自引用 class：来自 C 接口的具体 python class

Question

我正在尝试设计一个可以在 Python 中轻松扩展的 C 接口（在 C:

中使用 ctypes). I've used the natural idiom

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。这是我目前所拥有的：

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

我真的很难在这里定义 OPENFUNC 的原型。技术上应该是：

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

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

奖励点：实际的功能实现是什么？例如：

def func( str ): return None

或

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

两者都给我：

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

这与另一个 issue 相关吗？如果是这样，我是否应该更改我最初的 C 设计，因为我将无法 return 从回调中指向 python_format 实例的指针？

Answer 1

在 documentation for ctypes.Structure._fields_ 中解释了如何做到这一点：

It is possible to define the _fields_ class variable after the class statement that defines the Structure subclass, this allows to create data types that directly or indirectly reference themselves

这意味着您可以添加：

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

然后在定义 OPENFUNC （和其他函数类型）之后：

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

然后可以这样定义python_format._fields_：

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

这是一个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

为了得到一个规范的答案，我会回答我自己的问题，感谢@eryksun 的指导。

首先，虽然这在 documentation 中并不清楚，但不能从回调函数中 return 复杂类型。因此，不能映射 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)

以上代码可以正常编译，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：

The TypeError message you get when trying to use a non-simple type as the result of a callback is less than helpful. A callback's result type has to have a setfunc in its StgDictObject (a ctypes extension of the regular PyDictObject). This requirement restricts you to using a simple type such as c_void_p[...]

因此，从今天开始，在 issue 5710 修复之前，这里唯一的解决方案是：

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),
  )

自引用 class：来自 C 接口的具体 python class

Self-referencing class: concrete python class from C interface

c

python

ctypes

creation

self-reference