补充角色

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'

Answer 2

As your example is written (and as Cyber has written his answer based on your example) you are not getting the complement. You're getting A -> C (instead of the complement T), T -> G instead of A, etc.

Using a dictionary as Cyber has done, it should look like this:

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

And in code, including a check for non-DNA characters:

original = "ATCGTCA"
bad_original = "ATCGTCAZ"

complement = {'A':'T', 'T':'A', 'C':'G', 'G':'C'}
for dna in (original, bad_original):
    try:
        output = "".join([complement[x] for x in dna])
    except KeyError:
        output = "Contains non-DNA characters"

    print output

Where "original" yields "TAGCAGT" and "bad_original" yields "Contains non-DNA characters".

Note that this is complement, not the reverse complement, which is usually of more interest.

More generally, if you are planning on using this for sequences of DNA, you should probably look into the BioPython module ( http://biopython.org/wiki/Seq#Complement_and_reverse_complement ), which will get you complement (and reverse complement) with more versatility, error checking, etc.