letters.py

#!/usr/bin/env python

letters = 'abcdefghijklmnopqrstuvwxyz '

def file2prob(filename):
    """
    Read a file and return a dictionary of letters and
    their probabilities
    """
    letter_dict  = { c: 0 for c in letters }
    letter_total = 0

    with open(filename) as fp:
        for c in fp.read():
            if c.lower() not in letter_dict:
                continue
            letter_dict[c.lower()] += 1
            letter_total += 1

    probs = { c: letter_dict[c]/letter_total for c in letter_dict }
    return probs

def file2pairs(filename):
    """
    Read a file and return a dictionary of letters and
    the probabilities of following letters. That is, the
    conditional probability of a letter given its
    predecessor.
    """
    letter_dict  = { c: { a: 0 for a in letters }
                     for c in letters }
    previous = None
    with open(filename) as fp:
        for c in fp.read():
            if c not in letter_dict:
                continue
            c = c.lower()
            if previous is None:
                previous = c
                continue
            letter_dict[previous][c] += 1
            previous = c

    probs = { c: { d: letter_dict[c][d]/sum(letter_dict[c].values())
                   for d in letters } for c in letters }
    return probs

if __name__ == '__main__':
    import sys
    
    if len(sys.argv) != 3:
        print("Usage: %s filename letter" % sys.argv[0])
        sys.exit(-1)
        
    filename = sys.argv[1]
    letter = sys.argv[2]

    probs = file2prob(filename)
    print(probs[letter])