自引用类：来自 C 接口的具体 python 类

Question

我正在尝试设计一个可以在 Python 中轻松扩展的 C 接口（使用ctypes）。我在 C 中使用了自然习语：

struct format {
    int (*can_open)(const char *filename);
    struct format * (*open)(const char *filename);
    void (*delete)(struct format *self);
    int (*read)(struct format *self, char *buf, size_t len);
};

如果我想直接从 C 扩展这个接口，它工作得很好：

struct derived /* concrete implementation */
{
    struct format base;
};

但我真正想做的是使用 ctypes 从 Python 实现这个接口。这是我到目前为止所拥有的：

CANOPENFUNC   = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
#OPENFUNC     = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_char_p)
#OPENFUNC     = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
#DELETEFUNC   = ctypes.CFUNCTYPE(None, ctypes.c_void_p)
#READFUNC     = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p)

def py_canopen_func( string ):
    print "py_canopen_func", string
    return 1

canopen_func   = CANOPENFUNC(py_canopen_func)
#open_func     = OPENFUNC(  py_open_func)
#delete_func   = DELETEFUNC(py_canopen_func)
#read_func     = READFUNC(py_canopen_func)

class python_format(ctypes.Structure):
  _fields_ = (
    ('can_open',  CANOPENFUNC),
    ('open',      OPENFUNC),
    ('delete',    DELETEFUNC),
    ('read',      READFUNC),
  )
  def __init__(self):
    self.can_open = canopen_func
    OPENFUNC    = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
    def py_open_func2( string ):
      print "py_open_func2", string
      return ctypes.byref(self)
    self.open   = OPENFUNC( py_open_func2 )
    #self.delete = delete_func
    #self.read = read_func

真的，我正在努力为OPENFUNC这里定义原型。从技术上讲，它应该是：

OPENFUNC    = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)

但是我需要先定义python_format，而这又需要定义OPENFUNC.

加分点：什么是实际的功能实现？例如：

def func( str ): return None

或者

def func( str ): i = python_format(); return ctypes.pointer(i)

两者都给了我：

class python_format(ctypes.Structure):
  pass
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
OPENFUNC( func )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: invalid result type for callback function

这与其他问题有关吗？如果是这样，我是否应该更改我的初始 C 设计，因为我将无法python_format从回调返回指向实例的指针？

Answer 1

在它的文档中ctypes.Structure._fields_解释了如何执行此操作：

可以在定义 Structure 子类的类语句之后_fields_定义类变量，这允许创建直接或间接引用自身的数据类型

这意味着您可以添加：

class python_format(ctypes.Structure):  # forward declaration
    pass

然后在定义OPENFUNC（以及其他函数类型）之后：

OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
DELETEFUNC = etc...

就可以python_format._fields_这样定义：

python_format._fields_ = (
    ('can_open',  CANOPENFUNC),
    ('open',      OPENFUNC),
    ('delete',    DELETEFUNC),
    ('read',      READFUNC),
  )

这是基于您的代码的更完整的示例：

import ctypes

class python_format(ctypes.Structure):  # forward declaration
    pass

CANOPENFUNC = ctypes.PYFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
OPENFUNC = ctypes.PYFUNCTYPE(ctypes.c_int,
                                 ctypes.POINTER(python_format),
                                 ctypes.c_char_p)
DELETEFUNC = ctypes.PYFUNCTYPE(None, ctypes.c_void_p)
READFUNC = ctypes.PYFUNCTYPE(ctypes.c_int, ctypes.c_void_p)

def py_canopen_func(self, string):
    print "py_canopen_func", string
    return 1

def py_open_func(self, string):
    print "py_open_func2", string
    # Return types from callbacks cannot be anything other than simple
    # datatypes (c_int, c_float, ..., c_void_p). For other datatypes
    # (STRUCTURE, POINTER, ...), ctypes returns the following error
    # "Invalid result type for callback function"
    # see http://bugs.python.org/issue5710
    return 1  # can't return ctypes.byref(self)

canopen_func = CANOPENFUNC(py_canopen_func)
open_func = OPENFUNC(py_open_func)
#delete_func = DELETEFUNC(py_canopen_func)
#read_func = READFUNC(py_canopen_func)

class python_format(ctypes.Structure):
    python_format._fields_ = (
        ('can_open', CANOPENFUNC),
        ('open', OPENFUNC),
        ('delete', DELETEFUNC),
        ('read', READFUNC),
      )

    def __init__(self):
        self.can_open = canopen_func
        self.open = open_func
        #self.delete = delete_func
        #self.read = read_func

pf = python_format()

Answer 2

为了得到规范的答案，我将回答我自己的问题，感谢@eryksun 的指导。

首先，虽然文档中尚不清楚这一点，但不能从回调函数返回复杂类型。因此，无法映射 C 函数指针：

struct format {
    struct format * (*open)(const char *filename);
};

到

class python_format:
  pass
OPENFUNC    = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
def py_canopen_func( string ):
    return None
open_func     = OPENFUNC(py_open_func)

上面的代码可以正常编译，但在运行时，会得到：

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: invalid result type for callback function

长答案是：

当您尝试使用非简单类型作为回调结果时收到的 TypeError 消息没有什么帮助。回调的结果类型必须在其 StgDictObject（常规 PyDictObject 的 ctypes 扩展）中具有 setfunc。此要求限制您只能使用简单类型，例如 c_void_p[...]

因此，截至今天，直到问题 5710（又名49960）得到修复为止，唯一的解决方案如下：

class python_format(ctypes.Structure):
  __self_ref = []
  def __init__(self):
    self.open      = self.get_open_func()
  # technically should be a @classmethod but since we are self-referencing
  # ourself, this is just a normal method:
  def get_open_func(self):
    def py_open_func( string ):
      python_format.__self_ref.append( self )
      return ctypes.addressof(self)
    return OPENFUNC( py_open_func )
OPENFUNC     = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_char_p)

# delay init required because `read_info` requires a forward declaration:
python_format._fields_ = (
    ('open',      OPENFUNC),
  )