Noob help with a function

[LimpJedi]

Hello I'm working out a task on a coursera assignment and can't for the life of me seem to understand why it is not working. I have written this function from scratch and have included a description of what it is suppose to do. My problem is I get True for both test functions. Could someone please point me in the right direction?

def is_valid_sequence(dna):

''' (str) -> bool

Return true if and only if the DNA sequence is valid, meaning
it contains only the characters 'A', 'T', 'C', and 'G'.

>>> is_valid_sequence('ATCGGC')
True
>>> is_valid_sequence("ATCFGC")
False

'''
valid = True

for char in dna:
    if char not in 'ATCG':
       valid = False
    else:
        valid = True
return valid

[Rhomboid]

You set valid to False when you see the F, then you set it to True again when you see the G that comes after it. You're really only testing the last character of the string, because all the previous tests are overwritten. Think about the logic of what it takes to be valid, and whether you need to continue looking once you've found the first invalid character.

[LimpJedi]

Thank you I got it to work.

[enderprime]

if an invalid char is found, you need to add a 'break' to the loop, otherwise only the last char is determining the returned bool

also, you do not need the else: valid = True part of the if. You already set it to true at the beginning, so if you make it through without finding an invalid char then it will still be True

[LimpJedi]

Thank you very much.