python3：类中的singledispatch，如何调度self-typ

Question

使用python3.4。这里我想使用singledispatch在__mul__方法中分派不同的类型。代码如下：

class Vector(object):

    ## some code not paste  
    @functools.singledispatch
    def __mul__(self, other):
        raise NotImplementedError("can't mul these type")

    @__mul__.register(int)
    @__mul__.register(object)                # Becasue can't use Vector , I have to use object 
    def _(self, other):
        result = Vector(len(self))           # start with vector of zeros
        for j in range(len(self)):
            result[j] = self[j]*other
        return result

    @__mul__.register(Vector)                # how can I use the self't type
    @__mul__.register(object)                # 
    def _(self, other):
        pass # need impl 

正如您看到的代码，我想要支持Vector*Vertor，这有名称错误

^{pr2}$

问题可能是如何在类的方法中使用类名a类型？我知道c++有字体类语句。python如何解决我的问题？奇怪的是，result = Vector(len(self))可以在方法体中使用Vector。在

---

看看http://lukasz.langa.pl/8/single-dispatch-generic-functions/ 我可以选择这种方式来实现：

import unittest
from functools import  singledispatch

class Vector(object):
    """Represent a vector in a multidimensional space."""

    def __init__(self, d):
        self._coords = [0 for i in range(0, d)]
        self.__init__mul__()


    def __init__mul__(self):
        __mul__registry = self.__mul__.registry
        self.__mul__ = singledispatch(__mul__registry[object])
        self.__mul__.register(int, self.mul_int)
        self.__mul__.register(Vector, self.mul_Vector)

    def __setitem__(self, key, value):
        self._coords[key] = value

    def __getitem__(self, item):
        return self._coords[item]

    def __len__(self):
        return len(self._coords)

    def __str__(self):
        return str(self._coords)

    @singledispatch
    def __mul__(self, other):
        print ("error type is ", type(other))
        print (type(other))
        raise NotImplementedError("can't mul these type")

    def mul_int(self,other):
         print ("other type is ", type(other))
         result = Vector(len(self))           # start with vector of zeros
         for j in range(len(self)):
             result[j] = self[j]*other
         return result

    def mul_Vector(self, other):
        print ("other type is ", type(other))
        #result = Vector(len(self))           # start with vector of zeros
        sum = 0
        for i in range(0,len(self)):
            sum += self._coords[i] * other._coords[i]
        return sum

class TestCase(unittest.TestCase):
    def test_singledispatch(self):
        # the following demonstrates usage of a few methods
        v = Vector(5)              # construct five-dimensional <0, 0, 0, 0, 0>
        for i in range(1,6):
            v[i-1] = i
        print(v.__mul__(3))
        print(v.__mul__(v))
        print(v*3)

if __name__ == "__main__":
    unittest.main()

答案很奇怪：

other type is  <class 'int'>
[3, 6, 9, 12, 15]
other type is  <class '__main__.Vector'>
55
error type is  <class 'int'>
Traceback (most recent call last):
  File "p_algorithms\vector.py", line 164, in <module>
    print(v*3)
  File "C:\Python34\lib\functools.py", line 710, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "p_algorithms\vector.py", line 111, in __mul__
    raise NotImplementedError("can't mul these type")

v.__mul__(3)可以工作，v*3不能工作。这在我的选项中很奇怪v*3和{}是一样的。在

---

在@Martijn Pieters的评论之后更新，我仍然希望在类中实现v*3。所以我试试这个

import unittest
from functools import  singledispatch

class Vector(object):

    @staticmethod
    def static_mul_int(self,other):
         print ("other type is ", type(other))
         result = Vector(len(self))           # start with vector of zeros
         for j in range(len(self)):
             result[j] = self[j]*other
         return result

    @singledispatch
    @staticmethod
    def __static_mul__(cls, other):
        print ("error type is ", type(other))
        print (type(other))
        raise NotImplementedError("can't mul these type")


    __mul__registry2 = __static_mul__.registry
    __mul__ = singledispatch(__mul__registry2[object])
    __mul__.register(int, static_mul_int)

    def __init__(self, d):
        self._coords = [0 for i in range(0, d)]
        self.__init__mul__()


    def __init__mul__(self):
        __mul__registry = self.__mul__.registry
        print ("__mul__registry",__mul__registry,__mul__registry[object])
        self.__mul__ = singledispatch(__mul__registry[object])
        self.__mul__.register(int, self.mul_int)
        print ("at last __mul__registry",self.__mul__.registry)

    # @singledispatch
    # def __mul__(self, other):
    #     print ("error type is ", type(other))
    #     print (type(other))
    #     raise NotImplementedError("can't mul these type")


    def mul_int(self,other):
         print ("other type is ", type(other))
         result = Vector(len(self))           # start with vector of zeros
         for j in range(len(self)):
             result[j] = self[j]*other
         return result

    def __setitem__(self, key, value):
        self._coords[key] = value

    def __getitem__(self, item):
        return self._coords[item]

    def __len__(self):
        return len(self._coords)

    def __str__(self):
        return str(self._coords)


class TestCase(unittest.TestCase):
    def test_singledispatch(self):
        # the following demonstrates usage of a few methods
        v = Vector(5)              # construct five-dimensional <0, 0, 0, 0, 0>
        for i in range(1,6):
            v[i-1] = i
        print(v.__mul__(3))
        print("type(v).__mul__'s registry:",type(v).__mul__.registry)
        type(v).__mul__(v, 3)
        print(v*3)

if __name__ == "__main__":
    unittest.main() 

这次。v.__mul__(3)有错误：

Traceback (most recent call last):
  File "test.py", line 73, in test_singledispatch
    type(v).__mul__(v, 3)
  File "/usr/lib/python3.4/functools.py", line 708, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
TypeError: 'staticmethod' object is not callable

对我来说，静态方法的作用应该类似于实例方法。在

Answer 1

{/strong>至少你不能使用所有的装饰方法。Python3.8添加了一个新的选项，仅用于方法：^{}。在

这里没有定义Vector并不重要；任何方法的第一个参数总是self，而这里的第二个参数将使用单个分派。在

因为decorator在类对象创建之前应用于函数对象，所以您可以将您的“方法”注册为函数，而不是在类主体的外部，这样您就可以访问Vector名称：

class Vector(object):

    @functools.singledispatch
    def __mul__(self, other):
        return NotImplemented

@Vector.__mul__.register(int)
@Vector.__mul__.register(Vector)                
def _(self, other):
    result = Vector(len(self))           # start with vector of zeros
    for j in range(len(self)):
        result[j] = self[j]*other
    return result

对于不受支持的类型，您需要返回NotImplementedsingleton，而不是引发异常。这样Python也会尝试逆操作。在

但是，由于调度无论如何都要在这里输入错误的参数（self），因此您必须想出自己的单一分派机制。在

如果你真的想使用@functools.singledispatch，你就必须委托给一个正则函数，并且参数的倒数为：

^{pr2}$

至于使用__init__mul__的更新：v * 3是而不是翻译成v.__mul__(3)。它被转换为type(v).__mul__(v, 3)，请参阅Python数据模型参考中的Special method lookup。这个总是绕过直接在实例上设置的任何方法。在

这里type(v)是Vector；Python查找函数，它在这里不会使用绑定方法。同样，由于functools.singledispatch总是在first参数上分派，所以不能直接对Vector的方法使用单个分派，因为第一个参数总是Vector实例。在

换句话说，Python将使用您在__init__mul__中的self上设置的方法；特殊方法是从不查找实例，请参阅数据模型文档中的Special method lookup。在

python3.8添加的functools.singledispatchmethod()选项使用类作为实现descriptor protocol的装饰器，就像方法一样。这样，它就可以在绑定之前处理dispatch（因此，在self之前将被添加到参数列表中），然后绑定singledispatch调度器返回的注册函数。source code for this implementation与较旧的Python版本完全兼容，因此您可以改用它：

from functools import singledispatch, update_wrapper

# Python 3.8 singledispatchmethod, backported
class singledispatchmethod:
    """Single-dispatch generic method descriptor.

    Supports wrapping existing descriptors and handles non-descriptor
    callables as instance methods.
    """

    def __init__(self, func):
        if not callable(func) and not hasattr(func, "__get__"):
            raise TypeError(f"{func!r} is not callable or a descriptor")

        self.dispatcher = singledispatch(func)
        self.func = func

    def register(self, cls, method=None):
        """generic_method.register(cls, func) -> func

        Registers a new implementation for the given *cls* on a *generic_method*.
        """
        return self.dispatcher.register(cls, func=method)

    def __get__(self, obj, cls):
        def _method(*args, **kwargs):
            method = self.dispatcher.dispatch(args[0].__class__)
            return method.__get__(obj, cls)(*args, **kwargs)

        _method.__isabstractmethod__ = self.__isabstractmethod__
        _method.register = self.register
        update_wrapper(_method, self.func)
        return _method

    @property
    def __isabstractmethod__(self):
        return getattr(self.func, '__isabstractmethod__', False)

并将其应用于您的Vector()类。在创建类之后，您仍然需要为单个分派注册您的Vector实现，因为只有这样才能为类注册分派：

class Vector(object):
    def __init__(self, d):
        self._coords = [0] * d

    def __setitem__(self, key, value):
        self._coords[key] = value

    def __getitem__(self, item):
        return self._coords[item]

    def __len__(self):
        return len(self._coords)

    def __repr__(self):
        return f"Vector({self._coords!r})"

    def __str__(self):
        return str(self._coords)

    @singledispatchmethod
    def __mul__(self, other):
        return NotImplemented

    @__mul__.register
    def _int_mul(self, other: int):
        result = Vector(len(self))
        for j in range(len(self)):
            result[j] = self[j] * other
        return result

@Vector.__mul__.register
def _vector_mul(self, other: Vector):
    return sum(sc * oc for sc, oc in zip(self._coords, other._coords))

当然，您也可以先创建一个子类并在此基础上进行分派，因为分派也适用于子类：

class _Vector(object):
    def __init__(self, d):
        self._coords = [0] * d

class Vector(_Vector):
    def __setitem__(self, key, value):
        self._coords[key] = value

    def __getitem__(self, item):
        return self._coords[item]

    def __len__(self):
        return len(self._coords)

    def __repr__(self):
        return f"{type(self).__name__}({self._coords!r})"

    def __str__(self):
        return str(self._coords)

    @singledispatchmethod
    def __mul__(self, other):
        return NotImplemented

    @__mul__.register
    def _int_mul(self, other: int):
        result = Vector(len(self))
        for j in range(len(self)):
            result[j] = self[j] * other
        return result

    @__mul__.register
    def _vector_mul(self, other: _Vector):
        return sum(sc * oc for sc, oc in zip(self._coords, other._coords))

Answer 2

这有点难看，因为您需要将Vector/Vector乘法的实现推迟到实际定义Vector之后。但其思想是单个分派函数需要第一个参数是任意类型的，因此Vector.__mul__将用self作为第二个参数调用该函数。在

import functools

class Vector:

    def __mul__(self, other):
        # Python has already dispatched Vector() * object() here, so
        # swap the arguments so that our single-dispatch works. Note
        # that in general if a*b != b*a, then the _mul_by_other
        # implementations need to compensate.
        return Vector._mul_by_other(other, self)

    @functools.singledispatch
    def _mul_by_other(x, y):
        raise NotImplementedError("Can't multiply vector by {}".format(type(x)))

    @_mul_by_other.register(int)
    def _(x, y):
        print("Multiply vector by int")

@Vector._mul_by_other.register(Vector)
def _(x, y):
    print("Multiply vector by another vector")

x = Vector()
y = Vector()
x * 3
x * y
try:
    x * "foo"
except NotImplementedError:
    print("Caught attempt to multiply by string")