python - Determining Letter Frequency Of Cipher Text

Question

I am trying to make a tool that finds the frequencies of letters in some type of cipher text. Lets suppose it is all lowercase a-z no numbers. The encoded message is in a txt file

I am trying to build a script to help in cracking of substitution or possibly transposition ciphers.

Code so far:

cipher = open('cipher.txt','U').read()
cipherfilter = cipher.lower()
cipherletters = list(cipherfilter)

alpha = list('abcdefghijklmnopqrstuvwxyz')
occurrences = {} 
for letter in alpha:
    occurrences[letter] = cipherfilter.count(letter)
for letter in occurrences:
    print letter, occurrences[letter]

All it does so far is show how many times a letter appears. How would I print the frequency of all letters found in this file.

Answer 1

import collections

d = collections.defaultdict(int)
for c in 'test':
    d[c] += 1

print d # defaultdict(<type 'int'>, {'s': 1, 'e': 1, 't': 2})

From a file:

myfile = open('test.txt')
for line in myfile:
    line = line.rstrip('
')
    for c in line:
        d[c] += 1

For the genius that is the defaultdict container, we must give thanks and praise. Otherwise we'd all be doing something silly like this:

s = "andnowforsomethingcompletelydifferent"
d = {}
for letter in s:
    if letter not in d:
        d[letter] = 1
    else:
        d[letter] += 1