python 3.3 dict : how to convert struct PyDictKeysObject to python class?

我正在尝试修改Brandon Rhodes代码Routines that examine the internals of a CPython dictionary，使其适用于CPython 3.3。

我相信我已经成功翻译了这个结构。

typedef PyDictKeyEntry *(*dict_lookup_func)
    (PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject ***value_addr);

struct _dictkeysobject {
    Py_ssize_t dk_refcnt;
    Py_ssize_t dk_size;
    dict_lookup_func dk_lookup;
    Py_ssize_t dk_usable;
    PyDictKeyEntry dk_entries[1];
};

我认为以下内容看起来不错:

from ctypes import Structure, c_ulong, POINTER, cast, py_object, CFUNCTYPE

LOOKUPFUNC = CFUNCTYPE(POINTER(PyDictKeyEntry), POINTER(PyDictObject),
                       py_object, c_ulong, POINTER(POINTER(py_object)))

class PyDictKeysObject(Structure):
"""A key object"""
_fields_ = [
    ('dk_refcnt', c_ssize_t),
    ('dk_size', c_ssize_t),
    ('dk_lookup', LOOKUPFUNC),
    ('dk_usable', c_ssize_t),
    ('dk_entries', PyDictKeyEntry * 1),
]

PyDictKeysObject._dk_entries = PyDictKeysObject.dk_entries
PyDictKeysObject.dk_entries = property(lambda s:
    cast(s._dk_entries, POINTER(PyDictKeyEntry * s.dk_size))[0])

这行代码现在可以使用了，其中d == {0: 0, 1: 1, 2: 2, 3: 3}:

obj = cast(id(d), POINTER(PyDictObject)).contents  # works!!`

这是我从C struct PyDictObject的翻译:

class PyDictObject(Structure):  # an incomplete type
    """A dictionary object."""

def __len__(self):
    """Return the number of dictionary entry slots."""
    pass

def slot_of(self, key):
    """Find and return the slot at which `key` is stored."""
    pass

def slot_map(self):
    """Return a mapping of keys to their integer slot numbers."""
    pass

PyDictObject._fields_ = [
    ('ob_refcnt', c_ssize_t),
    ('ob_type', c_void_p),
    ('ma_used', c_ssize_t),
    ('ma_keys', POINTER(PyDictKeysObject)),
    ('ma_values', POINTER(py_object)),  # points to array of ptrs
]

最佳答案

我的问题是访问在Cpython 3.3中实现的python字典下的C结构。我从cpython/Objects/dictobject.c和Include/dictobject.h中提供的C结构开始。定义字典涉及三个C结构:PyDictObject，PyDictKeysObject和PyDictKeyEntry。每个C结构到python的正确翻译如下。这些注释表明我需要在哪里进行修复。感谢@eryksun在整个过程中指导我!!

class PyDictKeyEntry(Structure):
"""An entry in a dictionary."""
    _fields_ = [
        ('me_hash', c_ulong),
        ('me_key', py_object),
        ('me_value', py_object),
    ]

class PyDictObject(Structure):
    """A dictionary object."""
    pass

LOOKUPFUNC = CFUNCTYPE(POINTER(PyDictKeyEntry), POINTER(PyDictObject), py_object, c_ulong, POINTER(POINTER(py_object)))

class PyDictKeysObject(Structure):
"""An object of key entries."""
    _fields_ = [
        ('dk_refcnt', c_ssize_t),
        ('dk_size', c_ssize_t),
        ('dk_lookup', LOOKUPFUNC),  # a function prototype per docs
        ('dk_usable', c_ssize_t),
        ('dk_entries', PyDictKeyEntry * 1),  # an array of size 1; size grows as keys are inserted into dictionary; this variable-sized field was the trickiest part to translate into python
    ]

PyDictObject._fields_ = [
    ('ob_refcnt', c_ssize_t),  # Py_ssize_t translates to c_ssize_t per ctypes docs
    ('ob_type', c_void_p),     # could not find this in the docs
    ('ma_used', c_ssize_t),
    ('ma_keys', POINTER(PyDictKeysObject)),
    ('ma_values', POINTER(py_object)),  # Py_Object* translates to py_object per ctypes docs
]

PyDictKeysObject._dk_entries = PyDictKeysObject.dk_entries
PyDictKeysObject.dk_entries = property(lambda s: cast(s._dk_entries, POINTER(PyDictKeyEntry * s.dk_size))[0])  # this line is called every time the attribute dk_entries is accessed by a PyDictKeyEntry instance; it returns an array of size dk_size starting at address _dk_entries. (POINTER creates a pointer to the entire array; the pointer is dereferenced (using [0]) to return the entire array); the code then accesses the ith element of the array)

以下函数提供对python字典基础的PyDictObject的访问:

def dictobject(d):
    """Return the PyDictObject lying behind the Python dict `d`."""
    if not isinstance(d, dict):
        raise TypeError('cannot create a dictobject from %r' % (d,))
    return cast(id(d), POINTER(PyDictObject)).contents

如果d是具有键值对的python字典，则obj是包含键值对的PyDictObject实例:

obj = cast(id(d), POINTER(PyDictObject)).contents

PyDictKeysObject的一个实例是:

key_obj = obj.ma_keys.contents

指向存储在字典插槽0中的键的指针是:

key_obj.dk_entries[0].me_key

使用这些类以及检测插入字典中每个键的哈希冲突的例程的程序位于here中。我的代码是Brandon Rhodes为python 2.x编写的代码的修改。他的代码是here。