Count frequency of letters in a text file

Use collections.Counter():

from collections import Counter
with open(file) as f:
    c = Counter()
    for line in f:
        c += Counter(line)

If the file is not so large, you can read all of it into memory as a string and convert it into a Counter object in one line of code:

c = Counter(f.read())

Example:

>>> c = Counter()
>>> c += Counter('aaabbbcccddd eee fff ggg')
>>> c
Counter({'a': 3, ' ': 3, 'c': 3, 'b': 3, 'e': 3, 'd': 3, 'g': 3, 'f': 3})
>>> c += Counter('aaabbbccc')
Counter({'a': 6, 'c': 6, 'b': 6, ' ': 3, 'e': 3, 'd': 3, 'g': 3, 'f': 3})

or use the count() method of strings:

from string import ascii_lowercase     # ascii_lowercase =='abcdefghijklmnopqrstuvwxyz'
with open(file) as f:
    text = f.read().strip()
    dic = {}
    for x in ascii_lowercase:
        dic[x] = text.count(x)

Use a dictionary - basically letters[char]++