Pangrams CodeEval challenge

I took a challenge on CodeEval. Although the code seems to work for the examples taken from the site, I feel it is not really pretty and must be more complicated than it should be.


Description:


The sentence 'A quick brown fox jumps over the lazy dog' contains
every single letter in the alphabet. Such sentences are called
pangrams. You are to write a program, which takes a sentence, and
returns all the letters it is missing (which prevent it from being a
pangram). You should ignore the case of the letters in sentence, and
your return should be all lower case letters, in alphabetical order.
You should also ignore all non US-ASCII characters.In case the input
sentence is already a pangram, print out the string NULL.

import sys
filepath = sys.argv[1]
f = open(filepath)
wholealphabet = ('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r',
                 's','t','u','v','w','x','y','z')
for line in f:
    sortedletters = list(set(line.lower()))
    i = 0
    while i != len(sortedletters):
        if wholealphabet.count(sortedletters[i]) != 0: 
            i = i + 1
        else:
            sortedletters.pop(i)
    missingletters = ""
    for letter in wholealphabet:
        if sortedletters.count(letter) == 0:
            missingletters +=letter
    if len(missingletters) == 0:
        print("NULL")
    else:
        print(missingletters)

One of Python's greatest strengths is its built-in capability to use sets directly. I don't feel you've used sets to their fullest extent here. I'd also like to point out the with statement, which you should probably use to handle file handles.

from __future__ import with_statement
import sys
from string import ascii_lowercase
filepath = sys.argv[1]
wholealphabet = frozenset(ascii_lowercase)

# Use with to handle file … handles
with open(filepath) as f:
    for line in f: # assume a line is a sentence, not exactly per spec?
        # sortedletters = list(set(line.lower())) # guaranteed to be *unsorted*
        missingletters = wholealphabet.difference(line.lower())
        if missingletters:
            print ''.join(sorted(missingletters))
        else:
            print 'NULL'

That's really all you need. Unless you want to reconsider the definition of a sentence. :)

StackExchange Code Review Q#6323, answer score: 13