阅读源码的版本: cmmit id: 9e4f2f3a6b8ee995c365e86d976937c141d867f8
在阅读Python的源码时，我们第一个想到的就是先去阅读Python的数据结构的源码。
那么，我们先去挨着看看。
数据结构
long
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struct _longobject {
PyObject_VAR_HEAD
digit ob_digit[1];
};
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list
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typedef struct {
PyObject_VAR_HEAD
/* Vector of pointers to list elements. list[0] is ob_item[0], etc. */
PyObject **ob_item;
/* ob_item contains space for 'allocated' elements. The number
* currently in use is ob_size.
* Invariants:
* 0 <= ob_size <= allocated
* len(list) == ob_size
* ob_item == NULL implies ob_size == allocated == 0
* list.sort() temporarily sets allocated to -1 to detect mutations.
*
* Items must normally not be NULL, except during construction when
* the list is not yet visible outside the function that builds it.
*/
Py_ssize_t allocated;
} PyListObject;
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tuple
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typedef struct {
PyObject_VAR_HEAD
/* ob_item contains space for 'ob_size' elements.
Items must normally not be NULL, except during construction when
the tuple is not yet visible outside the function that builds it. */
PyObject *ob_item[1];
} PyTupleObject;
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纵观上面几种数据结构，我们发现，在struct的开头都嵌入了一个叫做PyObject_HEAD的宏，那么这个宏定义到底是什么呢？
PyObject_VAR_HEAD 定义
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/* 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.
*/
/* PyObject_VAR_HEAD定义所有可变长度对象的首段信息。
* 这些对象在结束部分会声明一个长度为1的数组，
* 但是实际上已经有充足的空间申请，以至于这个数组实际上只有ob_size个元素。
* 注意这个ob_size是一个元素数量。
*/
#define PyObject_VAR_HEAD PyVarObject ob_base;
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PyVarObject 定义
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typedef struct {
PyObject ob_base;
Py_ssize_t ob_size; /* Number of items in variable part */
} PyVarObject;
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PyObject 定义
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/* Nothing is actually declared to be a PyObject, but every pointer to
* a Python object can be cast to a PyObject*. This is inheritance built
* by hand. Similarly every pointer to a variable-size Python object can,
* in addition, be cast to PyVarObject*.
*/
typedef struct _object {
_PyObject_HEAD_EXTRA
Py_ssize_t ob_refcnt;
struct _typeobject *ob_type;
} PyObject;
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#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
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所以，我们最终可以解开这个所谓的PyObject_VAR_HEAD宏到底是什么了：
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struc PyObject_VAR_HEAD
{
struct _object *_ob_next;
struct _object *_ob_prev;
Py_ssize_t ob_refcnt;
struct _typeobject *ob_type;
Py_ssize_t ob_size;
}
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