Count occurences of all 3-character sequences of lowercase letters in a file.

counter3.py

import sys
import string

fname = sys.argv[1]
pairs = {}

char = None
next_char = None
next_next_char = None

chars = []

# add all the lowercase basic alphabet items to pairs
for ch in string.ascii_lowercase:
  for ch2 in string.ascii_lowercase:
    for ch3 in string.ascii_lowercase:
      pairs[ch + ch2 + ch3] = 0

with open(fname) as f:
  line = f.readline()
  while line:
    for ch in line:
      last_char = next_char
      next_char = next_next_char
      next_next_char = ch
      if last_char and next_char and next_next_char and last_char.strip() != '' and next_char.strip() != '' and next_next_char.strip() != '':
        if last_char + next_char + next_next_char in pairs:
          pairs[last_char + next_char + next_next_char ] += 1
        else:
          pairs[last_char + next_char + next_next_char ] = 1
    line = f.readline()

for key, val in pairs.items():
  print(str(val) + ' ' + key