python – ctypes为c_ulong重新实现rshift

我通过ctypes访问C库,我遇到了以下问题：

我正在使用ctypeslib生成一个“包装器”(使用ctypes访问库的ctypes命令).
C库包含在此步骤中转换为python函数的宏.
(为了尽可能独立于库内部,我想在python中使用这些宏中的一些.)

其中一个宏看起来像这样：

# using the ctypes types
myuint16_t = c_ushort
myuint32_t = c_ulong

def mymacro(x): return (myuint16_t)((myuint32_t)(x) >> 16) # macro

我想以下列方式在一个单独的模块中使用生成的函数(在函数内部)：

return wrapper.mymacro(valueToBeConverted) # valueToBeConverted is an int

但使用此行我收到以下错误：

....   
def mymacro(x): return (myuint16_t)((myuint32_t)(x) >> 16) # macro
TypeError: unsupported operand type(s) for >>: 'c_ulong' and 'int'

(我知道转移c_ulong的常用方法是c_ulongvar.value>> x但是每次C库中的某些更改时我都必须修补生成的包装.所以我尽量避免这种情况).

似乎c_ulong的__rshift__实现不能在这里使用.

print c_ulong.__rshift__
# throws AttributeError: type object 'c_ulong' has no attribute '__rshift__'

嗯,看起来很奇怪……所以我决定重新实现c_ulong的__rshift__方法让它工作：

from ctypes import *
from types import MethodType

def rshift(self, val):
    print self.value >> val

# create an unbound method which applies to all (even existing) instances
c_ulong.__rshift__ = MethodType(rshift, None, c_ulong)

a = c_ulong(1)
a >> 16

但它并没有解决问题.我仍然收到一个错误：

a >> 16
TypeError: unsupported operand type(s) for >>: 'c_ulong' and 'int'

是否有可能__rshift__方法只能用于同一个类的两个实例？我尝试了以下方法：

def rshift(self, val):
    print self.value >> int(val.value)

a = c_ulong(1)
a >> c_ulong(16) 

它的工作原理.但这也意味着我仍然需要修补生成的包装器.

所以：有人知道这里的诀窍吗？

更新：

@eryksun的解决方案奏效了.我在用：

from ctypes import *
# from types import MethodType

def _rshift(self, other):
    if hasattr(other, 'value'):
        other = other.value
    return c_ulong(self.value >> other)

def _lshift(self, other):
    if hasattr(other, 'value'):
        other = other.value
    return c_ulong(self.value << other)

def _coerce(self, other):
    try:
        return self, self.__class__(other)
    except TypeError:
        return NotImplemented

# Add the functions to the type. A method is created when
# accessed as an attribute of an instance.
c_ulong.__lshift__ = _lshift
c_ulong.__rshift__ = _rshift
c_ulong.__coerce__ = _coerce

解决方法:

由于_ctypes._SimpleCData类型没有Py_TPFLAGS_CHECKTYPES标志,因此2.x子类被视为在二进制操作中使用__coerce__的旧式数字.有关调用方案和函数binary_op1中的实现,请参见Objects/abstract.c.

出于演示目的,可以在类型对象上切换此标志,您只需要定义(隐含很多void *)直到tp_flags字段.

破解PyTypeObject

from ctypes import *
import _ctypes

Py_TPFLAGS_CHECKTYPES = 1 << 4

class PyTypeObject(Structure):
    _fields_ = (('ob_refcnt', c_ssize_t),
                ('ob_type', c_void_p),
                ('ob_size', c_ssize_t),
                ('tp_name', c_char_p),
                ('tp_basicsize', c_ssize_t),
                ('tp_itemsize', c_ssize_t),
                ('tp_dealloc', c_void_p),
                ('tp_print', c_void_p),
                ('tp_getattr', c_void_p),
                ('tp_setattr', c_void_p),
                ('tp_compare', c_void_p),
                ('tp_repr', c_void_p),
                ('tp_as_number', c_void_p),
                ('tp_as_sequence', c_void_p),
                ('tp_as_mapping', c_void_p),
                ('tp_hash', c_void_p),
                ('tp_call', c_void_p),
                ('tp_str', c_void_p),
                ('tp_getattro', c_void_p),
                ('tp_setattro', c_void_p),
                ('tp_as_buffer', c_void_p),
                ('tp_flags', c_long))

接下来,创建一个unsigned long子类,并使用from_address工厂为它创建一个PyTypeObject.获取内置id的地址,这是CPython特有的实现细节：

class c_ulong(_ctypes._SimpleCData):
    _type_ = "L"

    def __rshift__(self, other):
        print '__rshift__', self, other
        if hasattr(other, 'value'):
            other = other.value
        return c_ulong(self.value >> other)

c_ulong_type = PyTypeObject.from_address(id(c_ulong))

演示

>>> a = c_ulong(16)
>>> b = c_ulong(2)

>>> a >> b
__rshift__ c_ulong(16L) c_ulong(2L)
c_ulong(4L)

>>> a >> 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for >>: 'c_ulong' and 'int'

最后一步失败了.现在设置标志：

>>> c_ulong_type.tp_flags |= Py_TPFLAGS_CHECKTYPES

>>> a >> 2
__rshift__ c_ulong(16L) 2
c_ulong(4L)

问题解决了？但这是一个黑客.再次尝试使用__coerce__.

实施__coerce__

class c_ulong(_ctypes._SimpleCData):
    _type_ = "L"

    def __rshift__(self, other):
        print '__rshift__', self, other
        if hasattr(other, 'value'):
            other = other.value
        return c_ulong(self.value >> other)

    def __coerce__(self, other):
        print '__coerce__', self, other
        try:
            return self, self.__class__(other)
        except TypeError:
            return NotImplemented

演示

>>> a = c_ulong(16)
>>> b = c_ulong(2)

>>> a >> 2
__coerce__ c_ulong(16L) 2
__rshift__ c_ulong(16L) c_ulong(2L)
c_ulong(4L)

>>> 16 >> b
__coerce__ c_ulong(2L) 16
__rshift__ c_ulong(16L) c_ulong(2L)
c_ulong(4L)

当然,如果无法创建c_ulong,它会失败,例如浮点数：

>>> a >> 2.0
__coerce__ c_ulong(16L) 2.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for >>: 'c_ulong' and 'float'

标签：python,operator-overloading,bit-shift,ctypes
来源： https://codeday.me/bug/20190703/1370749.html