2. Defining Extension Types: Tutorial
Python allows the writer of a C extension module to define new types that
can be manipulated from Python code, much like the built-in str
and list types. The code for all extension types follows a
pattern, but there are some details that you need to understand before you
can get started. This document is a gentle introduction to the topic.
2.1. The Basics
The CPython runtime sees all Python objects as variables of type
PyObject*, which serves as a “base type” for all Python objects.
The PyObject structure itself only contains the object’s
reference count and a pointer to the object’s “type object”.
This is where the action is; the type object determines which (C) functions
get called by the interpreter when, for instance, an attribute gets looked up
on an object, a method called, or it is multiplied by another object. These
C functions are called “type methods”.
So, if you want to define a new extension type, you need to create a new type object.
This sort of thing can only be explained by example, so here’s a minimal, but
complete, module that defines a new type named Custom inside a C
extension module custom:
Note
What we’re showing here is the traditional way of defining static
extension types. It should be adequate for most uses. The C API also
allows defining heap-allocated extension types using the
PyType_FromSpec() function, which isn’t covered in this tutorial.
#define PY_SSIZE_T_CLEAN
#include <Python.h>
typedef struct {
PyObject_HEAD
/* Type-specific fields go here. */
} CustomObject;
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom.Custom",
.tp_doc = "Custom objects",
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT,
.tp_new = PyType_GenericNew,
};
static PyModuleDef custommodule = {
PyModuleDef_HEAD_INIT,
.m_name = "custom",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
Now that’s quite a bit to take in at once, but hopefully bits will seem familiar from the previous chapter. This file defines three things:
- What a
Customobject contains: this is theCustomObjectstruct, which is allocated once for eachCustominstance. - How the
Customtype behaves: this is theCustomTypestruct, which defines a set of flags and function pointers that the interpreter inspects when specific operations are requested. - How to initialize the
custommodule: this is thePyInit_customfunction and the associatedcustommodulestruct.
The first bit is:
typedef struct {
PyObject_HEAD
} CustomObject;
This is what a Custom object will contain. PyObject_HEAD is mandatory
at the start of each object struct and defines a field called ob_base
of type PyObject, containing a pointer to a type object and a
reference count (these can be accessed using the macros Py_REFCNT
and Py_TYPE respectively). The reason for the macro is to
abstract away the layout and to enable additional fields in debug builds.
Note
There is no semicolon above after the PyObject_HEAD macro.
Be wary of adding one by accident: some compilers will complain.
Of course, objects generally store additional data besides the standard
PyObject_HEAD boilerplate; for example, here is the definition for
standard Python floats:
typedef struct {
PyObject_HEAD
double ob_fval;
} PyFloatObject;
The second bit is the definition of the type object.
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom.Custom",
.tp_doc = "Custom objects",
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT,
.tp_new = PyType_GenericNew,
};
Note
We recommend using C99-style designated initializers as above, to
avoid listing all the PyTypeObject fields that you don’t care
about and also to avoid caring about the fields’ declaration order.
The actual definition of PyTypeObject in object.h has
many more fields than the definition above. The
remaining fields will be filled with zeros by the C compiler, and it’s
common practice to not specify them explicitly unless you need them.
We’re going to pick it apart, one field at a time:
PyVarObject_HEAD_INIT(NULL, 0)
This line is mandatory boilerplate to initialize the ob_base
field mentioned above.
.tp_name = "custom.Custom",
The name of our type. This will appear in the default textual representation of our objects and in some error messages, for example:
>>> "" + custom.Custom()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "custom.Custom") to str
Note that the name is a dotted name that includes both the module name and the
name of the type within the module. The module in this case is custom and
the type is Custom, so we set the type name to custom.Custom.
Using the real dotted import path is important to make your type compatible
with the pydoc and pickle modules.
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
This is so that Python knows how much memory to allocate when creating
new Custom instances. tp_itemsize is
only used for variable-sized objects and should otherwise be zero.
Note
If you want your type to be subclassable from Python, and your type has the same
tp_basicsize as its base type, you may have problems with multiple
inheritance. A Python subclass of your type will have to list your type first
in its __bases__, or else it will not be able to call your type’s
__new__() method without getting an error. You can avoid this problem by
ensuring that your type has a larger value for tp_basicsize than its
base type does. Most of the time, this will be true anyway, because either your
base type will be object, or else you will be adding data members to
your base type, and therefore increasing its size.
We set the class flags to Py_TPFLAGS_DEFAULT.
.tp_flags = Py_TPFLAGS_DEFAULT,
All types should include this constant in their flags. It enables all of the members defined until at least Python 3.3. If you need further members, you will need to OR the corresponding flags.
We provide a doc string for the type in tp_doc.
.tp_doc = "Custom objects",
To enable object creation, we have to provide a tp_new
handler. This is the equivalent of the Python method __new__(), but
has to be specified explicitly. In this case, we can just use the default
implementation provided by the API function PyType_GenericNew().
.tp_new = PyType_GenericNew,
Everything else in the file should be familiar, except for some code in
PyInit_custom():
if (PyType_Ready(&CustomType) < 0)
return;
This initializes the Custom type, filling in a number of members
to the appropriate default values, including ob_type that we initially
set to NULL.
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
This adds the type to the module dictionary. This allows us to create
Custom instances by calling the Custom class:
>>> import custom
>>> mycustom = custom.Custom()
That’s it! All that remains is to build it; put the above code in a file called
custom.c and:
from distutils.core import setup, Extension
setup(name="custom", version="1.0",
ext_modules=[Extension("custom", ["custom.c"])])
in a file called setup.py; then typing
$ python setup.py build
at a shell should produce a file custom.so in a subdirectory; move to
that directory and fire up Python — you should be able to import custom and
play around with Custom objects.
That wasn’t so hard, was it?
Of course, the current Custom type is pretty uninteresting. It has no data and doesn’t do anything. It can’t even be subclassed.
Note
While this documentation showcases the standard distutils module
for building C extensions, it is recommended in real-world use cases to
use the newer and better-maintained setuptools library. Documentation
on how to do this is out of scope for this document and can be found in
the Python Packaging User’s Guide.
2.2. Adding data and methods to the Basic example
Let’s extend the basic example to add some data and methods. Let’s also make
the type usable as a base class. We’ll create a new module, custom2 that
adds these capabilities:
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_XDECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_XDECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
"last name"},
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom2.Custom",
.tp_doc = "Custom objects",
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
.tp_new = Custom_new,
.tp_init = (initproc) Custom_init,
.tp_dealloc = (destructor) Custom_dealloc,
.tp_members = Custom_members,
.tp_methods = Custom_methods,
};
static PyModuleDef custommodule = {
PyModuleDef_HEAD_INIT,
.m_name = "custom2",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom2(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
This version of the module has a number of changes.
We’ve added an extra include:
#include <structmember.h>
This include provides declarations that we use to handle attributes, as described a bit later.
The Custom type now has three data attributes in its C struct,
first, last, and number. The first and last variables are Python
strings containing first and last names. The number attribute is a C integer.
The object structure is updated accordingly:
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
Because we now have data to manage, we have to be more careful about object allocation and deallocation. At a minimum, we need a deallocation method:
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
which is assigned to the tp_dealloc member:
.tp_dealloc = (destructor) Custom_dealloc,
This method first clears the reference counts of the two Python attributes.
Py_XDECREF() correctly handles the case where its argument is
NULL (which might happen here if tp_new failed midway). It then
calls the tp_free member of the object’s type
(computed by Py_TYPE(self)) to free the object’s memory. Note that
the object’s type might not be CustomType, because the object may
be an instance of a subclass.
Note
The explicit cast to destructor above is needed because we defined
Custom_dealloc to take a CustomObject * argument, but the tp_dealloc
function pointer expects to receive a PyObject * argument. Otherwise,
the compiler will emit a warning. This is object-oriented polymorphism,
in C!
We want to make sure that the first and last names are initialized to empty
strings, so we provide a tp_new implementation:
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
and install it in the tp_new member:
.tp_new = Custom_new,
The tp_new handler is responsible for creating (as opposed to initializing)
objects of the type. It is exposed in Python as the __new__() method.
It is not required to define a tp_new member, and indeed many extension
types will simply reuse PyType_GenericNew() as done in the first
version of the Custom type above. In this case, we use the tp_new
handler to initialize the first and last attributes to non-NULL
default values.
tp_new is passed the type being instantiated (not necessarily CustomType,
if a subclass is instantiated) and any arguments passed when the type was
called, and is expected to return the instance created. tp_new handlers
always accept positional and keyword arguments, but they often ignore the
arguments, leaving the argument handling to initializer (a.k.a. tp_init
in C or __init__ in Python) methods.
Note
tp_new shouldn’t call tp_init explicitly, as the interpreter
will do it itself.
The tp_new implementation calls the tp_alloc
slot to allocate memory:
self = (CustomObject *) type->tp_alloc(type, 0);
Since memory allocation may fail, we must check the tp_alloc
result against NULL before proceeding.
Note
We didn’t fill the tp_alloc slot ourselves. Rather
PyType_Ready() fills it for us by inheriting it from our base class,
which is object by default. Most types use the default allocation
strategy.
Note
If you are creating a co-operative tp_new (one
that calls a base type’s tp_new or __new__()),
you must not try to determine what method to call using method resolution
order at runtime. Always statically determine what type you are going to
call, and call its tp_new directly, or via
type->tp_base->tp_new. If you do not do this, Python subclasses of your
type that also inherit from other Python-defined classes may not work correctly.
(Specifically, you may not be able to create instances of such subclasses
without getting a TypeError.)
We also define an initialization function which accepts arguments to provide initial values for our instance:
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_XDECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_XDECREF(tmp);
}
return 0;
}
by filling the tp_init slot.
.tp_init = (initproc) Custom_init,
The tp_init slot is exposed in Python as the
__init__() method. It is used to initialize an object after it’s
created. Initializers always accept positional and keyword arguments,
and they should return either 0 on success or -1 on error.
Unlike the tp_new handler, there is no guarantee that tp_init
is called at all (for example, the pickle module by default
doesn’t call __init__() on unpickled instances). It can also be
called multiple times. Anyone can call the __init__() method on
our objects. For this reason, we have to be extra careful when assigning
the new attribute values. We might be tempted, for example to assign the
first member like this:
if (first) {
Py_XDECREF(self->first);
Py_INCREF(first);
self->first = first;
}
But this would be risky. Our type doesn’t restrict the type of the
first member, so it could be any kind of object. It could have a
destructor that causes code to be executed that tries to access the
first member; or that destructor could release the
Global interpreter Lock and let arbitrary code run in other
threads that accesses and modifies our object.
To be paranoid and protect ourselves against this possibility, we almost always reassign members before decrementing their reference counts. When don’t we have to do this?
- when we absolutely know that the reference count is greater than 1;
- when we know that deallocation of the object [1] will neither release the GIL nor cause any calls back into our type’s code;
- when decrementing a reference count in a
tp_deallochandler on a type which doesn’t support cyclic garbage collection [2].
We want to expose our instance variables as attributes. There are a number of ways to do that. The simplest way is to define member definitions:
static PyMemberDef Custom_members[] = {
{"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
"last name"},
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
and put the definitions in the tp_members slot:
.tp_members = Custom_members,
Each member definition has a member name, type, offset, access flags and documentation string. See the Generic Attribute Management section below for details.
A disadvantage of this approach is that it doesn’t provide a way to restrict the
types of objects that can be assigned to the Python attributes. We expect the
first and last names to be strings, but any Python objects can be assigned.
Further, the attributes can be deleted, setting the C pointers to NULL. Even
though we can make sure the members are initialized to non-NULL values, the
members can be set to NULL if the attributes are deleted.
We define a single method, Custom.name(), that outputs the objects name as the
concatenation of the first and last names.
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
The method is implemented as a C function that takes a Custom (or
Custom subclass) instance as the first argument. Methods always take an
instance as the first argument. Methods often take positional and keyword
arguments as well, but in this case we don’t take any and don’t need to accept
a positional argument tuple or keyword argument dictionary. This method is
equivalent to the Python method:
def name(self):
return "%s %s" % (self.first, self.last)
Note that we have to check for the possibility that our first and
last members are NULL. This is because they can be deleted, in which
case they are set to NULL. It would be better to prevent deletion of these
attributes and to restrict the attribute values to be strings. We’ll see how to
do that in the next section.
Now that we’ve defined the method, we need to create an array of method definitions:
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
(note that we used the METH_NOARGS flag to indicate that the method
is expecting no arguments other than self)
and assign it to the tp_methods slot:
.tp_methods = Custom_methods,
Finally, we’ll make our type usable as a base class for subclassing. We’ve
written our methods carefully so far so that they don’t make any assumptions
about the type of the object being created or used, so all we need to do is
to add the Py_TPFLAGS_BASETYPE to our class flag definition:
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
We rename PyInit_custom() to PyInit_custom2(), update the
module name in the PyModuleDef struct, and update the full class
name in the PyTypeObject struct.
Finally, we update our setup.py file to build the new module:
from distutils.core import setup, Extension
setup(name="custom", version="1.0",
ext_modules=[
Extension("custom", ["custom.c"]),
Extension("custom2", ["custom2.c"]),
])
2.3. Providing finer control over data attributes
In this section, we’ll provide finer control over how the first and
last attributes are set in the Custom example. In the previous
version of our module, the instance variables first and last
could be set to non-string values or even deleted. We want to make sure that
these attributes always contain strings.
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
tmp = self->first;
Py_INCREF(value);
self->first = value;
Py_DECREF(tmp);
return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
Py_INCREF(self->last);
return self->last;
}
static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The last attribute value must be a string");
return -1;
}
tmp = self->last;
Py_INCREF(value);
self->last = value;
Py_DECREF(tmp);
return 0;
}
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom3.Custom",
.tp_doc = "Custom objects",
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
.tp_new = Custom_new,
.tp_init = (initproc) Custom_init,
.tp_dealloc = (destructor) Custom_dealloc,
.tp_members = Custom_members,
.tp_methods = Custom_methods,
.tp_getset = Custom_getsetters,
};
static PyModuleDef custommodule = {
PyModuleDef_HEAD_INIT,
.m_name = "custom3",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom3(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
To provide greater control, over the first and last attributes,
we’ll use custom getter and setter functions. Here are the functions for
getting and setting the first attribute:
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
tmp = self->first;
Py_INCREF(value);
self->first = value;
Py_DECREF(tmp);
return 0;
}
The getter function is passed a Custom object and a “closure”, which is
a void pointer. In this case, the closure is ignored. (The closure supports an
advanced usage in which definition data is passed to the getter and setter. This
could, for example, be used to allow a single set of getter and setter functions
that decide the attribute to get or set based on data in the closure.)
The setter function is passed the Custom object, the new value, and the
closure. The new value may be NULL, in which case the attribute is being
deleted. In our setter, we raise an error if the attribute is deleted or if its
new value is not a string.
We create an array of PyGetSetDef structures:
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
and register it in the tp_getset slot:
.tp_getset = Custom_getsetters,
The last item in a PyGetSetDef structure is the “closure” mentioned
above. In this case, we aren’t using a closure, so we just pass NULL.
We also remove the member definitions for these attributes:
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
We also need to update the tp_init handler to only
allow strings [3] to be passed:
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
With these changes, we can assure that the first and last members are
never NULL so we can remove checks for NULL values in almost all cases.
This means that most of the Py_XDECREF() calls can be converted to
Py_DECREF() calls. The only place we can’t change these calls is in
the tp_dealloc implementation, where there is the possibility that the
initialization of these members failed in tp_new.
We also rename the module initialization function and module name in the
initialization function, as we did before, and we add an extra definition to the
setup.py file.
2.4. Supporting cyclic garbage collection
Python has a cyclic garbage collector (GC) that can identify unneeded objects even when their reference counts are not zero. This can happen when objects are involved in cycles. For example, consider:
>>> l = []
>>> l.append(l)
>>> del l
In this example, we create a list that contains itself. When we delete it, it still has a reference from itself. Its reference count doesn’t drop to zero. Fortunately, Python’s cyclic garbage collector will eventually figure out that the list is garbage and free it.
In the second version of the Custom example, we allowed any kind of
object to be stored in the first or last attributes [4].
Besides, in the second and third versions, we allowed subclassing
Custom, and subclasses may add arbitrary attributes. For any of
those two reasons, Custom objects can participate in cycles:
>>> import custom3
>>> class Derived(custom3.Custom): pass
...
>>> n = Derived()
>>> n.some_attribute = n
To allow a Custom instance participating in a reference cycle to
be properly detected and collected by the cyclic GC, our Custom type
needs to fill two additional slots and to enable a flag that enables these slots:
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
Py_VISIT(self->first);
Py_VISIT(self->last);
return 0;
}
static int
Custom_clear(CustomObject *self)
{
Py_CLEAR(self->first);
Py_CLEAR(self->last);
return 0;
}
static void
Custom_dealloc(CustomObject *self)
{
PyObject_GC_UnTrack(self);
Custom_clear(self);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
Py_INCREF(value);
Py_CLEAR(self->first);
self->first = value;
return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
Py_INCREF(self->last);
return self->last;
}
static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The last attribute value must be a string");
return -1;
}
Py_INCREF(value);
Py_CLEAR(self->last);
self->last = value;
return 0;
}
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom4.Custom",
.tp_doc = "Custom objects",
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
.tp_new = Custom_new,
.tp_init = (initproc) Custom_init,
.tp_dealloc = (destructor) Custom_dealloc,
.tp_traverse = (traverseproc) Custom_traverse,
.tp_clear = (inquiry) Custom_clear,
.tp_members = Custom_members,
.tp_methods = Custom_methods,
.tp_getset = Custom_getsetters,
};
static PyModuleDef custommodule = {
PyModuleDef_HEAD_INIT,
.m_name = "custom4",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom4(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
First, the traversal method lets the cyclic GC know about subobjects that could participate in cycles:
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
int vret;
if (self->first) {
vret = visit(self->first, arg);
if (vret != 0)
return vret;
}
if (self->last) {
vret = visit(self->last, arg);
if (vret != 0)
return vret;
}
return 0;
}
For each subobject that can participate in cycles, we need to call the
visit() function, which is passed to the traversal method. The
visit() function takes as arguments the subobject and the extra argument
arg passed to the traversal method. It returns an integer value that must be
returned if it is non-zero.
Python provides a Py_VISIT() macro that automates calling visit
functions. With Py_VISIT(), we can minimize the amount of boilerplate
in Custom_traverse:
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
Py_VISIT(self->first);
Py_VISIT(self->last);
return 0;
}
Note
The tp_traverse implementation must name its
arguments exactly visit and arg in order to use Py_VISIT().
Second, we need to provide a method for clearing any subobjects that can participate in cycles:
static int
Custom_clear(CustomObject *self)
{
Py_CLEAR(self->first);
Py_CLEAR(self->last);
return 0;
}
Notice the use of the Py_CLEAR() macro. It is the recommended and safe
way to clear data attributes of arbitrary types while decrementing
their reference counts. If you were to call Py_XDECREF() instead
on the attribute before setting it to NULL, there is a possibility
that the attribute’s destructor would call back into code that reads the
attribute again (especially if there is a reference cycle).
Note
You could emulate Py_CLEAR() by writing:
PyObject *tmp;
tmp = self->first;
self->first = NULL;
Py_XDECREF(tmp);
Nevertheless, it is much easier and less error-prone to always
use Py_CLEAR() when deleting an attribute. Don’t
try to micro-optimize at the expense of robustness!
The deallocator Custom_dealloc may call arbitrary code when clearing
attributes. It means the circular GC can be triggered inside the function.
Since the GC assumes reference count is not zero, we need to untrack the object
from the GC by calling PyObject_GC_UnTrack() before clearing members.
Here is our reimplemented deallocator using PyObject_GC_UnTrack()
and Custom_clear:
static void
Custom_dealloc(CustomObject *self)
{
PyObject_GC_UnTrack(self);
Custom_clear(self);
Py_TYPE(self)->tp_free((PyObject *) self);
}
Finally, we add the Py_TPFLAGS_HAVE_GC flag to the class flags:
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
That’s pretty much it. If we had written custom tp_alloc or
tp_free handlers, we’d need to modify them for cyclic
garbage collection. Most extensions will use the versions automatically provided.
2.5. Subclassing other types
It is possible to create new extension types that are derived from existing
types. It is easiest to inherit from the built in types, since an extension can
easily use the PyTypeObject it needs. It can be difficult to share
these PyTypeObject structures between extension modules.
In this example we will create a SubList type that inherits from the
built-in list type. The new type will be completely compatible with
regular lists, but will have an additional increment() method that
increases an internal counter:
>>> import sublist
>>> s = sublist.SubList(range(3))
>>> s.extend(s)
>>> print(len(s))
6
>>> print(s.increment())
1
>>> print(s.increment())
2
#define PY_SSIZE_T_CLEAN
#include <Python.h>
typedef struct {
PyListObject list;
int state;
} SubListObject;
static PyObject *
SubList_increment(SubListObject *self, PyObject *unused)
{
self->state++;
return PyLong_FromLong(self->state);
}
static PyMethodDef SubList_methods[] = {
{"increment", (PyCFunction) SubList_increment, METH_NOARGS,
PyDoc_STR("increment state counter")},
{NULL},
};
static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
static PyTypeObject SubListType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "sublist.SubList",
.tp_doc = "SubList objects",
.tp_basicsize = sizeof(SubListObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
.tp_init = (initproc) SubList_init,
.tp_methods = SubList_methods,
};
static PyModuleDef sublistmodule = {
PyModuleDef_HEAD_INIT,
.m_name = "sublist",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_sublist(void)
{
PyObject *m;
SubListType.tp_base = &PyList_Type;
if (PyType_Ready(&SubListType) < 0)
return NULL;
m = PyModule_Create(&sublistmodule);
if (m == NULL)
return NULL;
Py_INCREF(&SubListType);
if (PyModule_AddObject(m, "SubList", (PyObject *) &SubListType) < 0) {
Py_DECREF(&SubListType);
Py_DECREF(m);
return NULL;
}
return m;
}
As you can see, the source code closely resembles the Custom examples in
previous sections. We will break down the main differences between them.
typedef struct {
PyListObject list;
int state;
} SubListObject;
The primary difference for derived type objects is that the base type’s
object structure must be the first value. The base type will already include
the PyObject_HEAD() at the beginning of its structure.
When a Python object is a SubList instance, its PyObject * pointer
can be safely cast to both PyListObject * and SubListObject *:
static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
We see above how to call through to the __init__ method of the base
type.
This pattern is important when writing a type with custom
tp_new and tp_dealloc
members. The tp_new handler should not actually
create the memory for the object with its tp_alloc,
but let the base class handle it by calling its own tp_new.
The PyTypeObject struct supports a tp_base
specifying the type’s concrete base class. Due to cross-platform compiler
issues, you can’t fill that field directly with a reference to
PyList_Type; it should be done later in the module initialization
function:
PyMODINIT_FUNC
PyInit_sublist(void)
{
PyObject* m;
SubListType.tp_base = &PyList_Type;
if (PyType_Ready(&SubListType) < 0)
return NULL;
m = PyModule_Create(&sublistmodule);
if (m == NULL)
return NULL;
Py_INCREF(&SubListType);
if (PyModule_AddObject(m, "SubList", (PyObject *) &SubListType) < 0) {
Py_DECREF(&SubListType);
Py_DECREF(m);
return NULL;
}
return m;
}
Before calling PyType_Ready(), the type structure must have the
tp_base slot filled in. When we are deriving an
existing type, it is not necessary to fill out the tp_alloc
slot with PyType_GenericNew() – the allocation function from the base
type will be inherited.
After that, calling PyType_Ready() and adding the type object to the
module is the same as with the basic Custom examples.
Footnotes
| [1] | This is true when we know that the object is a basic type, like a string or a float. |
| [2] | We relied on this in the tp_dealloc handler
in this example, because our type doesn’t support garbage collection. |
| [3] | We now know that the first and last members are strings, so perhaps we could be less careful about decrementing their reference counts, however, we accept instances of string subclasses. Even though deallocating normal strings won’t call back into our objects, we can’t guarantee that deallocating an instance of a string subclass won’t call back into our objects. |
| [4] | Also, even with our attributes restricted to strings instances, the user
could pass arbitrary str subclasses and therefore still create
reference cycles. |