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
and
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.
The Basics
The
runtime sees all Python objects as variables of type
*, which serves as a “base type” for all Python objects. The
structure itself only contains the object’s
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
function, which isn’t covered in this tutorial.
#define PY_SSIZE_T_CLEAN#include<Python.h>typedefstruct{PyObject_HEAD/* Type-specific fields go here. */}CustomObject;staticPyTypeObjectCustomType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="custom.Custom",.tp_doc=PyDoc_STR("Custom objects"),.tp_basicsize=sizeof(CustomObject),.tp_itemsize=0,.tp_flags=Py_TPFLAGS_DEFAULT,.tp_new=PyType_GenericNew,};staticintcustom_module_exec(PyObject*m){if(PyType_Ready(&CustomType)<0){return-1;}if(PyModule_AddObjectRef(m,"Custom",(PyObject*)&CustomType)<0){return-1;}return0;}staticPyModuleDef_Slotcustom_module_slots[]={{Py_mod_exec,custom_module_exec},// Just use this while using static types{Py_mod_multiple_interpreters,Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},{0,NULL}};staticPyModuleDefcustom_module={.m_base=PyModuleDef_HEAD_INIT,.m_name="custom",.m_doc="Example module that creates an extension type.",.m_size=0,.m_slots=custom_module_slots,};PyMODINIT_FUNCPyInit_custom(void){returnPyModuleDef_Init(&custom_module);}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 the CustomObject struct, which is allocated once for each Custom instance.
How the Customtype behaves: this is the CustomType struct, which defines a set of flags and function pointers that the interpreter inspects when specific operations are requested.
How to define and execute the custom module: this is the PyInit_custom function and the associated custom_module struct for defining the module, and the custom_module_exec function to set up a fresh module object.
The first bit is:
typedefstruct{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
, containing a pointer to a type object and a reference count (these can be accessed using the macros
and
respectively). The reason for the macro is to abstract away the layout and to enable additional fields in
.
Note
There is no semicolon above after the
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:
typedefstruct{PyObject_HEADdoubleob_fval;}PyFloatObject;The second bit is the definition of the type object.
staticPyTypeObjectCustomType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="custom.Custom",.tp_doc=PyDoc_STR("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
fields that you don’t care about and also to avoid caring about the fields’ declaration order.
The actual definition of
in object.h has many more
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:
.ob_base=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 strNote 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
and
modules.
.tp_basicsize=sizeof(CustomObject),.tp_itemsize=0,This is so that Python knows how much memory to allocate when creating new Custom instances.
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
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
, or else it will not be able to call your type’s
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
, or else you will be adding data members to your base type, and therefore increasing its size.
We set the class flags to
.
.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=PyDoc_STR("Custom objects"),To enable object creation, we have to provide a
handler. This is the equivalent of the Python method
, but has to be specified explicitly. In this case, we can just use the default implementation provided by the API function
.
.tp_new=PyType_GenericNew,Everything else in the file should be familiar, except for some code in custom_module_exec():
if(PyType_Ready(&CustomType)<0){return-1;}This initializes the Custom type, filling in a number of members to the appropriate default values, including
that we initially set to NULL.
if(PyModule_AddObjectRef(m,"Custom",(PyObject*)&CustomType)<0){return-1;}This adds the type to the module dictionary. This allows us to create Custom instances by calling the Custom class:
>>> importcustom>>> mycustom=custom.Custom()That’s it! All that remains is to build it; put the above code in a file called custom.c,
[build-system]requires=["setuptools"]build-backend="setuptools.build_meta"[project]name="custom"version="1"in a file called pyproject.toml, and
fromsetuptoolsimportExtension,setupsetup(ext_modules=[Extension("custom",["custom.c"])])in a file called setup.py; then typing
$ python-mpipinstall. in a shell should produce a file custom.so in a subdirectory and install it; now fire up Python — you should be able to importcustom 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.
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<stddef.h>/* for offsetof() */typedefstruct{PyObject_HEADPyObject*first;/* first name */PyObject*last;/* last name */intnumber;}CustomObject;staticvoidCustom_dealloc(PyObject*op){CustomObject*self=(CustomObject*)op;Py_XDECREF(self->first);Py_XDECREF(self->last);Py_TYPE(self)->tp_free(self);}staticPyObject*Custom_new(PyTypeObject*type,PyObject*args,PyObject*kwds){CustomObject*self;self=(CustomObject*)type->tp_alloc(type,0);if(self!=NULL){self->first=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->first==NULL){Py_DECREF(self);returnNULL;}self->last=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->last==NULL){Py_DECREF(self);returnNULL;}self->number=0;}return(PyObject*)self;}staticintCustom_init(PyObject*op,PyObject*args,PyObject*kwds){CustomObject*self=(CustomObject*)op;staticchar*kwlist[]={"first","last","number",NULL};PyObject*first=NULL,*last=NULL;if(!PyArg_ParseTupleAndKeywords(args,kwds,"|OOi",kwlist,&first,&last,&self->number))return-1;if(first){Py_XSETREF(self->first,Py_NewRef(first));}if(last){Py_XSETREF(self->last,Py_NewRef(last));}return0;}staticPyMemberDefCustom_members[]={{"first",Py_T_OBJECT_EX,offsetof(CustomObject,first),0,"first name"},{"last",Py_T_OBJECT_EX,offsetof(CustomObject,last),0,"last name"},{"number",Py_T_INT,offsetof(CustomObject,number),0,"custom number"},{NULL}/* Sentinel */};staticPyObject*Custom_name(PyObject*op,PyObject*Py_UNUSED(dummy)){CustomObject*self=(CustomObject*)op;if(self->first==NULL){PyErr_SetString(PyExc_AttributeError,"first");returnNULL;}if(self->last==NULL){PyErr_SetString(PyExc_AttributeError,"last");returnNULL;}returnPyUnicode_FromFormat("%S %S",self->first,self->last);}staticPyMethodDefCustom_methods[]={{"name",Custom_name,METH_NOARGS,"Return the name, combining the first and last name"},{NULL}/* Sentinel */};staticPyTypeObjectCustomType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="custom2.Custom",.tp_doc=PyDoc_STR("Custom objects"),.tp_basicsize=sizeof(CustomObject),.tp_itemsize=0,.tp_flags=Py_TPFLAGS_DEFAULT|Py_TPFLAGS_BASETYPE,.tp_new=Custom_new,.tp_init=Custom_init,.tp_dealloc=Custom_dealloc,.tp_members=Custom_members,.tp_methods=Custom_methods,};staticintcustom_module_exec(PyObject*m){if(PyType_Ready(&CustomType)<0){return-1;}if(PyModule_AddObjectRef(m,"Custom",(PyObject*)&CustomType)<0){return-1;}return0;}staticPyModuleDef_Slotcustom_module_slots[]={{Py_mod_exec,custom_module_exec},{Py_mod_multiple_interpreters,Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},{0,NULL}};staticPyModuleDefcustom_module={.m_base=PyModuleDef_HEAD_INIT,.m_name="custom2",.m_doc="Example module that creates an extension type.",.m_size=0,.m_slots=custom_module_slots,};PyMODINIT_FUNCPyInit_custom2(void){returnPyModuleDef_Init(&custom_module);}This version of the module has a number of changes.
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:
typedefstruct{PyObject_HEADPyObject*first;/* first name */PyObject*last;/* last name */intnumber;}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:
staticvoidCustom_dealloc(PyObject*op){CustomObject*self=(CustomObject*)op;Py_XDECREF(self->first);Py_XDECREF(self->last);Py_TYPE(self)->tp_free(self);}which is assigned to the
member:
.tp_dealloc=Custom_dealloc,This method first clears the reference counts of the two Python attributes.
correctly handles the case where its argument is NULL (which might happen here if tp_new failed midway). It then calls the
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 CustomObject* above is needed because we defined Custom_dealloc to take a PyObject* argument, as the tp_dealloc function pointer expects to receive a PyObject* argument. By assigning to the tp_dealloc slot of a type, we declare that it can only be called with instances of our CustomObject class, so the cast to (CustomObject*) is safe. This is object-oriented polymorphism, in C!
In existing code, or in previous versions of this tutorial, you might see similar functions take a pointer to the subtype object structure (CustomObject*) directly, like this:
Custom_dealloc(CustomObject*self){Py_XDECREF(self->first);Py_XDECREF(self->last);Py_TYPE(self)->tp_free((PyObject*)self);}....tp_dealloc=(destructor)Custom_dealloc,This does the same thing on all architectures that CPython supports, but according to the C standard, it invokes undefined behavior.
We want to make sure that the first and last names are initialized to empty strings, so we provide a tp_new implementation:
staticPyObject*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);returnNULL;}self->last=PyUnicode_FromString("");if(self->last==NULL){Py_DECREF(self);returnNULL;}self->number=0;}return(PyObject*)self;}and install it in the
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
method. It is not required to define a tp_new member, and indeed many extension types will simply reuse
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
slot to allocate memory:
self=(CustomObject*)type->tp_alloc(type,0);Since memory allocation may fail, we must check the
result against NULL before proceeding.
Note
We didn’t fill the
slot ourselves. Rather
fills it for us by inheriting it from our base class, which is
by default. Most types use the default allocation strategy.
Note
If you are creating a co-operative
(one that calls a base type’s tp_new or
), 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
.)
We also define an initialization function which accepts arguments to provide initial values for our instance:
staticintCustom_init(PyObject*op,PyObject*args,PyObject*kwds){CustomObject*self=(CustomObject*)op;staticchar*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);}return0;}by filling the
slot.
.tp_init=Custom_init,The
slot is exposed in Python as the
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
module by default doesn’t call
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 detach the
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
will neither detach the
nor cause any calls back into our type’s code;
when decrementing a reference count in a
handler on a type which doesn’t support cyclic garbage collection
.
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:
staticPyMemberDefCustom_members[]={{"first",Py_T_OBJECT_EX,offsetof(CustomObject,first),0,"first name"},{"last",Py_T_OBJECT_EX,offsetof(CustomObject,last),0,"last name"},{"number",Py_T_INT,offsetof(CustomObject,number),0,"custom number"},{NULL}/* Sentinel */};and put the definitions in the
slot:
.tp_members=Custom_members,Each member definition has a member name, type, offset, access flags and documentation string. See the
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.
staticPyObject*Custom_name(PyObject*op,PyObject*Py_UNUSED(dummy)){CustomObject*self=(CustomObject*)op;if(self->first==NULL){PyErr_SetString(PyExc_AttributeError,"first");returnNULL;}if(self->last==NULL){PyErr_SetString(PyExc_AttributeError,"last");returnNULL;}returnPyUnicode_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:
defname(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:
staticPyMethodDefCustom_methods[]={{"name",Custom_name,METH_NOARGS,"Return the name, combining the first and last name"},{NULL}/* Sentinel */};(note that we used the
flag to indicate that the method is expecting no arguments other than self)
and assign it to the
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
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
struct, and update the full class name in the
struct.
Finally, we update our setup.py file to include the new module,
fromsetuptoolsimportExtension,setupsetup(ext_modules=[Extension("custom",["custom.c"]),Extension("custom2",["custom2.c"]),])and then we re-install so that we can importcustom2:
$ python-mpipinstall. 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<stddef.h>/* for offsetof() */typedefstruct{PyObject_HEADPyObject*first;/* first name */PyObject*last;/* last name */intnumber;}CustomObject;staticvoidCustom_dealloc(PyObject*op){CustomObject*self=(CustomObject*)op;Py_XDECREF(self->first);Py_XDECREF(self->last);Py_TYPE(self)->tp_free(self);}staticPyObject*Custom_new(PyTypeObject*type,PyObject*args,PyObject*kwds){CustomObject*self;self=(CustomObject*)type->tp_alloc(type,0);if(self!=NULL){self->first=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->first==NULL){Py_DECREF(self);returnNULL;}self->last=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->last==NULL){Py_DECREF(self);returnNULL;}self->number=0;}return(PyObject*)self;}staticintCustom_init(PyObject*op,PyObject*args,PyObject*kwds){CustomObject*self=(CustomObject*)op;staticchar*kwlist[]={"first","last","number",NULL};PyObject*first=NULL,*last=NULL;if(!PyArg_ParseTupleAndKeywords(args,kwds,"|UUi",kwlist,&first,&last,&self->number))return-1;if(first){Py_SETREF(self->first,Py_NewRef(first));}if(last){Py_SETREF(self->last,Py_NewRef(last));}return0;}staticPyMemberDefCustom_members[]={{"number",Py_T_INT,offsetof(CustomObject,number),0,"custom number"},{NULL}/* Sentinel */};staticPyObject*Custom_getfirst(PyObject*op,void*closure){CustomObject*self=(CustomObject*)op;returnPy_NewRef(self->first);}staticintCustom_setfirst(PyObject*op,PyObject*value,void*closure){CustomObject*self=(CustomObject*)op;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_SETREF(self->first,Py_NewRef(value));return0;}staticPyObject*Custom_getlast(PyObject*op,void*closure){CustomObject*self=(CustomObject*)op;returnPy_NewRef(self->last);}staticintCustom_setlast(PyObject*op,PyObject*value,void*closure){CustomObject*self=(CustomObject*)op;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_SETREF(self->last,Py_NewRef(value));return0;}staticPyGetSetDefCustom_getsetters[]={{"first",Custom_getfirst,Custom_setfirst,"first name",NULL},{"last",Custom_getlast,Custom_setlast,"last name",NULL},{NULL}/* Sentinel */};staticPyObject*Custom_name(PyObject*op,PyObject*Py_UNUSED(dummy)){CustomObject*self=(CustomObject*)op;returnPyUnicode_FromFormat("%S %S",self->first,self->last);}staticPyMethodDefCustom_methods[]={{"name",Custom_name,METH_NOARGS,"Return the name, combining the first and last name"},{NULL}/* Sentinel */};staticPyTypeObjectCustomType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="custom3.Custom",.tp_doc=PyDoc_STR("Custom objects"),.tp_basicsize=sizeof(CustomObject),.tp_itemsize=0,.tp_flags=Py_TPFLAGS_DEFAULT|Py_TPFLAGS_BASETYPE,.tp_new=Custom_new,.tp_init=Custom_init,.tp_dealloc=Custom_dealloc,.tp_members=Custom_members,.tp_methods=Custom_methods,.tp_getset=Custom_getsetters,};staticintcustom_module_exec(PyObject*m){if(PyType_Ready(&CustomType)<0){return-1;}if(PyModule_AddObjectRef(m,"Custom",(PyObject*)&CustomType)<0){return-1;}return0;}staticPyModuleDef_Slotcustom_module_slots[]={{Py_mod_exec,custom_module_exec},{Py_mod_multiple_interpreters,Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},{0,NULL}};staticPyModuleDefcustom_module={.m_base=PyModuleDef_HEAD_INIT,.m_name="custom3",.m_doc="Example module that creates an extension type.",.m_size=0,.m_slots=custom_module_slots,};PyMODINIT_FUNCPyInit_custom3(void){returnPyModuleDef_Init(&custom_module);}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:
staticPyObject*Custom_getfirst(PyObject*op,void*closure){CustomObject*self=(CustomObject*)op;Py_INCREF(self->first);returnself->first;}staticintCustom_setfirst(PyObject*op,PyObject*value,void*closure){CustomObject*self=(CustomObject*)op;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);return0;}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
structures:
staticPyGetSetDefCustom_getsetters[]={{"first",Custom_getfirst,Custom_setfirst,"first name",NULL},{"last",Custom_getlast,Custom_setlast,"last name",NULL},{NULL}/* Sentinel */};and register it in the
slot:
.tp_getset=Custom_getsetters,The last item in a
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:
staticPyMemberDefCustom_members[]={{"number",Py_T_INT,offsetof(CustomObject,number),0,"custom number"},{NULL}/* Sentinel */};We also need to update the
handler to only allow strings
to be passed:
staticintCustom_init(PyObject*op,PyObject*args,PyObject*kwds){CustomObject*self=(CustomObject*)op;staticchar*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);}return0;}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
calls can be converted to
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.
Supporting cyclic garbage collection
Python has a
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)>>> dellIn 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
. 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:
>>> importcustom3>>> classDerived(custom3.Custom):pass...>>> n=Derived()>>> n.some_attribute=nTo 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<stddef.h>/* for offsetof() */typedefstruct{PyObject_HEADPyObject*first;/* first name */PyObject*last;/* last name */intnumber;}CustomObject;staticintCustom_traverse(PyObject*op,visitprocvisit,void*arg){CustomObject*self=(CustomObject*)op;Py_VISIT(self->first);Py_VISIT(self->last);return0;}staticintCustom_clear(PyObject*op){CustomObject*self=(CustomObject*)op;Py_CLEAR(self->first);Py_CLEAR(self->last);return0;}staticvoidCustom_dealloc(PyObject*op){PyObject_GC_UnTrack(op);(void)Custom_clear(op);Py_TYPE(op)->tp_free(op);}staticPyObject*Custom_new(PyTypeObject*type,PyObject*args,PyObject*kwds){CustomObject*self;self=(CustomObject*)type->tp_alloc(type,0);if(self!=NULL){self->first=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->first==NULL){Py_DECREF(self);returnNULL;}self->last=Py_GetConstant(Py_CONSTANT_EMPTY_STR);if(self->last==NULL){Py_DECREF(self);returnNULL;}self->number=0;}return(PyObject*)self;}staticintCustom_init(PyObject*op,PyObject*args,PyObject*kwds){CustomObject*self=(CustomObject*)op;staticchar*kwlist[]={"first","last","number",NULL};PyObject*first=NULL,*last=NULL;if(!PyArg_ParseTupleAndKeywords(args,kwds,"|UUi",kwlist,&first,&last,&self->number))return-1;if(first){Py_SETREF(self->first,Py_NewRef(first));}if(last){Py_SETREF(self->last,Py_NewRef(last));}return0;}staticPyMemberDefCustom_members[]={{"number",Py_T_INT,offsetof(CustomObject,number),0,"custom number"},{NULL}/* Sentinel */};staticPyObject*Custom_getfirst(PyObject*op,void*closure){CustomObject*self=(CustomObject*)op;returnPy_NewRef(self->first);}staticintCustom_setfirst(PyObject*op,PyObject*value,void*closure){CustomObject*self=(CustomObject*)op;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_XSETREF(self->first,Py_NewRef(value));return0;}staticPyObject*Custom_getlast(PyObject*op,void*closure){CustomObject*self=(CustomObject*)op;returnPy_NewRef(self->last);}staticintCustom_setlast(PyObject*op,PyObject*value,void*closure){CustomObject*self=(CustomObject*)op;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_XSETREF(self->last,Py_NewRef(value));return0;}staticPyGetSetDefCustom_getsetters[]={{"first",Custom_getfirst,Custom_setfirst,"first name",NULL},{"last",Custom_getlast,Custom_setlast,"last name",NULL},{NULL}/* Sentinel */};staticPyObject*Custom_name(PyObject*op,PyObject*Py_UNUSED(dummy)){CustomObject*self=(CustomObject*)op;returnPyUnicode_FromFormat("%S %S",self->first,self->last);}staticPyMethodDefCustom_methods[]={{"name",Custom_name,METH_NOARGS,"Return the name, combining the first and last name"},{NULL}/* Sentinel */};staticPyTypeObjectCustomType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="custom4.Custom",.tp_doc=PyDoc_STR("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=Custom_init,.tp_dealloc=Custom_dealloc,.tp_traverse=Custom_traverse,.tp_clear=Custom_clear,.tp_members=Custom_members,.tp_methods=Custom_methods,.tp_getset=Custom_getsetters,};staticintcustom_module_exec(PyObject*m){if(PyType_Ready(&CustomType)<0){return-1;}if(PyModule_AddObjectRef(m,"Custom",(PyObject*)&CustomType)<0){return-1;}return0;}staticPyModuleDef_Slotcustom_module_slots[]={{Py_mod_exec,custom_module_exec},{Py_mod_multiple_interpreters,Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},{0,NULL}};staticPyModuleDefcustom_module={.m_base=PyModuleDef_HEAD_INIT,.m_name="custom4",.m_doc="Example module that creates an extension type.",.m_size=0,.m_slots=custom_module_slots,};PyMODINIT_FUNCPyInit_custom4(void){returnPyModuleDef_Init(&custom_module);}First, the traversal method lets the cyclic GC know about subobjects that could participate in cycles:
staticintCustom_traverse(PyObject*op,visitprocvisit,void*arg){CustomObject*self=(CustomObject*)op;intvret;if(self->first){vret=visit(self->first,arg);if(vret!=0)returnvret;}if(self->last){vret=visit(self->last,arg);if(vret!=0)returnvret;}return0;}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
macro that automates calling visit functions. With Py_VISIT(), we can minimize the amount of boilerplate in Custom_traverse:
staticintCustom_traverse(PyObject*op,visitprocvisit,void*arg){CustomObject*self=(CustomObject*)op;Py_VISIT(self->first);Py_VISIT(self->last);return0;}Note
The
implementation must name its arguments exactly visit and arg in order to use
.
Second, we need to provide a method for clearing any subobjects that can participate in cycles:
staticintCustom_clear(PyObject*op){CustomObject*self=(CustomObject*)op;Py_CLEAR(self->first);Py_CLEAR(self->last);return0;}Notice the use of the
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
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
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
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
before clearing members. Here is our reimplemented deallocator using PyObject_GC_UnTrack() and Custom_clear:
staticvoidCustom_dealloc(PyObject*op){PyObject_GC_UnTrack(op);(void)Custom_clear(op);Py_TYPE(op)->tp_free(op);}Finally, we add the
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
or
handlers, we’d need to modify them for cyclic garbage collection. Most extensions will use the versions automatically provided.
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
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
type. The new type will be completely compatible with regular lists, but will have an additional increment() method that increases an internal counter:
>>> importsublist>>> 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>typedefstruct{PyListObjectlist;intstate;}SubListObject;staticPyObject*SubList_increment(PyObject*op,PyObject*Py_UNUSED(dummy)){SubListObject*self=(SubListObject*)op;self->state++;returnPyLong_FromLong(self->state);}staticPyMethodDefSubList_methods[]={{"increment",SubList_increment,METH_NOARGS,PyDoc_STR("increment state counter")},{NULL},};staticintSubList_init(PyObject*op,PyObject*args,PyObject*kwds){SubListObject*self=(SubListObject*)op;if(PyList_Type.tp_init(op,args,kwds)<0)return-1;self->state=0;return0;}staticPyTypeObjectSubListType={.ob_base=PyVarObject_HEAD_INIT(NULL,0).tp_name="sublist.SubList",.tp_doc=PyDoc_STR("SubList objects"),.tp_basicsize=sizeof(SubListObject),.tp_itemsize=0,.tp_flags=Py_TPFLAGS_DEFAULT|Py_TPFLAGS_BASETYPE,.tp_init=SubList_init,.tp_methods=SubList_methods,};staticintsublist_module_exec(PyObject*m){SubListType.tp_base=&PyList_Type;if(PyType_Ready(&SubListType)<0){return-1;}if(PyModule_AddObjectRef(m,"SubList",(PyObject*)&SubListType)<0){return-1;}return0;}staticPyModuleDef_Slotsublist_module_slots[]={{Py_mod_exec,sublist_module_exec},{Py_mod_multiple_interpreters,Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},{0,NULL}};staticPyModuleDefsublist_module={.m_base=PyModuleDef_HEAD_INIT,.m_name="sublist",.m_doc="Example module that creates an extension type.",.m_size=0,.m_slots=sublist_module_slots,};PyMODINIT_FUNCPyInit_sublist(void){returnPyModuleDef_Init(&sublist_module);}As you can see, the source code closely resembles the Custom examples in previous sections. We will break down the main differences between them.
typedefstruct{PyListObjectlist;intstate;}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
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*:
staticintSubList_init(PyObject*op,PyObject*args,PyObject*kwds){SubListObject*self=(SubListObject*)op;if(PyList_Type.tp_init(op,args,kwds)<0)return-1;self->state=0;return0;}We see above how to call through to the
method of the base type.
This pattern is important when writing a type with custom
and
members. The tp_new handler should not actually create the memory for the object with its
, but let the base class handle it by calling its own tp_new.
The
struct supports a
specifying the type’s concrete base class. Due to cross-platform compiler issues, you can’t fill that field directly with a reference to
; it should be done in the
function:
staticintsublist_module_exec(PyObject*m){SubListType.tp_base=&PyList_Type;if(PyType_Ready(&SubListType)<0){return-1;}if(PyModule_AddObjectRef(m,"SubList",(PyObject*)&SubListType)<0){return-1;}return0;}Before calling
, the type structure must have the
slot filled in. When we are deriving an existing type, it is not necessary to fill out the
slot with
– the allocation function from the base type will be inherited.
After that, calling
and adding the type object to the module is the same as with the basic Custom examples.
Footnotes