Question

The string module provides a very useful maketrans function. It is easeir to show than to explain what this does.

from string import maketrans

dna_to_rna = maketrans('ACTG', 'ACUG')

dna = 'gattaca'
print dna.upper().translate(dna_to_rna).lower()

gauuaca

# Incidentally the translate function is useful for getting rid of unwanted characters in a string

from string import punctuation
print punctuation

!"#$%&'()*+,-./:;<=>?@[]^_`{|}~

import os

# Alice in Wonderland from Project Gutenberg

if not os.path.exists('alice.txt'):
    ! wget http://www.gutenberg.org/cache/epub/11/pg11.txt -O alice.txt

from collections import Counter

# Remove
alice = open('alice.txt').read()
words = alice.translate(None, punctuation).lower().split()
word_counts = Counter(words)
for item in word_counts.most_common(10):
    print item
print 'alice', word_counts['alice']

('the', 1804)
('and', 912)
('to', 801)
('a', 684)
('of', 625)
('it', 541)
('she', 538)
('said', 462)
('you', 429)
('in', 428)
alice 385