count the number of each characters in <stdin>

count-acgt.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from collections import Counter
import fileinput


def _main():
    """
    Usage example:
    % echo AAAAATATGAAACGT | python count-atgc.py
    [('A', 9), ('T', 3), ('G', 2), ('C', 1)]
    """

    c = Counter()

    for line in fileinput.input():
        for base in line.rstrip():
            c[base] += 1

    print c.most_common()


if __name__ == '__main__':
    _main()