letter frequency snippet

"""This script compares letter frequency in an English-language text
from Project Gutenberg with a standard letter
frequency metric.

The text selected is 'Turkish Literature' at
http://www.gutenberg.org/files/56464/56464-0.txt

Standard for letter frequency is from the words in the Oxford
Concise Dictionary as per the Wikipedia article 'Letter
Frequency in English' at https://en.wikipedia.org/wiki/Letter_frequency
"""
from string import ascii_lowercase

freqdict = {'a': 8.167, 'b': 1.492, 'c': 2.782, 'd': 4.253, 'e': 12.702,
            'f': 2.228, 'g': 2.015, 'h': 6.094, 'i': 6.966, 'j': 0.153,
            'k': 0.772, 'l': 4.025, 'm': 2.406, 'n': 6.749, 'o': 7.507,
            'p': 1.929, 'q': 0.095, 'r': 5.987, 's': 6.327, 't': 9.056,
            'u': 2.758, 'v': 0.987, 'w': 2.360, 'x': 0.150, 'y': 1.974,
            'z': 0.074}


bookfreq = dict()
for letter in ascii_lowercase:
    bookfreq[letter] = 0

filename = "source.txt"
total = 0
with open(filename, 'r') as handle:
    while True:
        line = handle.readline()
        if line == '':
            break
        else:
            for char in line:
                if char in ascii_lowercase:
                    bookfreq[char] += 1
                    total += 1

for key in bookfreq:
    '''Establishes percentage letter frequency in sample text.'''
    bookfreq[key] = round(((bookfreq[key] / total) * 100), 3)

# print(bookfreq)

compare = dict()
for key in bookfreq:
    '''Percentage deviation of sample text from standard, per letter.'''
    compare[key] = round(((1 - (bookfreq[key] / freqdict[key])) * 100), 3)

print(compare)