A slightly better spellchecker using a Trie.

spellchecker_trie.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
Written by Joe Hillenbrand <joehillen@gmail.com> 2012

All rights reserved!
"""

import sys

class Node:
    '''
    Node is a store of a single character and a dict of child nodes.
    '''
    def __init__(self, value):
        self.value = value
        self.children = dict()
        self.end = False # indicates if this is an exit node

    def __getitem__(self, key):
        if key in self.children:
            return self.children[key]
        return None

    def __setitem__(self, key, value):
        self.children[key] = value
    
    def __contains__(self, value):
        return value in self.children

    def __str__(self):
        return str(self.value)

class Trie:
    def __init__(self):
        self.root = Node('')

    def add(self, word):
        '''
        Traverse the trie and add new nodes as we go.
        '''
        word = word.strip()
        n = self.root # n is for "node"
        for l in word:
            nxt = n[l] # nxt is for the next node in the trie
            if nxt is not None:
                n = nxt
            else:
                n[l] = Node(l)
                n = n[l]
        n.end = True

    def __contains__(self, word):
        '''
        Traverse the trie to find a word.
        '''
        n = self.root # n is for "node"
        for l in word:
            if l not in n:
                return False
            n = n[l]
        if n.end == True:
            return True
        return False

class SpellCheck:

    def __init__(self):
        '''
        Load in the dictionary.
        '''
        self.words = Trie()
        with open('/usr/share/dict/words','r') as f:
            for word in f:
                self.words.add(word)
                

    def spellcheck(self, word):
        '''
        The following are tests using `doctest`. To run the tests do:

            python spellcheck-trie.py -t

        TESTS
        >>> s.spellcheck('sandwich')
        'sandwich'

        >>> s.spellcheck('little')
        'little'

        >>> s.spellcheck('aapple')
        'apple'

        >>> s.spellcheck('sheeeeep')
        'sheep'

        >>> s.spellcheck('peepple')
        'people'

        >>> s.spellcheck('sheeple')
        'NO SUGGESTION'

        >>> s.spellcheck('inSIDE')
        'inside'

        >>> s.spellcheck('jjoobbb')
        'job'

        >>> s.spellcheck('weke')
        'wake'

        >>> s.spellcheck('CUNsperrICY')
        'conspiracy'

        >>> s.spellcheck('ffoaoaoaoaoaoaaoaoaoaoaoadd')
        'food'

        >>> s.spellcheck('FOOD')
        'food'
        
        PATHOLOGICAL TESTS
        >>> s.spellcheck('')
        'NO SUGGESTION'
        >>> s.spellcheck('1337')
        'NO SUGGESTION'
        >>> s.spellcheck('woot!')
        'NO SUGGESTION'
        >>> s.spellcheck('two words')
        'NO SUGGESTION'
        >>> s.spellcheck(u'ಠ_ಠ')
        'NO SUGGESTION'
        >>> s.spellcheck('fo0bar')
        'NO SUGGESTION'
        '''

        # Try the word
        if word in self.words:
            return word

        # Try lowercase
        word = word.lower()
        if word in self.words:
            return word

        vowels = 'aeiou' # let's not consider 'y' a vowel in this case

        def recurse(path, word, node):
            '''
            This is a recursive function for finding spelling corrections.

            The tranversal is explicitly part of the spellchecker because
            how the trie is traversed is determined by the spelling correction 
            methods, in this case lowercase, vowel replace, and dedup.
            '''

            # base cases
            if node is None:
                return None
            if word == '':
                if node.end == True:
                    return path
                if node.end == False:
                    return None

            # try the letter
            ltr = word[0]
            if ltr in node:
                result = recurse(path + ltr, word[1:], node[ltr])
                if result:
                    return result

            # try lowercase
            ltr = ltr.lower()
            if ltr in node:
                result = recurse(path + ltr, word[1:], node[ltr])
                if result:
                    return result

            # try skipping duplicates
            if len(word) > 1 and ltr == word[1]:
                result = recurse(path, word[1:], node)
                if result:
                    return result

            # try replacing vowels
            if ltr in vowels:
                for v in vowels:
                    if v != ltr:
                        result = recurse(path + v, word[1:], node[v])
                        if result:
                            return result

            return None

        result = recurse('', word, self.words.root)
        if result:
            return result
        return 'NO SUGGESTION'


if __name__ == '__main__':

    if len(sys.argv) > 1 and sys.argv[1] == '-t':
        t = Trie()
        # Test trie
        with open('/usr/share/dict/words','r') as f:
            for word in f:
                word = word.strip()
                t.add(word)
                try:
                    assert(word in t)
                except AssertionError:
                    print word, "not in trie"
                    sys.exit(1)
        
        # Run doctests
        s = SpellCheck()
        import doctest
        doctest.testmod(extraglobs={'s': s })
        sys.exit(0)

    s = SpellCheck()
    while True:
        word = raw_input('> ')
        print s.spellcheck(word)