Parsing user queries

Parsing user queries

Parsing user queries

Overview

The job of a query parser is to convert a query string submitted by a user into query objects (objects from the whoosh.query module).

For example, the user query:

rendering shading

might be parsed into query objects like this:

And([Term("content", u"rendering"), Term("content", u"shading")])

Whoosh includes a powerful, modular parser for user queries in the whoosh.qparser module. The default parser implements a query language similar to the one that ships with Lucene. However, by changing plugins or using functions such as whoosh.qparser.MultifieldParser(), whoosh.qparser.SimpleParser() or whoosh.qparser.DisMaxParser(), you can change how the parser works, get a simpler parser or change the query language syntax.

(In previous versions of Whoosh, the query parser was based on pyparsing. The new hand-written parser is less brittle and more flexible.)

Note

Remember that you can directly create query objects programmatically using the objects in the whoosh.query module. If you are not processing actual user queries, this is preferable to building a query string just to parse it.

Using the default parser

To create a whoosh.qparser.QueryParser object, pass it the name of the default field to search and the schema of the index you’ll be searching.

from whoosh.qparser import QueryParser

parser = QueryParser("content", schema=myindex.schema)

Tip

You can instantiate a QueryParser object without specifying a schema, however the parser will not process the text of the user query. This is useful for debugging, when you want to see how QueryParser will build a query, but don’t want to make up a schema just for testing.

Once you have a QueryParser object, you can call parse() on it to parse a query string into a query object:

>>> parser.parse(u"alpha OR beta gamma")
And([Or([Term('content', u'alpha'), Term('content', u'beta')]), Term('content', u'gamma')])

See the query language reference for the features and syntax of the default parser’s query language.

Common customizations

Searching for any terms instead of all terms by default

If the user doesn’t explicitly specify AND or OR clauses:

physically based rendering

…by default, the parser treats the words as if they were connected by AND, meaning all the terms must be present for a document to match:

physically AND based AND rendering

To change the parser to use OR instead, so that any of the terms may be present for a document to match, i.e.:

physically OR based OR rendering

…configure the QueryParser using the group keyword argument like this:

from whoosh import qparser

parser = qparser.QueryParser(fieldname, schema=myindex.schema,
                             group=qparser.OrGroup)

The Or query lets you specify that documents that contain more of the query terms score higher. For example, if the user searches for foo bar, a document with four occurances of foo would normally outscore a document that contained one occurance each of foo and bar. However, users usually expect documents that contain more of the words they searched for to score higher. To configure the parser to produce Or groups with this behavior, use the factory() class method of OrGroup:

og = qparser.OrGroup.factory(0.9)
parser = qparser.QueryParser(fieldname, schema, group=og)

where the argument to factory() is a scaling factor on the bonus (between 0 and 1).

Letting the user search multiple fields by default

The default QueryParser configuration takes terms without explicit fields and assigns them to the default field you specified when you created the object, so for example if you created the object with:

parser = QueryParser("content", schema=myschema)

And the user entered the query:

three blind mice

The parser would treat it as:

content:three content:blind content:mice

However, you might want to let the user search multiple fields by default. For example, you might want “unfielded” terms to search both the title and content fields.

In that case, you can use a whoosh.qparser.MultifieldParser. This is just like the normal QueryParser, but instead of a default field name string, it takes a sequence of field names:

from whoosh.qparser import MultifieldParser

mparser = MultifieldParser(["title", "content"], schema=myschema)

When this MultifieldParser instance parses three blind mice, it treats it as:

(title:three OR content:three) (title:blind OR content:blind) (title:mice OR content:mice)

Simplifying the query language

Once you have a parser:

parser = qparser.QueryParser("content", schema=myschema)

you can remove features from it using the remove_plugin_class() method.

For example, to remove the ability of the user to specify fields to search:

parser.remove_plugin_class(qparser.FieldsPlugin)

To remove the ability to search for wildcards, which can be harmful to query performance:

parser.remove_plugin_class(qparser.WildcardPlugin)

See qparser module for information about the plugins included with Whoosh’s query parser.

Changing the AND, OR, ANDNOT, ANDMAYBE, and NOT syntax

The default parser uses English keywords for the AND, OR, ANDNOT, ANDMAYBE, and NOT functions:

parser = qparser.QueryParser("content", schema=myschema)

You can replace the default OperatorsPlugin object to replace the default English tokens with your own regular expressions.

The whoosh.qparser.OperatorsPlugin implements the ability to use AND, OR, NOT, ANDNOT, and ANDMAYBE clauses in queries. You can instantiate a new OperatorsPlugin and use the And, Or, Not, AndNot, and AndMaybe keyword arguments to change the token patterns:

# Use Spanish equivalents instead of AND and OR
op = qparser.OperatorsPlugin(And=" Y ", Or=" O ")
parser.replace_plugin(op)

Further, you may change the syntax of the NOT operator:

np = qparser.OperatorsPlugin(Not=' NO ')
parser.replace_plugin(np)

The arguments can be pattern strings or precompiled regular expression objects.

For example, to change the default parser to use typographic symbols instead of words for the AND, OR, ANDNOT, ANDMAYBE, and NOT functions:

parser = qparser.QueryParser("content", schema=myschema)
# These are regular expressions, so we have to escape the vertical bar
op = qparser.OperatorsPlugin(And="&", Or="\\|", AndNot="&!", AndMaybe="&~", Not="\\-")
parser.replace_plugin(op)

Adding less-than, greater-than, etc.

Normally, the way you match all terms in a field greater than “apple” is with an open ended range:

field:{apple to]

The whoosh.qparser.GtLtPlugin lets you specify the same search like this:

field:>apple

The plugin lets you use >, <, >=, <=, =>, or =< after a field specifier, and translates the expression into the equivalent range:

date:>='31 march 2001'

date:[31 march 2001 to]

Adding fuzzy term queries

Fuzzy queries are good for catching misspellings and similar words. The whoosh.qparser.FuzzyTermPlugin lets you search for “fuzzy” terms, that is, terms that don’t have to match exactly. The fuzzy term will match any similar term within a certain number of “edits” (character insertions, deletions, and/or transpositions – this is called the “Damerau-Levenshtein edit distance”).

To add the fuzzy plugin:

parser = qparser.QueryParser("fieldname", my_index.schema)
parser.add_plugin(qparser.FuzzyTermPlugin())

Once you add the fuzzy plugin to the parser, you can specify a fuzzy term by adding a ~ followed by an optional maximum edit distance. If you don’t specify an edit distance, the default is 1.

For example, the following “fuzzy” term query:

cat~

would match cat and all terms in the index within one “edit” of cat, for example cast (insert s), at (delete c), and act (transpose c and a).

If you wanted cat to match bat, it requires two edits (delete c and insert b) so you would need to set the maximum edit distance to 2:

cat~2

Because each additional edit you allow increases the number of possibilities that must be checked, edit distances greater than 2 can be very slow.

It is often useful to require that the first few characters of a fuzzy term match exactly. This is called a prefix. You can set the length of the prefix by adding a slash and a number after the edit distance. For example, to use a maximum edit distance of 2 and a prefix length of 3:

johannson~2/3

You can specify a prefix without specifying an edit distance:

johannson~/3

The default prefix distance is 0.

Allowing complex phrase queries

The default parser setup allows phrase (proximity) queries such as:

"whoosh search library"

The default phrase query tokenizes the text between the quotes and creates a search for those terms in proximity.

If you want to do more complex proximity searches, you can replace the phrase plugin with the whoosh.qparser.SequencePlugin, which allows any query between the quotes. For example:

"(john OR jon OR jonathan~) peters*"

The sequence syntax lets you add a “slop” factor just like the regular phrase:

"(john OR jon OR jonathan~) peters*"~2

To replace the default phrase plugin with the sequence plugin:

parser = qparser.QueryParser("fieldname", my_index.schema)
parser.remove_plugin_class(qparser.PhrasePlugin)
parser.add_plugin(qparser.SequencePlugin())

Alternatively, you could keep the default phrase plugin and give the sequence plugin different syntax by specifying a regular expression for the start/end marker when you create the sequence plugin. The regular expression should have a named group slop for the slop factor. For example:

parser = qparser.QueryParser("fieldname", my_index.schema)
parser.add_plugin(qparser.SequencePlugin("!(~(?P<slop>[1-9][0-9]*))?"))

This would allow you to use regular phrase queries and sequence queries at the same time:

"regular phrase" AND !sequence query~2!

Advanced customization

QueryParser arguments

QueryParser supports two extra keyword arguments:

group

The query class to use to join sub-queries when the user doesn’t explicitly specify a boolean operator, such as AND or OR. This lets you change the default operator from AND to OR.

This will be the whoosh.qparser.AndGroup or whoosh.qparser.OrGroup class (not an instantiated object) unless you’ve written your own custom grouping syntax you want to use.

termclass

The query class to use to wrap single terms.

This must be a whoosh.query.Query subclass (not an instantiated object) that accepts a fieldname string and term text unicode string in its __init__ method. The default is whoosh.query.Term.

This is useful if you want to change the default term class to whoosh.query.Variations, or if you’ve written a custom term class you want the parser to use instead of the ones shipped with Whoosh.

>>> from whoosh.qparser import QueryParser, OrGroup
>>> orparser = QueryParser("content", schema=myschema, group=OrGroup)

Configuring plugins

The query parser’s functionality is provided by a set of plugins. You can remove plugins to remove functionality, add plugins to add functionality, or replace default plugins with re-configured or rewritten versions.

The whoosh.qparser.QueryParser.add_plugin(), whoosh.qparser.QueryParser.remove_plugin_class(), and whoosh.qparser.QueryParser.replace_plugin() methods let you manipulate the plugins in a QueryParser object.

See qparser module for information about the available plugins.

Creating custom operators

  • Decide whether you want a PrefixOperator, PostfixOperator, or InfixOperator.
  • Create a new whoosh.qparser.syntax.GroupNode subclass to hold nodes affected by your operator. This object is responsible for generating a whoosh.query.Query object corresponding to the syntax.
  • Create a regular expression pattern for the operator’s query syntax.
  • Create an OperatorsPlugin.OpTagger object from the above information.
  • Create a new OperatorsPlugin instance configured with your custom operator(s).
  • Replace the default OperatorsPlugin in your parser with your new instance.

For example, if you were creating a BEFORE operator:

from whoosh import qparser, query

optype = qparser.InfixOperator
pattern = " BEFORE "

class BeforeGroup(qparser.GroupNode):
    merging = True
    qclass = query.Ordered

Create an OpTagger for your operator:

btagger = qparser.OperatorPlugin.OpTagger(pattern, BeforeGroup,
                                          qparser.InfixOperator)

By default, infix operators are left-associative. To make a right-associative infix operator, do this:

btagger = qparser.OperatorPlugin.OpTagger(pattern, BeforeGroup,
                                          qparser.InfixOperator,
                                          leftassoc=False)

Create an OperatorsPlugin instance with your new operator, and replace the default operators plugin in your query parser:

qp = qparser.QueryParser("text", myschema)
my_op_plugin = qparser.OperatorsPlugin([(btagger, 0)])
qp.replace_plugin(my_op_plugin)

Note that the list of operators you specify with the first argument is IN ADDITION TO the default operators (AND, OR, etc.). To turn off one of the default operators, you can pass None to the corresponding keyword argument:

cp = qparser.OperatorsPlugin([(optagger, 0)], And=None)

If you want ONLY your list of operators and none of the default operators, use the clean keyword argument:

cp = qparser.OperatorsPlugin([(optagger, 0)], clean=True)

Operators earlier in the list bind more closely than operators later in the list.