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  <div class="section" id="common-object-structures">
<span id="common-structs"></span><h1>Common Object Structures<a class="headerlink" href="#common-object-structures" title="Permalink to this headline">¶</a></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>
<p>All Python objects ultimately share a small number of fields at the beginning
of the object&#8217;s representation in memory.  These are represented by the
<a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject</span></tt></a> and <a class="reference internal" href="#PyVarObject" title="PyVarObject"><tt class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></tt></a> 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 class="type">
<dt id="PyObject">
<tt class="descname">PyObject</tt><a class="headerlink" href="#PyObject" title="Permalink to this definition">¶</a></dt>
<dd><p>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 &#8220;release&#8221; build, it contains only the object&#8217;s
reference count and a pointer to the corresponding type object.  It
corresponds to the fields defined by the expansion of the <tt class="docutils literal"><span class="pre">PyObject_HEAD</span></tt>
macro.</p>
</dd></dl>

<dl class="type">
<dt id="PyVarObject">
<tt class="descname">PyVarObject</tt><a class="headerlink" href="#PyVarObject" title="Permalink to this definition">¶</a></dt>
<dd><p>This is an extension of <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject</span></tt></a> that adds the <tt class="xref py py-attr docutils literal"><span class="pre">ob_size</span></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 <tt class="docutils literal"><span class="pre">PyObject_VAR_HEAD</span></tt> macro.</p>
</dd></dl>

<p>These macros are used in the definition of <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject</span></tt></a> and
<a class="reference internal" href="#PyVarObject" title="PyVarObject"><tt class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></tt></a>:</p>
<dl class="macro">
<dt id="PyObject_HEAD">
<tt class="descname">PyObject_HEAD</tt><a class="headerlink" href="#PyObject_HEAD" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to the declarations of the fields of the
<a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject</span></tt></a> 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 <tt class="xref c c-macro docutils literal"><span class="pre">Py_TRACE_REFS</span></tt>.  By default, that macro is
not defined, and <a class="reference internal" href="#PyObject_HEAD" title="PyObject_HEAD"><tt class="xref c c-macro docutils literal"><span class="pre">PyObject_HEAD</span></tt></a> expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">Py_ssize_t</span> <span class="n">ob_refcnt</span><span class="p">;</span>
<span class="n">PyTypeObject</span> <span class="o">*</span><span class="n">ob_type</span><span class="p">;</span>
</pre></div>
</div>
<p>When <tt class="xref c c-macro docutils literal"><span class="pre">Py_TRACE_REFS</span></tt> is defined, it expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject</span> <span class="o">*</span><span class="n">_ob_next</span><span class="p">,</span> <span class="o">*</span><span class="n">_ob_prev</span><span class="p">;</span>
<span class="n">Py_ssize_t</span> <span class="n">ob_refcnt</span><span class="p">;</span>
<span class="n">PyTypeObject</span> <span class="o">*</span><span class="n">ob_type</span><span class="p">;</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="PyObject_VAR_HEAD">
<tt class="descname">PyObject_VAR_HEAD</tt><a class="headerlink" href="#PyObject_VAR_HEAD" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to the declarations of the fields of the
<a class="reference internal" href="#PyVarObject" title="PyVarObject"><tt class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></tt></a> 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:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject_HEAD</span>
<span class="n">Py_ssize_t</span> <span class="n">ob_size</span><span class="p">;</span>
</pre></div>
</div>
<p>Note that <a class="reference internal" href="#PyObject_HEAD" title="PyObject_HEAD"><tt class="xref c c-macro docutils literal"><span class="pre">PyObject_HEAD</span></tt></a> is part of the expansion, and that its own
expansion varies depending on the definition of <tt class="xref c c-macro docutils literal"><span class="pre">Py_TRACE_REFS</span></tt>.</p>
</dd></dl>

<dl class="macro">
<dt id="PyObject_HEAD_INIT">
<tt class="descname">PyObject_HEAD_INIT</tt><big>(</big>type<big>)</big><a class="headerlink" href="#PyObject_HEAD_INIT" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to initialization values for a new
<a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject</span></tt></a> type.  This macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">_PyObject_EXTRA_INIT</span>
<span class="mi">1</span><span class="p">,</span> <span class="n">type</span><span class="p">,</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="PyVarObject_HEAD_INIT">
<tt class="descname">PyVarObject_HEAD_INIT</tt><big>(</big>type, size<big>)</big><a class="headerlink" href="#PyVarObject_HEAD_INIT" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to initialization values for a new
<a class="reference internal" href="#PyVarObject" title="PyVarObject"><tt class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></tt></a> type, including the <tt class="xref py py-attr docutils literal"><span class="pre">ob_size</span></tt> field.
This macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">_PyObject_EXTRA_INIT</span>
<span class="mi">1</span><span class="p">,</span> <span class="n">type</span><span class="p">,</span> <span class="n">size</span><span class="p">,</span>
</pre></div>
</div>
</dd></dl>

<dl class="type">
<dt id="PyCFunction">
<tt class="descname">PyCFunction</tt><a class="headerlink" href="#PyCFunction" title="Permalink to this definition">¶</a></dt>
<dd><p>Type of the functions used to implement most Python callables in C.
Functions of this type take two <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a> parameters and return
one such value.  If the return value is <em>NULL</em>, an exception shall have
been set.  If not <em>NULL</em>, the return value is interpreted as the return
value of the function as exposed in Python.  The function must return a new
reference.</p>
</dd></dl>

<dl class="type">
<dt id="PyCFunctionWithKeywords">
<tt class="descname">PyCFunctionWithKeywords</tt><a class="headerlink" href="#PyCFunctionWithKeywords" title="Permalink to this definition">¶</a></dt>
<dd><p>Type of the functions used to implement Python callables in C that take
keyword arguments: they take three <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a> parameters and return
one such value.  See <a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a> above for the meaning of the return
value.</p>
</dd></dl>

<dl class="type">
<dt id="PyMethodDef">
<tt class="descname">PyMethodDef</tt><a class="headerlink" href="#PyMethodDef" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure used to describe a method of an extension type.  This structure has
four fields:</p>
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="21%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Field</th>
<th class="head">C Type</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">ml_name</span></tt></td>
<td>char *</td>
<td>name of the method</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">ml_meth</span></tt></td>
<td>PyCFunction</td>
<td>pointer to the C
implementation</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">ml_flags</span></tt></td>
<td>int</td>
<td>flag bits indicating how the
call should be constructed</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">ml_doc</span></tt></td>
<td>char *</td>
<td>points to the contents of the
docstring</td>
</tr>
</tbody>
</table>
</dd></dl>

<p>The <tt class="xref py py-attr docutils literal"><span class="pre">ml_meth</span></tt> is a C function pointer.  The functions may be of different
types, but they always return <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a>.  If the function is not of
the <a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a>, the compiler will require a cast in the method table.
Even though <a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a> defines the first parameter as
<a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a>, it is common that the method implementation uses a the
specific C type of the <em>self</em> object.</p>
<p>The <tt class="xref py py-attr docutils literal"><span class="pre">ml_flags</span></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 <a class="reference internal" href="#METH_VARARGS" title="METH_VARARGS"><tt class="xref py py-const docutils literal"><span class="pre">METH_VARARGS</span></tt></a> and
<a class="reference internal" href="#METH_KEYWORDS" title="METH_KEYWORDS"><tt class="xref py py-const docutils literal"><span class="pre">METH_KEYWORDS</span></tt></a> can be combined (but note that <a class="reference internal" href="#METH_KEYWORDS" title="METH_KEYWORDS"><tt class="xref py py-const docutils literal"><span class="pre">METH_KEYWORDS</span></tt></a>
alone is equivalent to <tt class="docutils literal"><span class="pre">METH_VARARGS</span> <span class="pre">|</span> <span class="pre">METH_KEYWORDS</span></tt>). Any of the calling
convention flags can be combined with a binding flag.</p>
<dl class="data">
<dt id="METH_VARARGS">
<tt class="descname">METH_VARARGS</tt><a class="headerlink" href="#METH_VARARGS" title="Permalink to this definition">¶</a></dt>
<dd><p>This is the typical calling convention, where the methods have the type
<a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a>. The function expects two <a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a> values.
The first one is the <em>self</em> object for methods; for module functions, it is
the module object.  The second parameter (often called <em>args</em>) is a tuple
object representing all arguments. This parameter is typically processed
using <a class="reference internal" href="arg.html#PyArg_ParseTuple" title="PyArg_ParseTuple"><tt class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTuple()</span></tt></a> or <a class="reference internal" href="arg.html#PyArg_UnpackTuple" title="PyArg_UnpackTuple"><tt class="xref c c-func docutils literal"><span class="pre">PyArg_UnpackTuple()</span></tt></a>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_KEYWORDS">
<tt class="descname">METH_KEYWORDS</tt><a class="headerlink" href="#METH_KEYWORDS" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods with these flags must be of type <a class="reference internal" href="#PyCFunctionWithKeywords" title="PyCFunctionWithKeywords"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunctionWithKeywords</span></tt></a>.
The function expects three parameters: <em>self</em>, <em>args</em>, and a dictionary of
all the keyword arguments.  The flag is typically combined with
<a class="reference internal" href="#METH_VARARGS" title="METH_VARARGS"><tt class="xref py py-const docutils literal"><span class="pre">METH_VARARGS</span></tt></a>, and the parameters are typically processed using
<a class="reference internal" href="arg.html#PyArg_ParseTupleAndKeywords" title="PyArg_ParseTupleAndKeywords"><tt class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTupleAndKeywords()</span></tt></a>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_NOARGS">
<tt class="descname">METH_NOARGS</tt><a class="headerlink" href="#METH_NOARGS" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods without parameters don&#8217;t need to check whether arguments are given if
they are listed with the <a class="reference internal" href="#METH_NOARGS" title="METH_NOARGS"><tt class="xref py py-const docutils literal"><span class="pre">METH_NOARGS</span></tt></a> flag.  They need to be of type
<a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a>.  The first parameter is typically named <em>self</em> and will
hold a reference to the module or object instance.  In all cases the second
parameter will be <em>NULL</em>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_O">
<tt class="descname">METH_O</tt><a class="headerlink" href="#METH_O" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods with a single object argument can be listed with the <a class="reference internal" href="#METH_O" title="METH_O"><tt class="xref py py-const docutils literal"><span class="pre">METH_O</span></tt></a>
flag, instead of invoking <a class="reference internal" href="arg.html#PyArg_ParseTuple" title="PyArg_ParseTuple"><tt class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTuple()</span></tt></a> with a <tt class="docutils literal"><span class="pre">&quot;O&quot;</span></tt> argument.
They have the type <a class="reference internal" href="#PyCFunction" title="PyCFunction"><tt class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></tt></a>, with the <em>self</em> parameter, and a
<a class="reference internal" href="#PyObject" title="PyObject"><tt class="xref c c-type docutils literal"><span class="pre">PyObject*</span></tt></a> parameter representing the single argument.</p>
</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 class="data">
<dt id="METH_CLASS">
<tt class="descname">METH_CLASS</tt><a class="headerlink" href="#METH_CLASS" title="Permalink to this definition">¶</a></dt>
<dd><p id="index-0">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 <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><tt class="xref py py-func docutils literal"><span class="pre">classmethod()</span></tt></a> built-in
function.</p>
</dd></dl>

<dl class="data">
<dt id="METH_STATIC">
<tt class="descname">METH_STATIC</tt><a class="headerlink" href="#METH_STATIC" title="Permalink to this definition">¶</a></dt>
<dd><p id="index-1">The method will be passed <em>NULL</em> 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 <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><tt class="xref py py-func docutils literal"><span class="pre">staticmethod()</span></tt></a> built-in function.</p>
</dd></dl>

<p>One other constant controls whether a method is loaded in place of another
definition with the same method name.</p>
<dl class="data">
<dt id="METH_COEXIST">
<tt class="descname">METH_COEXIST</tt><a class="headerlink" href="#METH_COEXIST" title="Permalink to this definition">¶</a></dt>
<dd><p>The method will be loaded in place of existing definitions.  Without
<em>METH_COEXIST</em>, the default is to skip repeated definitions.  Since slot
wrappers are loaded before the method table, the existence of a
<em>sq_contains</em> slot, for example, would generate a wrapped method named
<a class="reference internal" href="../reference/datamodel.html#object.__contains__" title="object.__contains__"><tt class="xref py py-meth docutils literal"><span class="pre">__contains__()</span></tt></a> 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.</p>
</dd></dl>

<dl class="type">
<dt id="PyMemberDef">
<tt class="descname">PyMemberDef</tt><a class="headerlink" href="#PyMemberDef" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure which describes an attribute of a type which corresponds to a C
struct member.  Its fields are:</p>
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="21%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Field</th>
<th class="head">C Type</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">name</span></tt></td>
<td>char *</td>
<td>name of the member</td>
</tr>
<tr><td><a class="reference internal" href="../library/functions.html#type" title="type"><tt class="xref py py-attr docutils literal"><span class="pre">type</span></tt></a></td>
<td>int</td>
<td>the type of the member in the
C struct</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">offset</span></tt></td>
<td>Py_ssize_t</td>
<td>the offset in bytes that the
member is located on the
type&#8217;s object struct</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">flags</span></tt></td>
<td>int</td>
<td>flag bits indicating if the
field should be read-only or
writable</td>
</tr>
<tr><td><tt class="xref py py-attr docutils literal"><span class="pre">doc</span></tt></td>
<td>char *</td>
<td>points to the contents of the
docstring</td>
</tr>
</tbody>
</table>
<p><a class="reference internal" href="../library/functions.html#type" title="type"><tt class="xref py py-attr docutils literal"><span class="pre">type</span></tt></a> can be one of many <tt class="docutils literal"><span class="pre">T_</span></tt> macros corresponding to various C
types.  When the member is accessed in Python, it will be converted to the
equivalent Python type.</p>
<table border="1" class="docutils">
<colgroup>
<col width="45%" />
<col width="55%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Macro name</th>
<th class="head">C type</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>T_SHORT</td>
<td>short</td>
</tr>
<tr><td>T_INT</td>
<td>int</td>
</tr>
<tr><td>T_LONG</td>
<td>long</td>
</tr>
<tr><td>T_FLOAT</td>
<td>float</td>
</tr>
<tr><td>T_DOUBLE</td>
<td>double</td>
</tr>
<tr><td>T_STRING</td>
<td>char *</td>
</tr>
<tr><td>T_OBJECT</td>
<td>PyObject *</td>
</tr>
<tr><td>T_OBJECT_EX</td>
<td>PyObject *</td>
</tr>
<tr><td>T_CHAR</td>
<td>char</td>
</tr>
<tr><td>T_BYTE</td>
<td>char</td>
</tr>
<tr><td>T_UBYTE</td>
<td>unsigned char</td>
</tr>
<tr><td>T_UINT</td>
<td>unsigned int</td>
</tr>
<tr><td>T_USHORT</td>
<td>unsigned short</td>
</tr>
<tr><td>T_ULONG</td>
<td>unsigned long</td>
</tr>
<tr><td>T_BOOL</td>
<td>char</td>
</tr>
<tr><td>T_LONGLONG</td>
<td>long long</td>
</tr>
<tr><td>T_ULONGLONG</td>
<td>unsigned long long</td>
</tr>
<tr><td>T_PYSSIZET</td>
<td>Py_ssize_t</td>
</tr>
</tbody>
</table>
<p><tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></tt> and <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></tt> differ in that
<tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></tt> returns <tt class="xref docutils literal"><span class="pre">None</span></tt> if the member is <em>NULL</em> and
<tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></tt> raises an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><tt class="xref py py-exc docutils literal"><span class="pre">AttributeError</span></tt></a>.  Try to use
<tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></tt> over <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></tt> because <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></tt>
handles use of the <a class="reference internal" href="../reference/simple_stmts.html#del"><tt class="xref std std-keyword docutils literal"><span class="pre">del</span></tt></a> statement on that attribute more correctly
than <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></tt>.</p>
<p><tt class="xref py py-attr docutils literal"><span class="pre">flags</span></tt> can be 0 for write and read access or <tt class="xref c c-macro docutils literal"><span class="pre">READONLY</span></tt> for
read-only access.  Using <tt class="xref c c-macro docutils literal"><span class="pre">T_STRING</span></tt> for <a class="reference internal" href="../library/functions.html#type" title="type"><tt class="xref py py-attr docutils literal"><span class="pre">type</span></tt></a> implies
<tt class="xref c c-macro docutils literal"><span class="pre">READONLY</span></tt>.  Only <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></tt> and <tt class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></tt>
members can be deleted.  (They are set to <em>NULL</em>).</p>
</dd></dl>

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