Python源代码学习-对象机制

2018-05-10

在Python中, 万物皆对象，而所有对象的共性内容都被定义在PyObject中。PyObject可谓是Python对象机制的基石。
本文将简要介绍Python对象机制的实现, 以及其涉及的数据结构和原理。

对象机制基石-`PyObject`

在介绍Python对象之前，首先有必要介绍一下PyObject和PyVarObject的定义。通过typedef定义了结构体_object新的数据类型名: PyObject,而PyObject的核心定义在宏PyObject_HEAD。而PyVarObject实际是在PyObject_HEAD的基础上增加了变长数量ob_size字段。

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11 // [object.h]
// 1. 指向定长python object对象的指针都可以转换为PyObject* 
// 2. 指向变长python object对象的指针都可以转换为PyVarObject*
typedef struct _object {
    PyObject_HEAD
} PyObject;
typedef struct {
    PyObject_VAR_HEAD
} PyVarObject;

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30 // PyObject_HEAD 定义
/* PyObject_HEAD defines the initial segment of every PyObject. */
#define PyObject_HEAD                   \
    _PyObject_HEAD_EXTRA                \
    Py_ssize_t ob_refcnt;               \
    struct _typeobject *ob_type;
// PyObject_VAR_HEAD 定义
/* PyObject_VAR_HEAD defines the initial segment of all variable-size
 * container objects.  These end with a declaration of an array with 1
 * element, but enough space is malloc'ed so that the array actually
 * has room for ob_size elements.  Note that ob_size is an element count,
 * not necessarily a byte count.
 */
#define PyObject_VAR_HEAD               \
    PyObject_HEAD                       \
    Py_ssize_t ob_size; /* Number of items in variable part */
// _PyObject_HEAD_EXTRA 定义
#ifdef Py_TRACE_REFS
/* Define pointers to support a doubly-linked list of all live heap objects. */
#define _PyObject_HEAD_EXTRA            \
    struct _object *_ob_next;           \
    struct _object *_ob_prev;
#define _PyObject_EXTRA_INIT 0, 0,
#else
#define _PyObject_HEAD_EXTRA
#define _PyObject_EXTRA_INIT
#endif

在release模式编译Python, 数据类型名PyObject定义如下:

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8 /*
 * _typeobject: Python内部一种特殊对象, 用来指定对象类型的类型对象
 * ob_refcnt: Python使用基于引用计数的垃圾回收机制
 */
typedef struct _object {
    Py_ssize_t ob_refcnt;        // 引用计数
    struct _typeobject *ob_type; // 指向结构体对象类型的类型对象指针
} PyObject;

对象类型

定长对象vs变长对象

对象机制基石-PyObject

对象机制基石-`PyObject`