# Python3 bindings for Xapian

Table of contents

- Python3 bindings for Xapian
  - Strings
  - Unicode
  - Exceptions
  - Iterators
  - MSet
  - Non-Class Functions
  - Query
    - MatchAll and MatchNothing
  - MatchDecider
  - ValueRangeProcessor
  - Apache and mod_python/mod_wsgi
  - Test Suite

Xapian’s Python3 bindings are packaged in the xapian module - to use them, you’ll need to add this to your code:

    import xapian

They currently require at least Python 3.2. We’ve not tested with Python 3.1 - test results and any patches needed are most welcome.

The Python API largely follows the C++ API - the differences and additions are noted below.

The examples subdirectory contains examples (based on the simple C++ example) showing how to use the Python bindings: simpleindex.py, simplesearch.py, simpleexpand.py. There’s also simplematchdecider.py which shows how to define a MatchDecider in Python.

## Strings

The Xapian C++ API is largely agnostic about character encoding, and uses the std::string type as an opaque container for a sequence of bytes. In places where the bytes represent text (for example, in the Stem, QueryParser and TermGenerator classes), UTF-8 encoding is used. In order to wrap this for Python, std::string is mapped to/from the Python bytes type.

As a convenience, you can also pass Python str objects as parameters where this is appropriate, which will be converted to UTF-8 encoded text. Where std::string is returned, it’s always mapped to bytes in Python, which you can convert to a Python str by calling .decode(‘utf-8’) on it like so:

    for i in doc.termlist():
      print(i.term.decode('utf-8'))

## Unicode

Currently Xapian doesn’t have built-in support for normalising Unicode, so if you want to normalise Unicode text, you’ll need to do so in Python. The standard unicodedata module provides a way to do this - you probably want the NFKC normalisation scheme, so normalising a query string prior to parsing it would look something like this:

    query_string = get_query_string()
    query_string = unicodedata.normalize('NFKC', query_string)
    qp = xapian.QueryParser()
    query_obj = qp.parse_query(query_string)

## Exceptions

Xapian exceptions are translated into Python exceptions with the same names and inheritance hierarchy as the C++ exception classes. The base class of all Xapian exceptions is the xapian.Error class, and this in turn is a child of the standard python exceptions.Exception class.

This means that programs can trap all xapian exceptions using except xapian.Error, and can trap all exceptions which don’t indicate that the program should terminate using except Exception.

## Iterators

The iterator classes in the Xapian C++ API are wrapped in a Pythonic style. The following are supported (where marked as “default iterator”, it means __iter__() does the right thing, so you can for instance use for term in document to iterate over terms in a Document object):

Python iterators
Class | Python Method | Equivalent C++ Method | Iterator type
--- | --- | --- | ---
MSet | default iterator | begin() | MSetIter
ESet | default iterator | begin() | ESetIter
Enquire | matching_terms() | get_matching_terms_begin() | TermIter
Query | default iterator | get_terms_begin() | TermIter
Database | allterms() (and default iterator) | allterms_begin() | TermIter
Database | postlist(term) | postlist_begin(term) | PostingIter
Database | termlist(docid) | termlist_begin(docid) | TermIter
Database | positionlist(docid, term) | positionlist_begin(docid, term) | PositionIter
Database | metadata_keys(prefix) | metadata_keys(prefix) | TermIter
Database | spellings() | spellings_begin(term) | TermIter
Database | synonyms(term) | synonyms_begin(term) | TermIter
Database | synonym_keys(prefix) | synonym_keys_begin(prefix) | TermIter
Document | values() | values_begin() | ValueIter
Document | termlist() (and default iterator) | termlist_begin() | TermIter
QueryParser | stoplist() | stoplist_begin() | TermIter
QueryParser | unstemlist(term) | unstem_begin(term) | TermIter
ValueCountMatchSpy | values() | values_begin() | TermIter
ValueCountMatchSpy | top_values() | top_values_begin() | TermIter

The pythonic iterators generally return Python objects, with properties available as attribute values, with lazy evaluation where appropriate. An exception is PositionIter (as returned by Database.positionlist), which returns an integer.

The lazy evaluation is mainly transparent, but does become visible in one situation: if you keep an object returned by an iterator, without evaluating its properties to force the lazy evaluation to happen, and then move the iterator forward, the object may no longer be able to efficiently perform the lazy evaluation. In this situation, an exception will be raised indicating that the information requested wasn’t available. This will only happen for a few of the properties - most are either not evaluated lazily (because the underlying Xapian implementation doesn’t evaluate them lazily, so there’s no advantage in lazy evaluation), or can be accessed even after the iterator has moved. The simplest work around is to evaluate any properties you wish to use which are affected by this before moving the iterator. The complete set of iterator properties affected by this is:

> - Database.allterms (also accessible as Database.__iter__): termfreq
> - Database.termlist: termfreq and positer
> - Document.termlist (also accessible as Document.__iter__): termfreq and positer
> - Database.postlist: positer

## MSet

MSet objects have some additional methods to simplify access (these work using the C++ array dereferencing):

MSet additional methods
Method name | Explanation
--- | ---
get_hit(i) | returns MSetItem at index i
get_document_percentage(i) | convert_to_percent(get_hit(i))
get_document(i) | get_hit(i).get_document()
get_docid(i) | get_hit(i).get_docid()

Two MSet objects are equal if they have the same number and maximum possible number of members, and if every document member of the first MSet exists at the same index in the second MSet, with the same weight.

## Non-Class Functions

The C++ API contains a few non-class functions (the Database factory functions, and some functions reporting version information), which are wrapped like so for Python 3:

> - Xapian::version_string() is wrapped as xapian.version_string()
> - Xapian::major_version() is wrapped as xapian.major_version()
> - Xapian::minor_version() is wrapped as xapian.minor_version()
> - Xapian::revision() is wrapped as xapian.revision()
> - Xapian::Remote::open() is wrapped as xapian.remote_open() (both the TCP and “program” versions are wrapped - the SWIG wrapper checks the parameter list to decide which to call).
> - Xapian::Remote::open_writable() is wrapped as xapian.remote_open_writable() (both the TCP and “program” versions are wrapped - the SWIG wrapper checks the parameter list to decide which to call).

The following were deprecated in the C++ API before the Python 3 bindings saw a stable release, so are not wrapped for Python 3:

> - Xapian::Auto::open_stub()
> - Xapian::Chert::open()
> - Xapian::InMemory::open()

The version of the bindings in use is available as xapian.__version__ (as recommended by PEP 396). This may not be the same as xapian.version_string() as the latter is the version of xapian-core (the C++ library) in use.

## Query

In C++ there’s a Xapian::Query constructor which takes a query operator and start/end iterators specifying a number of terms or queries, plus an optional parameter. In Python, this is wrapped to accept any Python sequence (for example a list or tuple) of terms or queries (or even a mixture of terms and queries). For example:

    subq = xapian.Query(xapian.Query.OP_AND, "hello", "world")
    q = xapian.Query(xapian.Query.OP_AND, [subq, "foo", xapian.Query("bar", 2)])

### MatchAll and MatchNothing

As of 1.1.1, these are wrapped as xapian.Query.MatchAll and xapian.Query.MatchNothing.

## MatchDecider

Custom MatchDeciders can be created in Python by subclassing xapian.MatchDecider and defining a __call__ method that will do the work. Make sure you call the base class constructor in your constructor. The simplest example (which does nothing useful) would be as follows:

    class mymatchdecider(xapian.MatchDecider):
      def __init__(self):
        xapian.MatchDecider.__init__(self)

      def __call__(self, doc):
        return 1

## ValueRangeProcessor

The ValueRangeProcessor class (and its subclasses) provide an operator() method (which is exposed in python as a __call__() method, making the class instances into callables). This method checks whether a beginning and end of a range are in a format understood by the ValueRangeProcessor, and if so, converts the beginning and end into strings which sort appropriately. ValueRangeProcessors can be defined in python (and then passed to the QueryParser), or there are several default built-in ones which can be used.

In C++ the operator() method takes two std::string arguments by reference, which the subclassed method can modify, and returns a value slot number. In Python, we wrap this by passing two bytes objects to __call__ and having it return a tuple of (value_slot, modified_begin, modified_end). For example:

    vrp = xapian.NumberValueRangeProcessor(0, '$', True)
    a = '$10'
    b = '20'
    slot, a, b = vrp(a, b)

You can implement your own ValueRangeProcessor in Python. The Python implementation should override the __call__() method with its own implementation, which returns a tuple as above. For example:

    class MyVRP(xapian.ValueRangeProcessor):
      def __init__(self):
        xapian.ValueRangeProcessor.__init__(self)
      def __call__(self, begin, end):
        return (7, "A"+begin, "B"+end)

## Apache and mod_python/mod_wsgi

Prior to Xapian 1.3.0, applications which use the xapian module had to be run in the main interpreter under mod_python and mod_wsgi. As of 1.3.0, the xapian module no longer uses Python’s simplified GIL state API, and so this restriction should no longer apply.

## Test Suite

The Python bindings come with a test suite, consisting of two test files: smoketest.py and pythontest.py. These are run by the make check command, or may be run manually. By default, they will display the names of any tests which failed, and then display a count of tests which run and which failed. The verbosity may be increased by setting the VERBOSE environment variable, for example:

    make check VERBOSE=1

Setting VERBOSE to 1 will display detailed information about failures, and a value of 2 will display further information about the progress of tests.