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<H1><A NAME="SECTION0012200000000000000000"></A><A NAME="common-structs"></A>
<BR>
10.2 Common Object Structures
</H1>

<P>
There are a large number of structures which are used in the
definition of object types for Python.  This section describes these
structures and how they are used.

<P>
All Python objects ultimately share a small number of fields at the
beginning of the object's representation in memory.  These are
represented by the <tt class="ctype">PyObject</tt> and <tt class="ctype">PyVarObject</tt> types,
which are defined, in turn, by the expansions of some macros also
used, whether directly or indirectly, in the definition of all other
Python objects.

<P>
<dl><dt><b><tt class="ctype"><a id='l2h-920' xml:id='l2h-920'>PyObject</a></tt></b></dt>
<dd>
  All object types are extensions of this type.  This is a type which
  contains the information Python needs to treat a pointer to an
  object as an object.  In a normal ``release'' build, it contains
  only the objects reference count and a pointer to the corresponding
  type object.  It corresponds to the fields defined by the
  expansion of the <code>PyObject_HEAD</code> macro.
</dl>

<P>
<dl><dt><b><tt class="ctype"><a id='l2h-921' xml:id='l2h-921'>PyVarObject</a></tt></b></dt>
<dd>
  This is an extension of <tt class="ctype">PyObject</tt> that adds the
  <tt class="member">ob_size</tt> field.  This is only used for objects that have
  some notion of <em>length</em>.  This type does not often appear in
  the Python/C API.  It corresponds to the fields defined by the
  expansion of the <code>PyObject_VAR_HEAD</code> macro.
</dl>

<P>
These macros are used in the definition of <tt class="ctype">PyObject</tt> and
<tt class="ctype">PyVarObject</tt>:

<P>
<dl><dt><b><tt id='l2h-922' xml:id='l2h-922' class="macro">PyObject_HEAD</tt></b></dt>
<dd>
  This is a macro which expands to the declarations of the fields of
  the <tt class="ctype">PyObject</tt> type; it is used when declaring new types which
  represent objects without a varying length.  The specific fields it
  expands to depend on the definition of
  Py_TRACE_REFS.  By default, that macro is not
  defined, and PyObject_HEAD expands to:
  <div class="verbatim"><pre>
    int ob_refcnt;
    PyTypeObject *ob_type;
</pre></div>
  When Py_TRACE_REFS is defined, it expands to:
  <div class="verbatim"><pre>
    PyObject *_ob_next, *_ob_prev;
    int ob_refcnt;
    PyTypeObject *ob_type;
</pre></div>
</dl>

<P>
<dl><dt><b><tt id='l2h-923' xml:id='l2h-923' class="macro">PyObject_VAR_HEAD</tt></b></dt>
<dd>
  This is a macro which expands to the declarations of the fields of
  the <tt class="ctype">PyVarObject</tt> type; it is used when declaring new types which
  represent objects with a length that varies from instance to
  instance.  This macro always expands to:
  <div class="verbatim"><pre>
    PyObject_HEAD
    int ob_size;
</pre></div>
  Note that PyObject_HEAD is part of the expansion, and
  that it's own expansion varies depending on the definition of
  Py_TRACE_REFS.
</dl>

<P>
PyObject_HEAD_INIT

<P>
<dl><dt><b><tt class="ctype"><a id='l2h-924' xml:id='l2h-924'>PyCFunction</a></tt></b></dt>
<dd>
  Type of the functions used to implement most Python callables in C.
  Functions of this type take two <tt class="ctype">PyObject*</tt> parameters and
  return one such value.  If the return value is <tt class="constant">NULL</tt>, an exception
  shall have been set.  If not <tt class="constant">NULL</tt>, the return value is interpreted
  as the return value of the function as exposed in Python.  The
  function must return a new reference.
</dl>

<P>
<dl><dt><b><tt class="ctype"><a id='l2h-925' xml:id='l2h-925'>PyMethodDef</a></tt></b></dt>
<dd>
  Structure used to describe a method of an extension type.  This
  structure has four fields:

<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="left"  >Field</th>
      <th class="left"  >C Type</th>
      <th class="left"  >Meaning</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="left"   valign="baseline"><tt class="member">ml_name</tt></td>
        <td class="left"  >char *</td>
        <td class="left"  >name of the method</td></tr>
    <tr><td class="left"   valign="baseline"><tt class="member">ml_meth</tt></td>
        <td class="left"  >PyCFunction</td>
        <td class="left"  >pointer to the C implementation</td></tr>
    <tr><td class="left"   valign="baseline"><tt class="member">ml_flags</tt></td>
        <td class="left"  >int</td>
        <td class="left"  >flag bits indicating how the call should be
                            constructed</td></tr>
    <tr><td class="left"   valign="baseline"><tt class="member">ml_doc</tt></td>
        <td class="left"  >char *</td>
        <td class="left"  >points to the contents of the docstring</td></tr></tbody>
</table></div>
</dl>

<P>
The <tt class="member">ml_meth</tt> is a C function pointer.  The functions may be of
different types, but they always return <tt class="ctype">PyObject*</tt>.  If the
function is not of the <tt class="ctype">PyCFunction</tt>, the compiler will require
a cast in the method table.  Even though <tt class="ctype">PyCFunction</tt> defines
the first parameter as <tt class="ctype">PyObject*</tt>, it is common that the method
implementation uses a the specific C type of the <var>self</var> object.

<P>
The <tt class="member">ml_flags</tt> field is a bitfield which can include the
following flags.  The individual flags indicate either a calling
convention or a binding convention.  Of the calling convention flags,
only <tt class="constant">METH_VARARGS</tt> and <tt class="constant">METH_KEYWORDS</tt> can be
combined (but note that <tt class="constant">METH_KEYWORDS</tt> alone is equivalent
to <code><tt class="constant">METH_VARARGS</tt> | <tt class="constant">METH_KEYWORDS</tt></code>).
Any of the calling convention flags can be combined with a
binding flag.

<P>
<dl><dt><b><tt id='l2h-926' xml:id='l2h-926'>METH_VARARGS</tt></b></dt>
<dd>
  This is the typical calling convention, where the methods have the
  type <tt class="ctype">PyCFunction</tt>. The function expects two
  <tt class="ctype">PyObject*</tt> values.  The first one is the <var>self</var> object for
  methods; for module functions, it has the value given to
  <tt class="cfunction">Py_InitModule4()</tt> (or <tt class="constant">NULL</tt> if
  <tt class="cfunction">Py_InitModule()</tt> was used).  The second parameter
  (often called <var>args</var>) is a tuple object representing all
  arguments. This parameter is typically processed using
  <tt class="cfunction">PyArg_ParseTuple()</tt> or <tt class="cfunction">PyArg_UnpackTuple</tt>.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-927' xml:id='l2h-927'>METH_KEYWORDS</tt></b></dt>
<dd>
  Methods with these flags must be of type
  <tt class="ctype">PyCFunctionWithKeywords</tt>.  The function expects three
  parameters: <var>self</var>, <var>args</var>, and a dictionary of all the
  keyword arguments.  The flag is typically combined with
  <tt class="constant">METH_VARARGS</tt>, and the parameters are typically processed
  using <tt class="cfunction">PyArg_ParseTupleAndKeywords()</tt>.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-928' xml:id='l2h-928'>METH_NOARGS</tt></b></dt>
<dd>
  Methods without parameters don't need to check whether arguments are
  given if they are listed with the <tt class="constant">METH_NOARGS</tt> flag.  They
  need to be of type <tt class="ctype">PyCFunction</tt>.  When used with object
  methods, the first parameter is typically named <code>self</code> and will
  hold a reference to the object instance.  In all cases the second
  parameter will be <tt class="constant">NULL</tt>.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-929' xml:id='l2h-929'>METH_O</tt></b></dt>
<dd>
  Methods with a single object argument can be listed with the
  <tt class="constant">METH_O</tt> flag, instead of invoking
  <tt class="cfunction">PyArg_ParseTuple()</tt> with a <code>"O"</code> argument. They have
  the type <tt class="ctype">PyCFunction</tt>, with the <var>self</var> parameter, and a
  <tt class="ctype">PyObject*</tt> parameter representing the single argument.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-930' xml:id='l2h-930'>METH_OLDARGS</tt></b></dt>
<dd>
  This calling convention is deprecated.  The method must be of type
  <tt class="ctype">PyCFunction</tt>.  The second argument is <tt class="constant">NULL</tt> if no arguments
  are given, a single object if exactly one argument is given, and a
  tuple of objects if more than one argument is given.  There is no
  way for a function using this convention to distinguish between a
  call with multiple arguments and a call with a tuple as the only
  argument.
</dd></dl>

<P>
These two constants are not used to indicate the calling convention
but the binding when use with methods of classes.  These may not be
used for functions defined for modules.  At most one of these flags
may be set for any given method.

<P>
<dl><dt><b><tt id='l2h-931' xml:id='l2h-931'>METH_CLASS</tt></b></dt>
<dd>
  The method will be passed the type object as the first parameter
  rather than an instance of the type.  This is used to create
  <em>class methods</em>, similar to what is created when using the
  <tt class="function">classmethod()</tt><a id='l2h-932' xml:id='l2h-932'></a> built-in
  function.

<span class="versionnote">New in version 2.3.</span>

</dd></dl>

<P>
<dl><dt><b><tt id='l2h-933' xml:id='l2h-933'>METH_STATIC</tt></b></dt>
<dd>
  The method will be passed <tt class="constant">NULL</tt> as the first parameter rather than
  an instance of the type.  This is used to create <em>static
  methods</em>, similar to what is created when using the
  <tt class="function">staticmethod()</tt><a id='l2h-934' xml:id='l2h-934'></a> built-in
  function.

<span class="versionnote">New in version 2.3.</span>

</dd></dl>

<P>
One other constant controls whether a method is loaded in place of
another definition with the same method name.

<P>
<dl><dt><b><tt id='l2h-935' xml:id='l2h-935'>METH_COEXIST</tt></b></dt>
<dd>
  The method will be loaded in place of existing definitions.  Without
  <var>METH_COEXIST</var>, the default is to skip repeated definitions.  Since
  slot wrappers are loaded before the method table, the existence of a
  <var>sq_contains</var> slot, for example, would generate a wrapped method
  named <tt class="method">__contains__()</tt> and preclude the loading of a
  corresponding PyCFunction with the same name.  With the flag defined,
  the PyCFunction will be loaded in place of the wrapper object and will
  co-exist with the slot.  This is helpful because calls to PyCFunctions
  are optimized more than wrapper object calls.

<span class="versionnote">New in version 2.4.</span>

</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-936' xml:id='l2h-936' class="cfunction">Py_FindMethod</tt></b>(</nobr></td><td>PyMethodDef table[],
                                            PyObject *<var>ob</var>, char *<var>name</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  Return a bound method object for an extension type implemented in
  C.  This can be useful in the implementation of a
  <tt class="member">tp_getattro</tt> or <tt class="member">tp_getattr</tt> handler that does not
  use the <tt class="cfunction">PyObject_GenericGetAttr()</tt> function.
</dd></dl>

<P>

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