(valid_binary_string("01010100"))
print(valid_binary_string("01010102"))

There are other ways to solve this problem that may be more efficient. For example, you could use sets to reduce the number of comparisons.

def valid_binary_string(binary_string):
    return all(i in '01' for i in set(binary_string))

print(valid_binary_string("01010100"))
print(valid_binary_string("01010102"))

If you use sets you can use set operations. For example, set(binary_string) - set('01') will return an empty set for any vailid binary string.

binary_digits = set('01')

def valid_binary_string(binary_string):
    return not set(binary_string)-binary_digits

print(valid_binary_string("01010100"))
print(valid_binary_string("01010102"))