More pythonic way to find a complementary DNA strand

Question

Here's what I have, but it seems kind of redundant. Maybe someone more experienced in Python knows of a way to clean this up? Should be pretty self explanatory what it does.

def complementary_strand(self, strand):
        ''' Takes a DNA strand string and finds its opposite base pair match. '''
        strand = strand.upper()
        newstrand = ""
        for i in range(0, len(strand)):
            if strand[i] == "T":
                newstrand += "A"

            if strand[i] == "A":
                newstrand += "T"

            if strand[i] == "G":
                newstrand += "C"

            if strand[i] == "C":
                newstrand += "G"

        return newstrand

Answer 1

Even better would be to craft a generator, instead:

TRANS = { "T": "A", "A": "T", "G": "C", "C": "G" }

def complementary_strand(self, strand):
    for base in strand.upper():
        yield TRANS[base]

Then you can use it any way you want, and an iterator is more efficient:

for base in strand.complementary_strand():
    # Do something

Answer 2

Something like

def complementary_strand(self, strand):
    return strand.upper().translate(maketrans("TAGC", "ATCG"))

Answer 3

Probably the most efficient way to do it, if the string is long enough:

import string

def complementary_strand(self, strand):
    return strand.translate(string.maketrans('TAGCtagc', 'ATCGATCG'))

This is making use of the translate and maketrans methods. You can also move the translate table creation outside the function:

import string
def __init__(self, ...):
    self.trans = string.maketrans('TAGCtagc', 'ATCGATCG')

def complementary_strand(self, strand):
    return strand.translate(self.trans)

Answer 4

def __init__(self, *args):
    # ... original __init__ method, and:
    self.trans = { "T": "A", "A": "T", "G": "C", "C": "G" }

def complementary_strand(self, strand):
    '''Takes a DNA strand string and returns its opposite base pair match.'''
    return ''.join([self.trans[base] for base in strand.upper()])