### 導航 - [索引](../genindex.xhtml "總目錄") - [模塊](../py-modindex.xhtml "Python 模塊索引") | - [下一頁](building.xhtml "4. 構建C/C++擴展") | - [上一頁](newtypes_tutorial.xhtml "2. 自定義擴展類型：教程") | -  - [Python](https://www.python.org/) ? - zh\_CN 3.7.3 [文檔](../index.xhtml) ? - [擴展和嵌入 Python 解釋器](index.xhtml) ? - $('.inline-search').show(0); | # 3. 定義擴展類型：已分類主題 本章節目標是提供一個各種你可以實現的類型方法及其功能的簡短介紹。 這是C類型 [`PyTypeObject`](../c-api/type.xhtml#c.PyTypeObject "PyTypeObject") 的定義，省略了只用于調試構建的字段: ``` typedef struct _typeobject { PyObject_VAR_HEAD const char *tp_name; /* For printing, in format "<module>.<name>" */ Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */ /* Methods to implement standard operations */ destructor tp_dealloc; printfunc tp_print; getattrfunc tp_getattr; setattrfunc tp_setattr; PyAsyncMethods *tp_as_async; /* formerly known as tp_compare (Python 2) or tp_reserved (Python 3) */ 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 */ unsigned long tp_flags; const char *tp_doc; /* Documentation string */ /* call function for all accessible objects */ traverseproc tp_traverse; /* delete references to contained objects */ inquiry tp_clear; /* rich comparisons */ richcmpfunc tp_richcompare; /* weak reference enabler */ Py_ssize_t tp_weaklistoffset; /* 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; Py_ssize_t 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; destructor tp_del; /* Type attribute cache version tag. Added in version 2.6 */ unsigned int tp_version_tag; destructor tp_finalize; } PyTypeObject; ``` 這里有 *很多* 方法。但是不要太擔心，如果你要定義一個類型，通常只需要實現少量的方法。 正如你猜到的一樣，我們正要一步一步詳細介紹各種處理程序。因為有大量的歷史包袱影響字段的排序，所以我們不會根據它們在結構體里定義的順序講解。通常非常容易找到一個包含你需要的字段的例子，然后改變值去適應你新的類型。 ``` const char *tp_name; /* For printing */ ``` 類型的名字 - 上一章提到過的，會出現在很多地方，幾乎全部都是為了診斷目的。嘗試選擇一個好名字，對于診斷很有幫助。 ``` Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */ ``` 這些字段告訴運行時在創造這個類型的新對象時需要分配多少內存。Python為了可變長度的結構（想下：字符串，元組）有些內置支持，這是 [`tp_itemsize`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_itemsize "PyTypeObject.tp_itemsize") 字段存在的原由。這部分稍后解釋。 ``` const char *tp_doc; ``` 這里你可以放置一段字符串（或者它的地址），當你想在Python腳本引用 `obj.__doc__` 時返回這段文檔字符串。 現在我們來看一下基本類型方法 - 大多數擴展類型將實現的方法。 ## 3.1. 終結和內存釋放 ``` destructor tp_dealloc; ``` 當您的類型實例的引用計數減少為零并且Python解釋器想要回收它時，將調用此函數。如果你的類型有內存可供釋放或執行其他清理，你可以把它放在這里。 對象本身也需要在這里釋放。 以下是此函數的示例： ``` 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()`](../c-api/exceptions.xhtml#c.PyErr_Fetch "PyErr_Fetch") and [`PyErr_Restore()`](../c-api/exceptions.xhtml#c.PyErr_Restore "PyErr_Restore") functions: ``` static void my_dealloc(PyObject *obj) { MyObject *self = (MyObject *) obj; PyObject *cbresult; if (self->my_callback != NULL) { PyObject *err_type, *err_value, *err_traceback; /* This saves the current exception state */ PyErr_Fetch(&err_type, &err_value, &err_traceback); cbresult = PyObject_CallObject(self->my_callback, NULL); if (cbresult == NULL) PyErr_WriteUnraisable(self->my_callback); else Py_DECREF(cbresult); /* This restores the saved exception state */ PyErr_Restore(err_type, err_value, err_traceback); Py_DECREF(self->my_callback); } Py_TYPE(obj)->tp_free((PyObject*)self); } ``` 注解 There are limitations to what you can safely do in a deallocator function. First, if your type supports garbage collection (using [`tp_traverse`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_traverse "PyTypeObject.tp_traverse")and/or [`tp_clear`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_clear "PyTypeObject.tp_clear")), some of the object's members can have been cleared or finalized by the time [`tp_dealloc`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_dealloc "PyTypeObject.tp_dealloc") is called. Second, in [`tp_dealloc`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_dealloc "PyTypeObject.tp_dealloc"), your object is in an unstable state: its reference count is equal to zero. Any call to a non-trivial object or API (as in the example above) might end up calling [`tp_dealloc`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_dealloc "PyTypeObject.tp_dealloc") again, causing a double free and a crash. 從 Python 3.4 開始，推薦不要在 [`tp_dealloc`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_dealloc "PyTypeObject.tp_dealloc") 放復雜的終結代碼，而是使用新的 [`tp_finalize`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_finalize "PyTypeObject.tp_finalize") 類型方法。 參見 [**PEP 442**](https://www.python.org/dev/peps/pep-0442) \[https://www.python.org/dev/peps/pep-0442\] 解釋了新的終結方案。 ## 3.2. 對象展示 In Python, there are two ways to generate a textual representation of an object: the [`repr()`](../library/functions.xhtml#repr "repr") function, and the [`str()`](../library/stdtypes.xhtml#str "str") function. (The [`print()`](../library/functions.xhtml#print "print")function just calls [`str()`](../library/stdtypes.xhtml#str "str").) These handlers are both optional. ``` reprfunc tp_repr; reprfunc tp_str; ``` The [`tp_repr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_repr "PyTypeObject.tp_repr") handler should return a string object containing a representation of the instance for which it is called. Here is a simple example: ``` static PyObject * newdatatype_repr(newdatatypeobject * obj) { return PyUnicode_FromFormat("Repr-ified_newdatatype{{size:%d}}", obj->obj_UnderlyingDatatypePtr->size); } ``` If no [`tp_repr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_repr "PyTypeObject.tp_repr") handler is specified, the interpreter will supply a representation that uses the type's [`tp_name`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_name "PyTypeObject.tp_name") and a uniquely-identifying value for the object. The [`tp_str`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_str "PyTypeObject.tp_str") handler is to [`str()`](../library/stdtypes.xhtml#str "str") what the [`tp_repr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_repr "PyTypeObject.tp_repr") handler described above is to [`repr()`](../library/functions.xhtml#repr "repr"); that is, it is called when Python code calls [`str()`](../library/stdtypes.xhtml#str "str") on an instance of your object. Its implementation is very similar to the [`tp_repr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_repr "PyTypeObject.tp_repr") function, but the resulting string is intended for human consumption. If [`tp_str`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_str "PyTypeObject.tp_str") is not specified, the [`tp_repr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_repr "PyTypeObject.tp_repr") handler is used instead. Here is a simple example: ``` static PyObject * newdatatype_str(newdatatypeobject * obj) { return PyUnicode_FromFormat("Stringified_newdatatype{{size:%d}}", obj->obj_UnderlyingDatatypePtr->size); } ``` ## 3.3. Attribute Management For every object which can support attributes, the corresponding type must provide the functions that control how the attributes are resolved. There needs to be a function which can retrieve attributes (if any are defined), and another to set attributes (if setting attributes is allowed). Removing an attribute is a special case, for which the new value passed to the handler is *NULL*. Python supports two pairs of attribute handlers; a type that supports attributes only needs to implement the functions for one pair. The difference is that one pair takes the name of the attribute as a `char*`, while the other accepts a [`PyObject*`](../c-api/structures.xhtml#c.PyObject "PyObject"). Each type can use whichever pair makes more sense for the implementation's convenience. ``` getattrfunc tp_getattr; /* char * version */ setattrfunc tp_setattr; /* ... */ getattrofunc tp_getattro; /* PyObject * version */ setattrofunc tp_setattro; ``` If accessing attributes of an object is always a simple operation (this will be explained shortly), there are generic implementations which can be used to provide the [`PyObject*`](../c-api/structures.xhtml#c.PyObject "PyObject") version of the attribute management functions. The actual need for type-specific attribute handlers almost completely disappeared starting with Python 2.2, though there are many examples which have not been updated to use some of the new generic mechanism that is available. ### 3.3.1. Generic Attribute Management Most extension types only use *simple* attributes. So, what makes the attributes simple? There are only a couple of conditions that must be met: 1. The name of the attributes must be known when [`PyType_Ready()`](../c-api/type.xhtml#c.PyType_Ready "PyType_Ready") is called. 2. No special processing is needed to record that an attribute was looked up or set, nor do actions need to be taken based on the value. Note that this list does not place any restrictions on the values of the attributes, when the values are computed, or how relevant data is stored. When [`PyType_Ready()`](../c-api/type.xhtml#c.PyType_Ready "PyType_Ready") is called, it uses three tables referenced by the type object to create [descriptor](../glossary.xhtml#term-descriptor)s which are placed in the dictionary of the type object. Each descriptor controls access to one attribute of the instance object. Each of the tables is optional; if all three are *NULL*, instances of the type will only have attributes that are inherited from their base type, and should leave the [`tp_getattro`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_getattro "PyTypeObject.tp_getattro") and [`tp_setattro`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_setattro "PyTypeObject.tp_setattro") fields *NULL* as well, allowing the base type to handle attributes. The tables are declared as three fields of the type object: ``` struct PyMethodDef *tp_methods; struct PyMemberDef *tp_members; struct PyGetSetDef *tp_getset; ``` If [`tp_methods`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_methods "PyTypeObject.tp_methods") is not *NULL*, it must refer to an array of [`PyMethodDef`](../c-api/structures.xhtml#c.PyMethodDef "PyMethodDef") structures. Each entry in the table is an instance of this structure: ``` typedef struct PyMethodDef { const char *ml_name; /* method name */ PyCFunction ml_meth; /* implementation function */ int ml_flags; /* flags */ const char *ml_doc; /* docstring */ } PyMethodDef; ``` One entry should be defined for each method provided by the type; no entries are needed for methods inherited from a base type. One additional entry is needed at the end; it is a sentinel that marks the end of the array. The `ml_name` field of the sentinel must be *NULL*. The second table is used to define attributes which map directly to data stored in the instance. A variety of primitive C types are supported, and access may be read-only or read-write. The structures in the table are defined as: ``` typedef struct PyMemberDef { const char *name; int type; int offset; int flags; const char *doc; } PyMemberDef; ``` For each entry in the table, a [descriptor](../glossary.xhtml#term-descriptor) will be constructed and added to the type which will be able to extract a value from the instance structure. The [`type`](../library/functions.xhtml#type "type") field should contain one of the type codes defined in the `structmember.h` header; the value will be used to determine how to convert Python values to and from C values. The `flags` field is used to store flags which control how the attribute can be accessed. The following flag constants are defined in `structmember.h`; they may be combined using bitwise-OR. 常數 意義 `READONLY` Never writable. `READ_RESTRICTED` Not readable in restricted mode. `WRITE_RESTRICTED` Not writable in restricted mode. `RESTRICTED` Not readable or writable in restricted mode. An interesting advantage of using the [`tp_members`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_members "PyTypeObject.tp_members") table to build descriptors that are used at runtime is that any attribute defined this way can have an associated doc string simply by providing the text in the table. An application can use the introspection API to retrieve the descriptor from the class object, and get the doc string using its `__doc__` attribute. As with the [`tp_methods`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_methods "PyTypeObject.tp_methods") table, a sentinel entry with a `name` value of *NULL* is required. ### 3.3.2. Type-specific Attribute Management For simplicity, only the `char*` version will be demonstrated here; the type of the name parameter is the only difference between the `char*`and [`PyObject*`](../c-api/structures.xhtml#c.PyObject "PyObject") flavors of the interface. This example effectively does the same thing as the generic example above, but does not use the generic support added in Python 2.2. It explains how the handler functions are called, so that if you do need to extend their functionality, you'll understand what needs to be done. The [`tp_getattr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_getattr "PyTypeObject.tp_getattr") handler is called when the object requires an attribute look-up. It is called in the same situations where the [`__getattr__()`](../reference/datamodel.xhtml#object.__getattr__ "object.__getattr__")method of a class would be called. Here is an example: ``` static PyObject * newdatatype_getattr(newdatatypeobject *obj, char *name) { if (strcmp(name, "data") == 0) { return PyLong_FromLong(obj->data); } PyErr_Format(PyExc_AttributeError, "'%.50s' object has no attribute '%.400s'", tp->tp_name, name); return NULL; } ``` The [`tp_setattr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_setattr "PyTypeObject.tp_setattr") handler is called when the [`__setattr__()`](../reference/datamodel.xhtml#object.__setattr__ "object.__setattr__") or [`__delattr__()`](../reference/datamodel.xhtml#object.__delattr__ "object.__delattr__") method of a class instance would be called. When an attribute should be deleted, the third parameter will be *NULL*. Here is an example that simply raises an exception; if this were really all you wanted, the [`tp_setattr`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_setattr "PyTypeObject.tp_setattr") handler should be set to *NULL*. ``` static int newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v) { PyErr_Format(PyExc_RuntimeError, "Read-only attribute: %s", name); return -1; } ``` ## 3.4. Object Comparison ``` richcmpfunc tp_richcompare; ``` The [`tp_richcompare`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_richcompare "PyTypeObject.tp_richcompare") handler is called when comparisons are needed. It is analogous to the [rich comparison methods](../reference/datamodel.xhtml#richcmpfuncs), like [`__lt__()`](../reference/datamodel.xhtml#object.__lt__ "object.__lt__"), and also called by [`PyObject_RichCompare()`](../c-api/object.xhtml#c.PyObject_RichCompare "PyObject_RichCompare") and [`PyObject_RichCompareBool()`](../c-api/object.xhtml#c.PyObject_RichCompareBool "PyObject_RichCompareBool"). This function is called with two Python objects and the operator as arguments, where the operator is one of `Py_EQ`, `Py_NE`, `Py_LE`, `Py_GT`, `Py_LT` or `Py_GT`. It should compare the two objects with respect to the specified operator and return `Py_True` or `Py_False` if the comparison is successful, `Py_NotImplemented` to indicate that comparison is not implemented and the other object's comparison method should be tried, or *NULL*if an exception was set. Here is a sample implementation, for a datatype that is considered equal if the size of an internal pointer is equal: ``` static PyObject * newdatatype_richcmp(PyObject *obj1, PyObject *obj2, int op) { PyObject *result; int c, size1, size2; /* code to make sure that both arguments are of type newdatatype omitted */ size1 = obj1->obj_UnderlyingDatatypePtr->size; size2 = obj2->obj_UnderlyingDatatypePtr->size; switch (op) { case Py_LT: c = size1 < size2; break; case Py_LE: c = size1 <= size2; break; case Py_EQ: c = size1 == size2; break; case Py_NE: c = size1 != size2; break; case Py_GT: c = size1 > size2; break; case Py_GE: c = size1 >= size2; break; } result = c ? Py_True : Py_False; Py_INCREF(result); return result; } ``` ## 3.5. Abstract Protocol Support Python supports a variety of *abstract* 'protocols;' the specific interfaces provided to use these interfaces are documented in [抽象對象層](../c-api/abstract.xhtml#abstract). A number of these abstract interfaces were defined early in the development of the Python implementation. In particular, the number, mapping, and sequence protocols have been part of Python since the beginning. Other protocols have been added over time. For protocols which depend on several handler routines from the type implementation, the older protocols have been defined as optional blocks of handlers referenced by the type object. For newer protocols there are additional slots in the main type object, with a flag bit being set to indicate that the slots are present and should be checked by the interpreter. (The flag bit does not indicate that the slot values are non-*NULL*. The flag may be set to indicate the presence of a slot, but a slot may still be unfilled.) ``` PyNumberMethods *tp_as_number; PySequenceMethods *tp_as_sequence; PyMappingMethods *tp_as_mapping; ``` If you wish your object to be able to act like a number, a sequence, or a mapping object, then you place the address of a structure that implements the C type [`PyNumberMethods`](../c-api/typeobj.xhtml#c.PyNumberMethods "PyNumberMethods"), [`PySequenceMethods`](../c-api/typeobj.xhtml#c.PySequenceMethods "PySequenceMethods"), or [`PyMappingMethods`](../c-api/typeobj.xhtml#c.PyMappingMethods "PyMappingMethods"), respectively. It is up to you to fill in this structure with appropriate values. You can find examples of the use of each of these in the `Objects` directory of the Python source distribution. ``` hashfunc tp_hash; ``` This function, if you choose to provide it, should return a hash number for an instance of your data type. Here is a simple example: ``` static Py_hash_t newdatatype_hash(newdatatypeobject *obj) { Py_hash_t result; result = obj->some_size + 32767 * obj->some_number; if (result == -1) result = -2; return result; } ``` `Py_hash_t` is a signed integer type with a platform-varying width. Returning `-1` from [`tp_hash`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_hash "PyTypeObject.tp_hash") indicates an error, which is why you should be careful to avoid returning it when hash computation is successful, as seen above. ``` ternaryfunc tp_call; ``` This function is called when an instance of your data type is "called", for example, if `obj1` is an instance of your data type and the Python script contains `obj1('hello')`, the [`tp_call`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_call "PyTypeObject.tp_call") handler is invoked. This function takes three arguments: 1. *self* is the instance of the data type which is the subject of the call. If the call is `obj1('hello')`, then *self* is `obj1`. 2. *args* is a tuple containing the arguments to the call. You can use [`PyArg_ParseTuple()`](../c-api/arg.xhtml#c.PyArg_ParseTuple "PyArg_ParseTuple") to extract the arguments. 3. *kwds* is a dictionary of keyword arguments that were passed. If this is non-*NULL* and you support keyword arguments, use [`PyArg_ParseTupleAndKeywords()`](../c-api/arg.xhtml#c.PyArg_ParseTupleAndKeywords "PyArg_ParseTupleAndKeywords") to extract the arguments. If you do not want to support keyword arguments and this is non-*NULL*, raise a [`TypeError`](../library/exceptions.xhtml#TypeError "TypeError") with a message saying that keyword arguments are not supported. Here is a toy `tp_call` implementation: ``` static PyObject * newdatatype_call(newdatatypeobject *self, PyObject *args, PyObject *kwds) { PyObject *result; const char *arg1; const char *arg2; const char *arg3; if (!PyArg_ParseTuple(args, "sss:call", &arg1, &arg2, &arg3)) { return NULL; } result = PyUnicode_FromFormat( "Returning -- value: [%d] arg1: [%s] arg2: [%s] arg3: [%s]\n", obj->obj_UnderlyingDatatypePtr->size, arg1, arg2, arg3); return result; } ``` ``` /* Iterators */ getiterfunc tp_iter; iternextfunc tp_iternext; ``` These functions provide support for the iterator protocol. Both handlers take exactly one parameter, the instance for which they are being called, and return a new reference. In the case of an error, they should set an exception and return *NULL*. [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter") corresponds to the Python [`__iter__()`](../reference/datamodel.xhtml#object.__iter__ "object.__iter__") method, while [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext")corresponds to the Python [`__next__()`](../library/stdtypes.xhtml#iterator.__next__ "iterator.__next__") method. Any [iterable](../glossary.xhtml#term-iterable) object must implement the [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter")handler, which must return an [iterator](../glossary.xhtml#term-iterator) object. Here the same guidelines apply as for Python classes: - For collections (such as lists and tuples) which can support multiple independent iterators, a new iterator should be created and returned by each call to [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter"). - Objects which can only be iterated over once (usually due to side effects of iteration, such as file objects) can implement [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter")by returning a new reference to themselves -- and should also therefore implement the [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext") handler. Any [iterator](../glossary.xhtml#term-iterator) object should implement both [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter")and [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext"). An iterator's [`tp_iter`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iter "PyTypeObject.tp_iter") handler should return a new reference to the iterator. Its [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext") handler should return a new reference to the next object in the iteration, if there is one. If the iteration has reached the end, [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext")may return *NULL* without setting an exception, or it may set [`StopIteration`](../library/exceptions.xhtml#StopIteration "StopIteration") *in addition* to returning *NULL*; avoiding the exception can yield slightly better performance. If an actual error occurs, [`tp_iternext`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_iternext "PyTypeObject.tp_iternext") should always set an exception and return *NULL*. ## 3.6. Weak Reference Support One of the goals of Python's weak reference implementation is to allow any type to participate in the weak reference mechanism without incurring the overhead on performance-critical objects (such as numbers). 參見 Documentation for the [`weakref`](../library/weakref.xhtml#module-weakref "weakref: Support for weak references and weak dictionaries.") module. For an object to be weakly referencable, the extension type must do two things: 1. Include a [`PyObject*`](../c-api/structures.xhtml#c.PyObject "PyObject") field in the C object structure dedicated to the weak reference mechanism. The object's constructor should leave it *NULL* (which is automatic when using the default [`tp_alloc`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_alloc "PyTypeObject.tp_alloc")). 2. Set the [`tp_weaklistoffset`](../c-api/typeobj.xhtml#c.PyTypeObject.tp_weaklistoffset "PyTypeObject.tp_weaklistoffset") type member to the offset of the aforementioned field in the C object structure, so that the interpreter knows how to access and modify that field. Concretely, here is how a trivial object structure would be augmented with the required field: ``` typedef struct { PyObject_HEAD PyObject *weakreflist; /* List of weak references */ } TrivialObject; ``` And the corresponding member in the statically-declared type object: ``` static PyTypeObject TrivialType = { PyVarObject_HEAD_INIT(NULL, 0) /* ... other members omitted for brevity ... */ .tp_weaklistoffset = offsetof(TrivialObject, weakreflist), }; ``` The only further addition is that `tp_dealloc` needs to clear any weak references (by calling `PyObject_ClearWeakRefs()`) if the field is non-*NULL*: ``` static void Trivial_dealloc(TrivialObject *self) { /* Clear weakrefs first before calling any destructors */ if (self->weakreflist != NULL) PyObject_ClearWeakRefs((PyObject *) self); /* ... remainder of destruction code omitted for brevity ... */ Py_TYPE(self)->tp_free((PyObject *) self); } ``` ## 3.7. 更多建議 In order to learn how to implement any specific method for your new data type, get the [CPython](../glossary.xhtml#term-cpython) source code. Go to the `Objects` directory, then search the C source files for `tp_` plus the function you want (for example, `tp_richcompare`). You will find examples of the function you want to implement. When you need to verify that an object is a concrete instance of the type you are implementing, use the [`PyObject_TypeCheck()`](../c-api/object.xhtml#c.PyObject_TypeCheck "PyObject_TypeCheck") function. A sample of its use might be something like the following: ``` if (!PyObject_TypeCheck(some_object, &MyType)) { PyErr_SetString(PyExc_TypeError, "arg #1 not a mything"); return NULL; } ``` 參見 下載CPython源代碼版本。<https://www.python.org/downloads/source/> GitHub上開發CPython源代碼的CPython項目。<https://github.com/python/cpython> ### 導航 - [索引](../genindex.xhtml "總目錄") - [模塊](../py-modindex.xhtml "Python 模塊索引") | - [下一頁](building.xhtml "4. 構建C/C++擴展") | - [上一頁](newtypes_tutorial.xhtml "2. 自定義擴展類型：教程") | -  - [Python](https://www.python.org/) ? - zh\_CN 3.7.3 [文檔](../index.xhtml) ? - [擴展和嵌入 Python 解釋器](index.xhtml) ? - $('.inline-search').show(0); | ? [版權所有](../copyright.xhtml) 2001-2019, Python Software Foundation. Python 軟件基金會是一個非盈利組織。 [請捐助。](https://www.python.org/psf/donations/) 最后更新于 5月 21, 2019. [發現了問題](../bugs.xhtml)？ 使用[Sphinx](http://sphinx.pocoo.org/)1.8.4 創建。