2. Defining New Types
As mentioned in the last chapter, Python allows the writer of an extension module to define new types that can be manipulated from Python code, much like strings and lists in core Python.
This is not hard; the code for all extension types follows a pattern, but there are some details that you need to understand before you can get started.
Note
The way new types are defined changed dramatically (and for the better) in Python 2.2. This document documents how to define new types for Python 2.2 and later. If you need to support older versions of Python, you will need to refer to older versions of this documentation .
2.1. The Basics
The Python runtime sees all Python objects as variables of type PyObject*. A PyObject is not a very magnificent object - it just contains the refcount and a pointer to the object’s “type object”. This is where the action is; the type object determines which (C) functions get called when, for instance, an attribute gets looked up on an object or it is multiplied by another object. These C functions are called “type methods”.
So, if you want to define a new object 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:
#include <Python.h>
typedef struct {
PyObject_HEAD
/* Type-specific fields go here. */
} noddy_NoddyObject;
static PyTypeObject noddy_NoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"noddy.Noddy", /* tp_name */
sizeof(noddy_NoddyObject), /* tp_basicsize */
0, /* tp_itemsize */
0, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT, /* tp_flags */
"Noddy objects", /* tp_doc */
};
static PyMethodDef noddy_methods[] = {
{NULL} /* Sentinel */
};
#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initnoddy(void)
{
PyObject* m;
noddy_NoddyType.tp_new = PyType_GenericNew;
if (PyType_Ready(&noddy_NoddyType) < 0)
return;
m = Py_InitModule3("noddy", noddy_methods,
"Example module that creates an extension type.");
Py_INCREF(&noddy_NoddyType);
PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType);
}
Now that’s quite a bit to take in at once, but hopefully bits will seem familiar from the last chapter.
The first bit that will be new is:
typedef struct {
PyObject_HEAD
} noddy_NoddyObject;
This is what a Noddy object will contain—in this case, nothing more than every Python object contains, namely a refcount and a pointer to a type object. These are the fields the PyObject_HEAD macro brings in. The reason for the macro is to standardize the layout and to enable special debugging fields in debug builds. Note that there is no semicolon after the PyObject_HEAD macro; one is included in the macro definition. Be wary of adding one by accident; it’s easy to do from habit, and your compiler might not complain, but someone else’s probably will! (On Windows, MSVC is known to call this an error and refuse to compile the code.)
For contrast, let’s take a look at the corresponding definition for standard Python integers:
typedef struct {
PyObject_HEAD
long ob_ival;
} PyIntObject;
Moving on, we come to the crunch — the type object.
static PyTypeObject noddy_NoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"noddy.Noddy", /* tp_name */
sizeof(noddy_NoddyObject), /* tp_basicsize */
0, /* tp_itemsize */
0, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT, /* tp_flags */
"Noddy objects", /* tp_doc */
};
Now if you go and look up the definition of PyTypeObject in object.h you’ll see that it has many more fields that 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.
This is so important that we’re going to pick the top of it apart still further:
PyVarObject_HEAD_INIT(NULL, 0)
This line is a bit of a wart; what we’d like to write is:
PyVarObject_HEAD_INIT(&PyType_Type, 0)
as the type of a type object is “type”, but this isn’t strictly conforming C and some compilers complain. Fortunately, this member will be filled in for us by PyType_Ready().
"noddy.Noddy", /* tp_name */
The name of our type. This will appear in the default textual representation of our objects and in some error messages, for example:
>>> "" + noddy.new_noddy()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot add type "noddy.Noddy" to string
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 noddy and the type is Noddy, so we set the type name to noddy.Noddy. One side effect of using an undotted name is that the pydoc documentation tool will not list the new type in the module documentation.
sizeof(noddy_NoddyObject), /* tp_basicsize */
This is so that Python knows how much memory to allocate when you call PyObject_New().
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.
0, /* tp_itemsize */
This has to do with variable length objects like lists and strings. Ignore this for now.
Skipping a number of type methods that we don’t provide, we set the class flags to Py_TPFLAGS_DEFAULT.
Py_TPFLAGS_DEFAULT, /* tp_flags */
All types should include this constant in their flags. It enables all of the members defined by the current version of Python.
We provide a doc string for the type in tp_doc.
"Noddy objects", /* tp_doc */
Now we get into the type methods, the things that make your objects different from the others. We aren’t going to implement any of these in this version of the module. We’ll expand this example later to have more interesting behavior.
For now, all we want to be able to do is to create new Noddy objects. To enable object creation, we have to provide a tp_new implementation. In this case, we can just use the default implementation provided by the API function PyType_GenericNew(). We’d like to just assign this to the tp_new slot, but we can’t, for portability sake, On some platforms or compilers, we can’t statically initialize a structure member with a function defined in another C module, so, instead, we’ll assign the tp_new slot in the module initialization function just before calling PyType_Ready():
noddy_NoddyType.tp_new = PyType_GenericNew;
if (PyType_Ready(&noddy_NoddyType) < 0)
return;
All the other type methods are NULL, so we’ll go over them later — that’s for a later section!
Everything else in the file should be familiar, except for some code in initnoddy():
if (PyType_Ready(&noddy_NoddyType) < 0)
return;
This initializes the Noddy type, filing in a number of members, including ob_type that we initially set to NULL.
PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType);
This adds the type to the module dictionary. This allows us to create Noddy instances by calling the Noddy class:
>>> import noddy
>>> mynoddy = noddy.Noddy()
That’s it! All that remains is to build it; put the above code in a file called noddy.c and
from distutils.core import setup, Extension
setup(name="noddy", version="1.0",
ext_modules=[Extension("noddy", ["noddy.c"])])
in a file called setup.py; then typing
$ python setup.py build
at a shell should produce a file noddy.so in a subdirectory; move to that directory and fire up Python — you should be able to import noddy and play around with Noddy objects.
That wasn’t so hard, was it?
Of course, the current Noddy type is pretty uninteresting. It has no data and doesn’t do anything. It can’t even be subclassed.
2.1.1. 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, noddy2 that adds these capabilities:
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} Noddy;
static void
Noddy_dealloc(Noddy* self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject*)self);
}
static PyObject *
Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
Noddy *self;
self = (Noddy *)type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyString_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyString_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *)self;
}
static int
Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
{
PyObject *first=NULL, *last=NULL, *tmp;
static char *kwlist[] = {"first", "last", "number", NULL};
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 Noddy_members[] = {
{"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
"last name"},
{"number", T_INT, offsetof(Noddy, number), 0,
"noddy number"},
{NULL} /* Sentinel */
};
static PyObject *
Noddy_name(Noddy* self)
{
static PyObject *format = NULL;
PyObject *args, *result;
if (format == NULL) {
format = PyString_FromString("%s %s");
if (format == NULL)
return NULL;
}
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
args = Py_BuildValue("OO", self->first, self->last);
if (args == NULL)
return NULL;
result = PyString_Format(format, args);
Py_DECREF(args);
return result;
}
static PyMethodDef Noddy_methods[] = {
{"name", (PyCFunction)Noddy_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject NoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"noddy.Noddy", /* tp_name */
sizeof(Noddy), /* tp_basicsize */
0, /* tp_itemsize */
(destructor)Noddy_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT |
Py_TPFLAGS_BASETYPE, /* tp_flags */
"Noddy objects", /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
Noddy_methods, /* tp_methods */
Noddy_members, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
(initproc)Noddy_init, /* tp_init */
0, /* tp_alloc */
Noddy_new, /* tp_new */
};
static PyMethodDef module_methods[] = {
{NULL} /* Sentinel */
};
#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initnoddy2(void)
{
PyObject* m;
if (PyType_Ready(&NoddyType) < 0)
return;
m = Py_InitModule3("noddy2", module_methods,
"Example module that creates an extension type.");
if (m == NULL)
return;
Py_INCREF(&NoddyType);
PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
}
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 name of the Noddy object structure has been shortened to Noddy. The type object name has been shortened to NoddyType.
The Noddy type now has three data attributes, first, last, and number. The first and last variables are Python strings containing first and last names. The number attribute is an integer.
The object structure is updated accordingly:
typedef struct {
PyObject_HEAD
PyObject *first;
PyObject *last;
int number;
} Noddy;
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
Noddy_dealloc(Noddy* self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject*)self);
}
which is assigned to the tp_dealloc member:
(destructor)Noddy_dealloc, /*tp_dealloc*/
This method decrements the reference counts of the two Python attributes. We use Py_XDECREF() here because the first and last members could be NULL. It then calls the tp_free member of the object’s type to free the object’s memory. Note that the object’s type might not be NoddyType, because the object may be an instance of a subclass.
We want to make sure that the first and last names are initialized to empty strings, so we provide a new method:
static PyObject *
Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
Noddy *self;
self = (Noddy *)type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyString_FromString("");
if (self->first == NULL)
{
Py_DECREF(self);
return NULL;
}
self->last = PyString_FromString("");
if (self->last == NULL)
{
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *)self;
}
and install it in the tp_new member:
Noddy_new, /* tp_new */
The new member is responsible for creating (as opposed to initializing) objects of the type. It is exposed in Python as the __new__() method. See the paper titled “Unifying types and classes in Python” for a detailed discussion of the __new__() method. One reason to implement a new method is to assure the initial values of instance variables. In this case, we use the new method to make sure that the initial values of the members first and last are not NULL. If we didn’t care whether the initial values were NULL, we could have used PyType_GenericNew() as our new method, as we did before. PyType_GenericNew() initializes all of the instance variable members to NULL.
The new method is a static method that is passed the type being instantiated and any arguments passed when the type was called, and that returns the new object created. New methods always accept positional and keyword arguments, but they often ignore the arguments, leaving the argument handling to initializer methods. Note that if the type supports subclassing, the type passed may not be the type being defined. The new method calls the tp_alloc slot to allocate memory. We don’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.
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 provide an initialization function:
static int
Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
{
PyObject *first=NULL, *last=NULL, *tmp;
static char *kwlist[] = {"first", "last", "number", NULL};
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.
(initproc)Noddy_init, /* tp_init */
The tp_init slot is exposed in Python as the __init__() method. It is used to initialize an object after it’s created. Unlike the new method, we can’t guarantee that the initializer is called. The initializer isn’t called when unpickling objects and it can be overridden. Our initializer accepts arguments to provide initial values for our instance. Initializers always accept positional and keyword arguments.
Initializers can 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 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. 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 not cause any calls back into our type’s code
when decrementing a reference count in a
tp_deallochandler when garbage-collections is not supported 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 Noddy_members[] = {
{"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
"last name"},
{"number", T_INT, offsetof(Noddy, number), 0,
"noddy number"},
{NULL} /* Sentinel */
};
and put the definitions in the tp_members slot:
Noddy_members, /* tp_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, name(), that outputs the objects name as the concatenation of the first and last names.
static PyObject *
Noddy_name(Noddy* self)
{
static PyObject *format = NULL;
PyObject *args, *result;
if (format == NULL) {
format = PyString_FromString("%s %s");
if (format == NULL)
return NULL;
}
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
args = Py_BuildValue("OO", self->first, self->last);
if (args == NULL)
return NULL;
result = PyString_Format(format, args);
Py_DECREF(args);
return result;
}
The method is implemented as a C function that takes a Noddy (or Noddy 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 Noddy_methods[] = {
{"name", (PyCFunction)Noddy_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
and assign them to the tp_methods slot:
Noddy_methods, /* tp_methods */
Note that we used the METH_NOARGS flag to indicate that the method is passed no arguments.
Finally, we’ll make our type usable as a base class. 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:
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /*tp_flags*/
We rename initnoddy() to initnoddy2() and update the module name passed to Py_InitModule3().
Finally, we update our setup.py file to build the new module:
from distutils.core import setup, Extension
setup(name="noddy", version="1.0",
ext_modules=[
Extension("noddy", ["noddy.c"]),
Extension("noddy2", ["noddy2.c"]),
])
2.1.2. 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 Noddy 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.
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first;
PyObject *last;
int number;
} Noddy;
static void
Noddy_dealloc(Noddy* self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject*)self);
}
static PyObject *
Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
Noddy *self;
self = (Noddy *)type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyString_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyString_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *)self;
}
static int
Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
{
PyObject *first=NULL, *last=NULL, *tmp;
static char *kwlist[] = {"first", "last", "number", NULL};
if (! PyArg_ParseTupleAndKeywords(args, kwds, "|SSi", 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 Noddy_members[] = {
{"number", T_INT, offsetof(Noddy, number), 0,
"noddy number"},
{NULL} /* Sentinel */
};
static PyObject *
Noddy_getfirst(Noddy *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Noddy_setfirst(Noddy *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (! PyString_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
Py_DECREF(self->first);
Py_INCREF(value);
self->first = value;
return 0;
}
static PyObject *
Noddy_getlast(Noddy *self, void *closure)
{
Py_INCREF(self->last);
return self->last;
}
static int
Noddy_setlast(Noddy *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
return -1;
}
if (! PyString_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The last attribute value must be a string");
return -1;
}
Py_DECREF(self->last);
Py_INCREF(value);
self->last = value;
return 0;
}
static PyGetSetDef Noddy_getseters[] = {
{"first",
(getter)Noddy_getfirst, (setter)Noddy_setfirst,
"first name",
NULL},
{"last",
(getter)Noddy_getlast, (setter)Noddy_setlast,
"last name",
NULL},
{NULL} /* Sentinel */
};
static PyObject *
Noddy_name(Noddy* self)
{
static PyObject *format = NULL;
PyObject *args, *result;
if (format == NULL) {
format = PyString_FromString("%s %s");
if (format == NULL)
return NULL;
}
args = Py_BuildValue("OO", self->first, self->last);
if (args == NULL)
return NULL;
result = PyString_Format(format, args);
Py_DECREF(args);
return result;
}
static PyMethodDef Noddy_methods[] = {
{"name", (PyCFunction)Noddy_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject NoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"noddy.Noddy", /* tp_name */
sizeof(Noddy), /* tp_basicsize */
0, /* tp_itemsize */
(destructor)Noddy_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT |
Py_TPFLAGS_BASETYPE, /* tp_flags */
"Noddy objects", /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
Noddy_methods, /* tp_methods */
Noddy_members, /* tp_members */
Noddy_getseters, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
(initproc)Noddy_init, /* tp_init */
0, /* tp_alloc */
Noddy_new, /* tp_new */
};
static PyMethodDef module_methods[] = {
{NULL} /* Sentinel */
};
#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initnoddy3(void)
{
PyObject* m;
if (PyType_Ready(&NoddyType) < 0)
return;
m = Py_InitModule3("noddy3", module_methods,
"Example module that creates an extension type.");
if (m == NULL)
return;
Py_INCREF(&NoddyType);
PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
}
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:
Noddy_getfirst(Noddy *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Noddy_setfirst(Noddy *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (! PyString_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
Py_DECREF(self->first);
Py_INCREF(value);
self->first = value;
return 0;
}
The getter function is passed a Noddy object and a “closure”, which is 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 Noddy 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 the attribute value is not a string.
We create an array of PyGetSetDef structures:
static PyGetSetDef Noddy_getseters[] = {
{"first",
(getter)Noddy_getfirst, (setter)Noddy_setfirst,
"first name",
NULL},
{"last",
(getter)Noddy_getlast, (setter)Noddy_setlast,
"last name",
NULL},
{NULL} /* Sentinel */
};
and register it in the tp_getset slot:
Noddy_getseters, /* tp_getset */
to register our attribute getters and setters.
The last item in a PyGetSetDef structure is the closure mentioned above. In this case, we aren’t using the closure, so we just pass NULL.
We also remove the member definitions for these attributes:
static PyMemberDef Noddy_members[] = {
{"number", T_INT, offsetof(Noddy, number), 0,
"noddy number"},
{NULL} /* Sentinel */
};
We also need to update the tp_init handler to only allow strings 3 to be passed:
static int
Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
{
PyObject *first=NULL, *last=NULL, *tmp;
static char *kwlist[] = {"first", "last", "number", NULL};
if (! PyArg_ParseTupleAndKeywords(args, kwds, "|SSi", 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 deallocator, where there is the possibility that the initialization of these members failed in the constructor.
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.
2.1.3. Supporting cyclic garbage collection
Python has a cyclic-garbage collector 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 Noddy example, we allowed any kind of object to be stored in the first or last attributes 4. This means that Noddy objects can participate in cycles:
>>> import noddy2
>>> n = noddy2.Noddy()
>>> l = [n]
>>> n.first = l
This is pretty silly, but it gives us an excuse to add support for the cyclic-garbage collector to the Noddy example. To support cyclic garbage collection, types need to fill two slots and set a class flag that enables these slots:
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first;
PyObject *last;
int number;
} Noddy;
static int
Noddy_traverse(Noddy *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;
}
static int
Noddy_clear(Noddy *self)
{
PyObject *tmp;
tmp = self->first;
self->first = NULL;
Py_XDECREF(tmp);
tmp = self->last;
self->last = NULL;
Py_XDECREF(tmp);
return 0;
}
static void
Noddy_dealloc(Noddy* self)
{
PyObject_GC_UnTrack(self);
Noddy_clear(self);
Py_TYPE(self)->tp_free((PyObject*)self);
}
static PyObject *
Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
Noddy *self;
self = (Noddy *)type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyString_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyString_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *)self;
}
static int
Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
{
PyObject *first=NULL, *last=NULL, *tmp;
static char *kwlist[] = {"first", "last", "number", NULL};
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 Noddy_members[] = {
{"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
"last name"},
{"number", T_INT, offsetof(Noddy, number), 0,
"noddy number"},
{NULL} /* Sentinel */
};
static PyObject *
Noddy_name(Noddy* self)
{
static PyObject *format = NULL;
PyObject *args, *result;
if (format == NULL) {
format = PyString_FromString("%s %s");
if (format == NULL)
return NULL;
}
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
args = Py_BuildValue("OO", self->first, self->last);
if (args == NULL)
return NULL;
result = PyString_Format(format, args);
Py_DECREF(args);
return result;
}
static PyMethodDef Noddy_methods[] = {
{"name", (PyCFunction)Noddy_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject NoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"noddy.Noddy", /* tp_name */
sizeof(Noddy), /* tp_basicsize */
0, /* tp_itemsize */
(destructor)Noddy_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT |
Py_TPFLAGS_BASETYPE |
Py_TPFLAGS_HAVE_GC, /* tp_flags */
"Noddy objects", /* tp_doc */
(traverseproc)Noddy_traverse, /* tp_traverse */
(inquiry)Noddy_clear, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
Noddy_methods, /* tp_methods */
Noddy_members, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
(initproc)Noddy_init, /* tp_init */
0, /* tp_alloc */
Noddy_new, /* tp_new */
};
static PyMethodDef module_methods[] = {
{NULL} /* Sentinel */
};
#ifndef PyMODINIT_FUNC /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initnoddy4(void)
{
PyObject* m;
if (PyType_Ready(&NoddyType) < 0)
return;
m = Py_InitModule3("noddy4", module_methods,
"Example module that creates an extension type.");
if (m == NULL)
return;
Py_INCREF(&NoddyType);
PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
}
The traversal method provides access to subobjects that could participate in cycles:
static int
Noddy_traverse(Noddy *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 2.4 and higher provide a Py_VISIT() macro that automates calling visit functions. With Py_VISIT(), Noddy_traverse() can be simplified:
static int
Noddy_traverse(Noddy *self, visitproc visit, void *arg)
{
Py_VISIT(self->first);
Py_VISIT(self->last);
return 0;
}
Note
Note that the tp_traverse implementation must name its arguments exactly visit and arg in order to use Py_VISIT(). This is to encourage uniformity across these boring implementations.
We also need to provide a method for clearing any subobjects that can participate in cycles.
static int
Noddy_clear(Noddy *self)
{
PyObject *tmp;
tmp = self->first;
self->first = NULL;
Py_XDECREF(tmp);
tmp = self->last;
self->last = NULL;
Py_XDECREF(tmp);
return 0;
}
Notice the use of a temporary variable in Noddy_clear(). We use the temporary variable so that we can set each member to NULL before decrementing its reference count. We do this because, as was discussed earlier, if the reference count drops to zero, we might cause code to run that calls back into the object. In addition, because we now support garbage collection, we also have to worry about code being run that triggers garbage collection. If garbage collection is run, our tp_traverse handler could get called. We can’t take a chance of having Noddy_traverse() called when a member’s reference count has dropped to zero and its value hasn’t been set to NULL.
Python 2.4 and higher provide a Py_CLEAR() that automates the careful decrementing of reference counts. With Py_CLEAR(), the Noddy_clear() function can be simplified:
static int
Noddy_clear(Noddy *self)
{
Py_CLEAR(self->first);
Py_CLEAR(self->last);
return 0;
}
Note that Noddy_dealloc() may call arbitrary functions through __del__ method or weakref callback. It means circular GC can be triggered inside the function. Since GC assumes reference count is not zero, we need to untrack the object from GC by calling PyObject_GC_UnTrack() before clearing members. Here is reimplemented deallocator which uses PyObject_GC_UnTrack() and Noddy_clear().
static void
Noddy_dealloc(Noddy* self)
{
PyObject_GC_UnTrack(self);
Noddy_clear(self);
Py_TYPE(self)->tp_free((PyObject*)self);
}
Finally, we add the Py_TPFLAGS_HAVE_GC flag to the class flags:
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC, /* tp_flags */
That’s pretty much it. If we had written custom tp_alloc or tp_free slots, we’d need to modify them for cyclic-garbage collection. Most extensions will use the versions automatically provided.
2.1.4. 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 Shoddy 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 shoddy
>>> s = shoddy.Shoddy(range(3))
>>> s.extend(s)
>>> print len(s)
6
>>> print s.increment()
1
>>> print s.increment()
2
#include <Python.h>
typedef struct {
PyListObject list;
int state;
} Shoddy;
static PyObject *
Shoddy_increment(Shoddy *self, PyObject *unused)
{
self->state++;
return PyInt_FromLong(self->state);
}
static PyMethodDef Shoddy_methods[] = {
{"increment", (PyCFunction)Shoddy_increment, METH_NOARGS,
PyDoc_STR("increment state counter")},
{NULL, NULL},
};
static int
Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
static PyTypeObject ShoddyType = {
PyVarObject_HEAD_INIT(NULL, 0)
"shoddy.Shoddy", /* tp_name */
sizeof(Shoddy), /* tp_basicsize */
0, /* tp_itemsize */
0, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT |
Py_TPFLAGS_BASETYPE, /* tp_flags */
0, /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
0, /* tp_iter */
0, /* tp_iternext */
Shoddy_methods, /* tp_methods */
0, /* tp_members */
0, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
(initproc)Shoddy_init, /* tp_init */
0, /* tp_alloc */
0, /* tp_new */
};
PyMODINIT_FUNC
initshoddy(void)
{
PyObject *m;
ShoddyType.tp_base = &PyList_Type;
if (PyType_Ready(&ShoddyType) < 0)
return;
m = Py_InitModule3("shoddy", NULL, "Shoddy module");
if (m == NULL)
return;
Py_INCREF(&ShoddyType);
PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
}
As you can see, the source code closely resembles the Noddy examples in previous sections. We will break down the main differences between them.
typedef struct {
PyListObject list;
int state;
} Shoddy;
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 Shoddy instance, its PyObject* pointer can be safely cast to both PyListObject* and Shoddy*.
static int
Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
In the __init__ method for our type, we can see how to call through to the __init__ method of the base type.
This pattern is important when writing a type with custom new and dealloc methods. The new method should not actually create the memory for the object with tp_alloc, that will be handled by the base class when calling its tp_new.
When filling out the PyTypeObject() for the Shoddy type, you see a slot for tp_base(). Due to cross platform compiler issues, you can’t fill that field directly with the PyList_Type(); it can be done later in the module’s init() function.
PyMODINIT_FUNC
initshoddy(void)
{
PyObject *m;
ShoddyType.tp_base = &PyList_Type;
if (PyType_Ready(&ShoddyType) < 0)
return;
m = Py_InitModule3("shoddy", NULL, "Shoddy module");
if (m == NULL)
return;
Py_INCREF(&ShoddyType);
PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
}
Before calling PyType_Ready(), the type structure must have the tp_base slot filled in. When we are deriving a new type, it is not necessary to fill out the tp_alloc slot with PyType_GenericNew() – the allocate 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 Noddy examples.
2.2. Type Methods
This section aims to give a quick fly-by on the various type methods you can implement and what they do.
Here is the definition of PyTypeObject, with some fields only used in debug builds omitted:
typedef struct _typeobject {
PyObject_VAR_HEAD
char *tp_name; /* For printing, in format "<module>.<name>" */
int tp_basicsize, tp_itemsize; /* For allocation */
/* Methods to implement standard operations */
destructor tp_dealloc;
printfunc tp_print;
getattrfunc tp_getattr;
setattrfunc tp_setattr;
cmpfunc tp_compare;
reprfunc tp_repr;
/* Method suites for standard classes */
PyNumberMethods *tp_as_number;
PySequenceMethods *tp_as_sequence;
PyMappingMethods *tp_as_mapping;
/* More standard operations (here for binary compatibility) */
hashfunc tp_hash;
ternaryfunc tp_call;
reprfunc tp_str;
getattrofunc tp_getattro;
setattrofunc tp_setattro;
/* Functions to access object as input/output buffer */
PyBufferProcs *tp_as_buffer;
/* Flags to define presence of optional/expanded features */
long tp_flags;
char *tp_doc; /* Documentation string */
/* Assigned meaning in release 2.0 */
/* call function for all accessible objects */
traverseproc tp_traverse;
/* delete references to contained objects */
inquiry tp_clear;
/* Assigned meaning in release 2.1 */
/* rich comparisons */
richcmpfunc tp_richcompare;
/* weak reference enabler */
long tp_weaklistoffset;
/* Added in release 2.2 */
/* Iterators */
getiterfunc tp_iter;
iternextfunc tp_iternext;
/* Attribute descriptor and subclassing stuff */
struct PyMethodDef *tp_methods;
struct PyMemberDef *tp_members;
struct PyGetSetDef *tp_getset;
struct _typeobject *tp_base;
PyObject *tp_dict;
descrgetfunc tp_descr_get;
descrsetfunc tp_descr_set;
long tp_dictoffset;
initproc tp_init;
allocfunc tp_alloc;
newfunc tp_new;
freefunc tp_free; /* Low-level free-memory routine */
inquiry tp_is_gc; /* For PyObject_IS_GC */
PyObject *tp_bases;
PyObject *tp_mro; /* method resolution order */
PyObject *tp_cache;
PyObject *tp_subclasses;
PyObject *tp_weaklist;
} PyTypeObject;
Now that’s a lot of methods. Don’t worry too much though - if you have a type you want to define, the chances are very good that you will only implement a handful of these.
As you probably expect by now, we’re going to go over this and give more information about the various handlers. We won’t go in the order they are defined in the structure, because there is a lot of historical baggage that impacts the ordering of the fields; be sure your type initialization keeps the fields in the right order! It’s often easiest to find an example that includes all the fields you need (even if they’re initialized to 0) and then change the values to suit your new type.
char *tp_name; /* For printing */
The name of the type - as mentioned in the last section, this will appear in various places, almost entirely for diagnostic purposes. Try to choose something that will be helpful in such a situation!
int tp_basicsize, tp_itemsize; /* For allocation */
These fields tell the runtime how much memory to allocate when new objects of this type are created. Python has some built-in support for variable length structures (think: strings, lists) which is where the tp_itemsize field comes in. This will be dealt with later.
char *tp_doc;
Here you can put a string (or its address) that you want returned when the Python script references obj.__doc__ to retrieve the doc string.
Now we come to the basic type methods—the ones most extension types will implement.
2.2.1. Finalization and De-allocation
destructor tp_dealloc;
This function is called when the reference count of the instance of your type is reduced to zero and the Python interpreter wants to reclaim it. If your type has memory to free or other clean-up to perform, you can put it here. The object itself needs to be freed here as well. Here is an example of this function:
static void
newdatatype_dealloc(newdatatypeobject * obj)
{
free(obj->obj_UnderlyingDatatypePtr);
Py_TYPE(obj)->tp_free(obj);
}
One important requirement of the deallocator function is that it leaves any pending exceptions alone. This is important since deallocators are frequently called as the interpreter unwinds the Python stack; when the stack is unwound due to an exception (rather than normal returns), nothing is done to protect the deallocators from seeing that an exception has already been set. Any actions which a deallocator performs which may cause additional Python code to be executed may detect that an exception has been set. This can lead to misleading errors from the interpreter. The proper way to protect against this is to save a pending exception before performing the unsafe action, and restoring it when done. This can be done using the PyErr_Fetch() and PyErr_Restore() functions:
static void
my_dealloc(PyObject *obj)
{
