Converting letters using a dictionary

Question

I'm trying to write a program that converts letters matching a key in a dictionary to the value associated with that key e.g. if the dictionary is {'A':'T', 'C':'G', 'T':'A', 'G':'C'} and the string is 'AAG' the output should be 'TTC'.

EDIT: This is what I've got now thanks to some of your answers:

def matching_codons(complements, poolA):
  answer = []
  codon = ''
  counter = 0

  for i in poolA:
    for a in i:
      codon+= complements[a]
      counter += 1
      if counter == 3:
        answer.append(codon)

Unfortunately this only translates the first 3 letters - how can I make it keep running through the loop?

Note: poolA is a list of strings e.g. ['AAG', 'TAC', 'CGG', 'GAT', 'TTG', 'GTG', 'CAT', 'GGC', 'ATT', 'TCT']

Note2: I can't hard-code anything like a translation table because technically the dictionary input can be changed

Answer 1

another solution is using maketrans from string

from string import maketrans
complementTrans = maketrans("ACTG", "TGAC")

poolA =  ['AAG', 'TAC', 'CGG', 'GAT', 'TTG', 'GTG', 'CAT', 'GGC', 'ATT', 'TCT']
[codon.translate(complementTrans) for codon in poolA]

you get:

['TTC', 'ATG', 'GCC', 'CTA', 'AAC', 'CAC', 'GTA', 'CCG', 'TAA', 'AGA']

Bonus

It is always better to use biopython library, for example

poolA =  ['AAG', 'TAC', 'CGG', 'GAT', 'TTG', 'GTG', 'CAT', 'GGC', 'ATT', 'TCT']

from Bio.Seq import Seq
from Bio.Alphabet import IUPAC

[str(Seq(seq, IUPAC.unambiguous_dna).complement()) for seq in poolA]

you get the same result

Bonus 2

Fixing your code, I remove unnecessary variable counter

poolA =  ['AAG', 'TAC', 'CGG', 'GAT', 'TTG', 'GTG', 'CAT', 'GGC', 'ATT', 'TCT']
complements = {'A':'T', 'C':'G', 'T':'A', 'G':'C'} 

def matching_codons(complements, poolA):
    answer = []
    for i in poolA:
        codon = ''     # inside of for, codon reset in each iteration
        for a in i:
            codon+= complements[a]
        answer.append(codon)  # outside of secondary for, codon have three leters to end iterations
    return answer

matching_codons(complements, poolA)