Getting complement of a character

Question

How can't the codes below work, in order to get the complement of the character entered? It seems like the loop never end, but let say, if I enter 'Z' as dna, why wouldn't it break and quit? Did I use the break or if wrongly? How about elif?

def get_complement(dna):

''' (ch) -> ch

Reverse the 'A' to 'T' or vice versa and 'C' to 'G' and vice versa too.
>>> get_complement('A')
'C'
>>> get_complement('G')
'T'

'''
if dna == 'A':
    print ('C')
    if dna == 'C':
        print ('A')
        if dna == 'T':
            print ('G')
            if dna == 'G' :
                print ('T')
                while  {'A', 'C', 'G', 'T'}.isnotsubset(set(dna)) :
                    break
                return ('')

Answer 1

You should set up a map, using a dictionary

complement = {'A': 'C', 'C': 'A', 'T': 'G', 'G': 'T'}

Then for some string you can do

original = "ATCGTCA"
"".join(complement[letter] for letter in original)

Output

'CGATGAC'

For just a single character:

complement['A']

Output

'C'