Python bindings for Xapian

The Python bindings for Xapian are packaged in the xapian module, and largely follow the C++ API, with the following differences and additions. Python strings and lists, etc., are converted automatically in the bindings, so generally it should just work as expected.

The examples subdirectory contains examples showing how to use the Python bindings based on the simple examples from xapian-examples: simpleindex.py, simplesearch.py, simpleexpand.py. There's also simplematchdecider.py which shows how to define a MatchDecider in Python.

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

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".

Unicode

The xapian Python bindings accept unicode strings as well as simple strings (ie, "str" type strings) at all places in the API which accept string data. Any unicode strings supplied will automatically be translated into UTF-8 simple strings before being passed to the Xapian core. The Xapian core is largely agnostic about character encoding, but in those places where it does process data in a character encoding dependent way it assumes that the data is in UTF-8. The Xapian Python bindings always return string data as simple strings.

Therefore, in order to avoid issues with character encodings, you should always pass text data to Xapian as unicode strings, or UTF-8 encoded simple strings. There is, however, no requirement for simple strings passed into Xapian to be valid UTF-8 encoded strings, unless they are being passed to a text processing routine (such as the query parser, or the stemming algorithms). For example, it is perfectly valid to pass arbitrary binary data in a simple string to the xapian.Document.set_data() method.

It is often useful to normalise unicode data before passing it to Xapian - Xapian currently has no built-in support for normalising unicode representations of data. The standard python module "unicodedata" provides support for normalising unicode: you probably want the "NFKC" normalisation scheme: in other words, use something like

   unicodedata.normalize('NFKC', u'foo')

to normalise the string "foo" before passing it to Xapian.

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):

The pythonic iterators return Python objects, with properties available as available as attribute values, with lazy evaluation where appropriate. An exception is the PositionIter object 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 simply 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

In older releases, the pythonic iterators returned lists representing the appropriate item when their next() method was called, except PositionIter which just returned a single value. For backward compatibility, this sequence API is still supported so existing code should continue to work, but its use is now deprecated and we plan to remove it in Xapian 1.1.0, so we recommend users migrate to the new API.

Non-Pythonic Iterators

Before the pythonic iterator wrappers were added, the python bindings provided thin wrappers around the C++ iterators. However, these iterators don't behave like most iterators do in Python, so the pythonic iterators were implemented to replace them. The non-pythonic iterators are still available to allow existing code to continue to work, but they're now deprecated and we plan to remove them in Xapian 1.2.0. The documentation below is provided to aid migration away from them.

All non-pythonic iterators support next() and equals() methods to move through and test iterators (as for all language bindings). MSetIterator and ESetIterator also support prev(). Python-wrapped iterators also support direct comparison, so something like:

   m=mset.begin()
   while m!=mset.end():
     # do something
     m.next()

C++ iterators are often dereferenced to get information, eg (*it). With Python these are all mapped to named methods, as follows:

Other methods, such as MSetIterator.get_document(), are available unchanged.

MSet

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

Additionally, the MSet has a property, mset.items, which returns a list of tuples representing the MSet; this may be more convenient than using an MSetIter. The members of the tuple are as follows.

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.

ESet

The ESet has a property, eset.items, which returns a list of tuples representing the ESet; this may be more convenient than using the ESetIterator. The members of the tuple are as follows.

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:

• 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::Auto::open_stub() is wrapped as xapian.open_stub()
• Xapian::Flint::open() is wrapped as xapian.flint_open()
• Xapian::InMemory::open() is wrapped as xapian.inmemory_open()
• Xapian::Quartz::open() is wrapped as xapian.quartz_open()
• 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).

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) to give the terms/queries, and you can specify a mixture of terms and queries if you wish. For example:

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

Enquire

There is an additional method get_matching_terms() which takes an MSetIterator and returns a list of terms in the current query which match the document given by that iterator. You may find this more convenient than using the TermIterator directly.

MatchDecider

Custom MatchDeciders can be created in Python; simply subclass xapian.MatchDecider, ensure you call the super-constructor, and define a __call__ method that will do the work. 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

ValueRangeProcessors

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.

Unfortunately, in C++ the operator() method takes two std::string arguments by reference, and returns values by modifying these arguments. This is not possible in Python, since strings are immutable objects. Instead, in the Python implementation, when the __call__ method is called, the resulting values of these arguments are returned as part of a tuple. The operator() method in C++ returns a value number; the return value of __call__ in python consists of a 3-tuple starting with this value number, followed by the returned "begin" value, followed by the returned "end" value. For example:

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

Additionally, a ValueRangeProcessor may be implemented in Python. The Python implementation should override the __call__() method with its own implementation, and, again, since it cannot return values by reference, it should return a tuple of (value number, begin, end). 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

There is an issue with python's threading support and mod-python, which can cause problems when using the Xapian Python bindings from mod-python. If you find that your mod-python application is hanging whenever a Xapian method is called, you have probably run into this problem.

By default, mod-python uses a separate instance of the python interpreter for each mod-python application. However, the Xapian python bindings do not know how to get and release control of the thread lock unless they are running in the "main" python interpreter; as a result, deadlocks can occur.

To avoid the problem, you need to use Python version 2.4 or later, and need to set the "PythonInterpreter main_interpreter" option in the apache configuration section for all mod_python scripts which use xapian: this ensures that the xapian bindings are called from the main_interpreter, and can handle the thread locks appropriately. An example configuration file using this option is available here.

In the longer term, we hope to be able to resolve this problem, so that arbitrary python interpreters can be used; but this is quite a tricky problem to fix, and is relatively low priority. More details can be found in Xapian's bug tracker, as bug #185.