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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN"
"http://www.w3.org/TR/REC-html40/strict.dtd">
<title>Rich Comparisons</title>
<div>
<img src="../../../c++boost.gif"
alt="c++boost.gif (8819 bytes)"
align="center"
width="277" height="86">
<hr>
<h1>Rich Comparisons</h1>
<hr>
In Python versions up to and including Python 2.0, support for
implementing comparisons on user-defined classes and extension types
was quite simple. Classes could implement a <tt>__cmp__</tt> method
that was given two instances of a class as arguments, and could only
return <tt>0</tt> if they were equal or <tt>+1</tt> or <tt>-1</tt> if
they were not. The method could not raise an exception or return
anything other than an integer value.
In Python 2.1, <b>Rich Comparisons</b> were added (see
<a href="http://python.sourceforge.net/peps/pep-0207.html">PEP 207</a>).
Python classes can now individually overload each of the &lt;, &lt;=,
&gt;, &gt;=, ==, and != operations.
<p>
For more detailed information, search for "rich comparison"
<a href="http://www.python.org/doc/current/ref/customization.html"
>here</a>.
<p>
Boost.Python supports both automatic overloading and manual overloading
of the Rich Comparison operators. The <b>compile-time</b> support is
independent of the Python version that is used when compiling
Boost.Python extension modules. That is, <tt>op_lt</tt> for example can
always be used, and the C++ <tt>operator&lt;</tt> will always be bound
to the Python method <tt>__lt__</tt>. However, the <b>run-time</b>
behavior will depend on the Python version.
<p>
With Python versions before 2.1, the Rich Comparison operators will not
be called by Python when any of the six comparison operators
(<tt>&lt;</tt>, <tt>&lt;=</tt>, <tt>==</tt>, <tt>!=</tt>,
<tt>&gt;</tt>, <tt>&gt;=</tt>) is used in an expression. The only way
to access the corresponding methods is to call them explicitly, e.g.
<tt>a.__lt__(b)</tt>. Only with Python versions 2.1 or higher will
expressions like <tt>a &lt; b</tt> work as expected.
<p>
To support Rich Comparisions, the Python C API was modified between
Python versions 2.0 and 2.1. A new slot was introduced in the
<tt>PyTypeObject</tt> structure: <tt>tp_richcompare</tt>. For backwards
compatibility, a flag (<tt>Py_TPFLAGS_HAVE_RICHCOMPARE</tt>) has to be
set to signal to the Python interpreter that Rich Comparisions are
supported by a particular type.
There is only one flag for all the six comparison operators.
When any of the six operators is wrapped automatically or
manually, Boost.Python will set this flag. Attempts to use comparison
operators at the Python level that are not defined at the C++ level
will then lead to an <tt>AttributeError</tt> when the Python 2.1
(or higher) interpreter tries, e.g., <tt>a.__lt__(b)</tt>. That
is, in general all six operators should be supplied. Automatically
wrapped operators and manually wrapped operators can be mixed. For
example:<pre>
boost::python::class_builder&lt;code&gt; py_code(this_module, "code");
py_code.def(boost::python::constructor&lt;&gt;());
py_code.def(boost::python::constructor&lt;int&gt;());
py_code.def(boost::python::operators&lt;( boost::python::op_eq
| boost::python::op_ne)&gt;());
py_code.def(NotImplemented, "__lt__");
py_code.def(NotImplemented, "__le__");
py_code.def(NotImplemented, "__gt__");
py_code.def(NotImplemented, "__ge__");
</pre>
<tt>NotImplemented</tt> is a simple free function that (currently) has
to be provided by the user. For example:<pre>
boost::python::ref
NotImplemented(const code&amp;, const code&amp;) {
return
boost::python::ref(Py_NotImplemented, boost::python::ref::increment_count);
}
</pre>
See also:
<ul>
<li><a href="../example/richcmp1.cpp"><tt>../example/richcmp1.cpp</tt></a>
<li><a href="../example/richcmp2.cpp"><tt>../example/richcmp2.cpp</tt></a>
<li><a href="../example/richcmp3.cpp"><tt>../example/richcmp3.cpp</tt></a>
</ul>
<hr>
&copy; Copyright Nicholas K. Sauter &amp; Ralf W. Grosse-Kunstleve 2001.
Permission to copy, use, modify, sell and distribute this document is
granted provided this copyright notice appears in all copies. This
document is provided "as is" without express or implied warranty, and
with no claim as to its suitability for any purpose.
<p>
Updated: July 2001
</div>