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<H2><A NAME="SECTION004250000000000000000">
2.2.5 Abstract Protocol Support</A>
</H2>

<P>
Python supports a variety of <em>abstract</em> `protocols;' the specific
interfaces provided to use these interfaces are documented in the
<em class="citetitle"><a
 href="../api/api.html"
 title="Python/C API Reference Manual"
 >Python/C API Reference Manual</a></em> in the
chapter ``<a class="ulink" href="../api/abstract.html"
  >Abstract Objects Layer</a>.''

<P>
A number of these abstract interfaces were defined early in the
development of the Python implementation.  In particular, the number,
mapping, and sequence protocols have been part of Python since the
beginning.  Other protocols have been added over time.  For protocols
which depend on several handler routines from the type implementation,
the older protocols have been defined as optional blocks of handlers
referenced by the type object.  For newer protocols there are
additional slots in the main type object, with a flag bit being set to
indicate that the slots are present and should be checked by the
interpreter.  (The flag bit does not indicate that the slot values are
non-<tt class="constant">NULL</tt>. The flag may be set to indicate the presence of a slot,
but a slot may still be unfilled.)

<P>
<div class="verbatim"><pre>
    PyNumberMethods   tp_as_number;
    PySequenceMethods tp_as_sequence;
    PyMappingMethods  tp_as_mapping;
</pre></div>

<P>
If you wish your object to be able to act like a number, a sequence,
or a mapping object, then you place the address of a structure that
implements the C type <tt class="ctype">PyNumberMethods</tt>,
<tt class="ctype">PySequenceMethods</tt>, or <tt class="ctype">PyMappingMethods</tt>, respectively.
It is up to you to fill in this structure with appropriate values. You
can find examples of the use of each of these in the <span class="file">Objects</span>
directory of the Python source distribution.

<P>
<div class="verbatim"><pre>
    hashfunc tp_hash;
</pre></div>

<P>
This function, if you choose to provide it, should return a hash
number for an instance of your data type. Here is a moderately
pointless example:

<P>
<div class="verbatim"><pre>
static long
newdatatype_hash(newdatatypeobject *obj)
{
    long result;
    result = obj-&gt;obj_UnderlyingDatatypePtr-&gt;size;
    result = result * 3;
    return result;
}
</pre></div>

<P>
<div class="verbatim"><pre>
    ternaryfunc tp_call;
</pre></div>

<P>
This function is called when an instance of your data type is "called",
for example, if <code>obj1</code> is an instance of your data type and the Python
script contains <code>obj1('hello')</code>, the <tt class="member">tp_call</tt> handler is
invoked.

<P>
This function takes three arguments:

<P>

<OL>
<LI><var>arg1</var> is the instance of the data type which is the subject of
    the call. If the call is <code>obj1('hello')</code>, then <var>arg1</var> is
    <code>obj1</code>.

<P>
</LI>
<LI><var>arg2</var> is a tuple containing the arguments to the call.  You
    can use <tt class="cfunction">PyArg_ParseTuple()</tt> to extract the arguments.

<P>
</LI>
<LI><var>arg3</var> is a dictionary of keyword arguments that were passed.
    If this is non-<tt class="constant">NULL</tt> and you support keyword arguments, use
    <tt class="cfunction">PyArg_ParseTupleAndKeywords()</tt> to extract the
    arguments.  If you do not want to support keyword arguments and
    this is non-<tt class="constant">NULL</tt>, raise a <tt class="exception">TypeError</tt> with a message
    saying that keyword arguments are not supported.
</LI>
</OL>

<P>
Here is a desultory example of the implementation of the call function.

<P>
<div class="verbatim"><pre>
/* Implement the call function.
 *    obj1 is the instance receiving the call.
 *    obj2 is a tuple containing the arguments to the call, in this
 *         case 3 strings.
 */
static PyObject *
newdatatype_call(newdatatypeobject *obj, PyObject *args, PyObject *other)
{
    PyObject *result;
    char *arg1;
    char *arg2;
    char *arg3;

    if (!PyArg_ParseTuple(args, "sss:call", &amp;arg1, &amp;arg2, &amp;arg3)) {
        return NULL;
    }
    result = PyString_FromFormat(
        "Returning -- value: [\%d] arg1: [\%s] arg2: [\%s] arg3: [\%s]\n",
        obj-&gt;obj_UnderlyingDatatypePtr-&gt;size,
        arg1, arg2, arg3);
    printf("\%s", PyString_AS_STRING(result));
    return result;
}
</pre></div>

<P>
XXX some fields need to be added here...

<P>
<div class="verbatim"><pre>
    /* Added in release 2.2 */
    /* Iterators */
    getiterfunc tp_iter;
    iternextfunc tp_iternext;
</pre></div>

<P>
These functions provide support for the iterator protocol.  Any object
which wishes to support iteration over its contents (which may be
generated during iteration) must implement the <code>tp_iter</code>
handler.  Objects which are returned by a <code>tp_iter</code> handler must
implement both the <code>tp_iter</code> and <code>tp_iternext</code> handlers.
Both handlers take exactly one parameter, the instance for which they
are being called, and return a new reference.  In the case of an
error, they should set an exception and return <tt class="constant">NULL</tt>.

<P>
For an object which represents an iterable collection, the
<code>tp_iter</code> handler must return an iterator object.  The iterator
object is responsible for maintaining the state of the iteration.  For
collections which can support multiple iterators which do not
interfere with each other (as lists and tuples do), a new iterator
should be created and returned.  Objects which can only be iterated
over once (usually due to side effects of iteration) should implement
this handler by returning a new reference to themselves, and should
also implement the <code>tp_iternext</code> handler.  File objects are an
example of such an iterator.

<P>
Iterator objects should implement both handlers.  The <code>tp_iter</code>
handler should return a new reference to the iterator (this is the
same as the <code>tp_iter</code> handler for objects which can only be
iterated over destructively).  The <code>tp_iternext</code> handler should
return a new reference to the next object in the iteration if there is
one.  If the iteration has reached the end, it may return <tt class="constant">NULL</tt>
without setting an exception or it may set <tt class="exception">StopIteration</tt>;
avoiding the exception can yield slightly better performance.  If an
actual error occurs, it should set an exception and return <tt class="constant">NULL</tt>.

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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