WIP: Python对象基础

[harveyqing]

00 Basics

• Python中，一切皆对象，类型也是一种对象

• Python包含内置对象和用户自定义对象

• Python中所有的类型对象都是被静态初始化的

• 对象一旦被创建，它在内存中的大小就是固定的

01 对象的基石 -- PyObject

定义于 [Include/object.h]，定义为：

typedef struct _object {
    _PyObject_HEAD_EXTRA
    Py_ssize_t ob_refcnt;
    struct _typeobject *ob_type;
} PyObject;

可知，PyObject是一个结构体，其包含三个部分：

_PyObject_HEAD_EXTRA

主要用于调试模式下的对象追踪：

#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

ob_refcnt

即 Object Reference Count，对象引用计数，类型为Py_ssize_t:

typedef intptr_t        Py_intptr_t;

/* Py_ssize_t is a signed integral type such that sizeof(Py_ssize_t) ==
 * sizeof(size_t).  C99 doesn't define such a thing directly (size_t is an
 * unsigned integral type).  See PEP 353 for details.
 */
#ifdef HAVE_SSIZE_T
typedef ssize_t         Py_ssize_t;
#elif SIZEOF_VOID_P == SIZEOF_SIZE_T
typedef Py_intptr_t     Py_ssize_t;
#else
#   error "Python needs a typedef for Py_ssize_t in pyport.h."
#endif

Py_ssize是一个与size_t等字节数的符号整型，C99中并未直接定义ssize_t（符号整型，定义了的size_t是无符号整型），特此定义此类型。 PEP 353

In Python 2.4, indices of sequences are restricted to the C type int. On 64-bit machines, sequences therefore cannot use the full address space, and are restricted to 2**31 elements. This PEP proposes to change this, introducing a platform-specific index type Py_ssize_t. An implementation of the proposed change is in http://svn.python.org/projects/python/branches/ssize_t .

可见，Py_ssize_t的引入是用以解决64位系统上计数、索引等数值范围的问题。

ob_refcnt主要是用来维护对象的引用计数，用以支持Python运行时的引用计数垃圾回收机制；引用计数的增、减主要依托于Py_INCREF和Py_DECREF两个宏：

/* Cast argument to PyObject* type. */
#define _PyObject_CAST(op) ((PyObject*)(op))

static inline void _Py_INCREF(PyObject *op)
{
    _Py_INC_REFTOTAL;  // 此宏仅用于调试模式下记录总的引用计数
    op->ob_refcnt++;
}

#define Py_INCREF(op) _Py_INCREF(_PyObject_CAST(op))


static inline void _Py_DECREF(const char *filename, int lineno,
                              PyObject *op)
{
    (void)filename; /* may be unused, shut up -Wunused-parameter */
    (void)lineno; /* may be unused, shut up -Wunused-parameter */
    _Py_DEC_REFTOTAL;
    if (--op->ob_refcnt != 0) {
#ifdef Py_REF_DEBUG
        if (op->ob_refcnt < 0) {
            _Py_NegativeRefcount(filename, lineno, op);
        }
#endif
    }
    else {
        _Py_Dealloc(op);
    }
}

#define Py_DECREF(op) _Py_DECREF(__FILE__, __LINE__, _PyObject_CAST(op))

void
_Py_Dealloc(PyObject *op)
{
    destructor dealloc = Py_TYPE(op)->tp_dealloc;  // 调用对象所属类型对象的``tp_dealloc``所指向的方法
#ifdef Py_TRACE_REFS
    _Py_ForgetReference(op);
#else
    _Py_INC_TPFREES(op);
#endif
    (*dealloc)(op);
}

#ifdef __cplusplus
}
#endif

ob_type

即Object Type，指明了对象所属类型，定义如下：

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;
    Py_ssize_t tp_vectorcall_offset;
    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;