Counting DNA Nucleotides

import re

a ="AAGACACTGATGTAACTCGCAGCGTGAGTGGTGTTGAATGAGCCTATCATGAGTATGGTAGCCACTCATTACCGGATGCGGTTAGACCGACCAAGAAATTCAACTAGATACGTTCGCCTCGGCGCGACGCTTGGTTGCTGGTGGATCGGCTCCAAGTTACCATCGTTAAGCCACGGTGACACCAATCTGTTAAAATGGCTTCGAGCTCCCTACGGTTTCCAATATGACATAGACTAGGAAATGATGACTACCTAGCCGCCCCGTCGTTACTTCGATCCTATAGACTATACGAGGTAACAATCAACGGAGTGATAGAGCCGGGTACAAAAGCGCGTGCTAAAATCCTGACTTGAATCGATTGCACTGCCGGCGGTTGAACTTACCTGGACCAGCCACTAATCCTCTTCCCGTCGGGTCGGTCACGCGTTACGTGCCCTAACCTGCAGTCGCTTTGACATGCGGCACCGAGATAAGGCTCGTATTCAGACCCGCCGGTATGCGAGTACAGCAGGGCAGGGTGACGGTCCCCAAGTGAACGCAACTATCGCCTGCACGCACTGGAGAAAGAGCTCGAACTTATATCACCCGACTTCAAGTTTTTAAATTGATCCACTGTGGTCCGGACCCCCAGGCCGCTTAGAGTGGCGCCTGTTTCCATGACTTTGTACCATCTAGTATACTATCGCGAGAACTGTGCACGGGTTTACTTCCACACATTTGTTGTCGATCCTGATCTCGCTTTTGAATCGCGGCGGCTGGATTCGGGGGCATTGTAATTGACCACAATATACCGACACGACTATCACGCTCGACTAGTAGCTGCGGAGGACTTACTTATAGTCTAATGTAAATTTCTATCAATCAAGGGATACTGTAGAAGTGGTGGACGCCCGATGTACACTAATCACATGGCTAGGAG"

counta=0
countb=0
countc=0
countd=0
for i in range(0,len(a)):
    if a[i]=='A':
        counta+=1
    if a[i]=='T':
        countb+=1
    if a[i]=='G':
        countc+=1
    if a[i]=='C':
        countd+=1

print(counta,countd, countc, countb)

A string is simply an ordered collection of symbols selected from some alphabet and formed into a word; the length of a string is the number of symbols that it contains.

An example of a length 21 DNA string (whose alphabet contains the symbols 'A', 'C', 'G', and 'T') is "ATGCTTCAGAAAGGTCTTACG."

Given: A DNA string s$s$ of length at most 1000 nt.

Return: Four integers (separated by spaces) counting the respective number of times that the symbols 'A', 'C', 'G', and 'T' occur in s$s$.