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# “Did you mean... ?” Correcting errors in user queries

Note

In Whoosh 1.9 the old spelling system based on a separate N-gram index was replaced with this significantly more convenient and powerful implementation.

## 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 file.

## Pulling suggestions from an indexed field

To enable spell checking on the contents of a field, use the `spelling=True` keyword argument on the field in the schema definition:

    schema = Schema(text=TEXT(spelling=True))

(If you have an existing index you want to enable spelling for, you can alter the schema in-place using the `whoosh.writing.add_spelling()` function to create the missing word graph files.)

Tip

You can get suggestions for fields without the `spelling` attribute, but calculating the suggestions will be slower.

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 word list:

    from whoosh.spelling import GraphCorrector

    corrector = GraphCorrector.from_word_list(word_list)

Creating a corrector directly from a word list can be slow for large word lists, so you can save a corrector’s graph to a more efficient on-disk form like this:

    graphfile = myindex.storage.create_file("words.graph")
    # to_file() automatically closes the file when it's finished
    corrector.to_file(graphfile)

To open the graph file again very quickly:

    graphfile = myindex.storage.open_file("words.graph")
    corrector = GraphCorrector.from_graph_file(graphfile)

## 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 = GraphCorrector.from_graph_file(wordfile)
    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.

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