“Did you mean… ?” Correcting errors in user queries

Overview

Whoosh can quickly suggest replacements for mis-typed words by returning a list of words from the index (or a dictionary) that are close to the mis-typed word:

with ix.searcher() as s:
    corrector = s.corrector("text")
    for mistyped_word in mistyped_words:
        print corrector.suggest(mistyped_word, limit=3)

See the whoosh.spelling.Corrector.suggest() method documentation for information on the arguments.

Currently the suggestion engine is more like a “typo corrector” than a real “spell checker” since it doesn’t do the kind of sophisticated phonetic matching or semantic/contextual analysis a good spell checker might. However, it is still very useful.

There are two main strategies for correcting words:

• Use the terms from an index field.
• Use words from a word list.

Pulling suggestions from an indexed field

In Whoosh 2.7 and later, spelling suggestions are available on all fields. However, if you have an analyzer that modifies the indexed words (such as stemming), you can add spelling=True to a field to have it store separate unmodified versions of the terms for spelling suggestions:

ana = analysis.StemmingAnalyzer()
schema = fields.Schema(text=TEXT(analyzer=ana, spelling=True))

You can then use the whoosh.searching.Searcher.corrector() method to get a corrector for a field:

corrector = searcher.corrector("content")

The advantage of using the contents of an index field is that when you are spell checking queries on that index, the suggestions are tailored to the contents of the index. The disadvantage is that if the indexed documents contain spelling errors, then the spelling suggestions will also be erroneous.

Pulling suggestions from a word list

There are plenty of word lists available on the internet you can use to populate the spelling dictionary.

(In the following examples, word_list can be a list of unicode strings, or a file object with one word on each line.)

To create a whoosh.spelling.Corrector object from a sorted word list:

from whoosh.spelling import ListCorrector

# word_list must be a sorted list of unicocde strings
corrector = ListCorrector(word_list)

Merging two or more correctors

You can combine suggestions from two sources (for example, the contents of an index field and a word list) using a whoosh.spelling.MultiCorrector:

c1 = searcher.corrector("content")
c2 = spelling.ListCorrector(word_list)
corrector = MultiCorrector([c1, c2])

Correcting user queries

You can spell-check a user query using the whoosh.searching.Searcher.correct_query() method:

from whoosh import qparser

# Parse the user query string
qp = qparser.QueryParser("content", myindex.schema)
q = qp.parse(qstring)

# Try correcting the query
with myindex.searcher() as s:
    corrected = s.correct_query(q, qstring)
    if corrected.query != q:
        print("Did you mean:", corrected.string)

The correct_query method returns an object with the following attributes:

query

A corrected whoosh.query.Query tree. You can test whether this is equal (==) to the original parsed query to check if the corrector actually changed anything.

string

A corrected version of the user’s query string.

tokens

A list of corrected token objects representing the corrected terms. You can use this to reformat the user query (see below).

You can use a whoosh.highlight.Formatter object to format the corrected query string. For example, use the HtmlFormatter to format the corrected string as HTML:

from whoosh import highlight

hf = highlight.HtmlFormatter()
corrected = s.correct_query(q, qstring, formatter=hf)

See the documentation for whoosh.searching.Searcher.correct_query() for information on the defaults and arguments.