在python中使用与实例和类方法相同的功能 [英] Using same function as instance and classmethod in python

问题描述

一个人可以做这样的事情：

One can do something like this:

class master:
    @combomethod 
    def foo(param):
        param.bar() # Param could be type as well as object

class slaveClass( master ):
    @classmethod 
    def bar(cls):
        print("This is class method")

slaveType = slaveClass
slaveType.foo()

class slaveInstance( master ):
    def __init__(self, data):
        self.data = data
    def bar(self):
        print("This is "+self.data+" method")


slaveType = slaveInstance("instance")
slaveType.foo()

combomethod 在 。

combomethod is defined in "Creating a method that is simultaneously an instance and class method".

我的问题是，为什么就是这样，默认的第一个参数不能使用d作为comboclass的参数？或者至少，为什么我不能将对象作为第一个参数传递给classmethod？我知道classmethod和instancemethods之间的区别，我知道装饰器，但我可能不明白 @classmethod 和 self 参数传递完成。有技术限制吗？或者，为什么还没有内置 combomethod ？

My question is, why is it like this, that default first parameter can't be used as parameter of comboclass? Or at least, why can't I pass object to classmethod as the first parameter? I know the difference between classmethod and instancemethods, and I know decorators, but I might not understand how built-in @classmethod and self parameter passing is made. Is there a technical limitation? Or, why isn't combomethod allready built in?

推荐答案

combomethod 在访问时不会创建方法对象，而是一个特殊包装的函数。像方法一样，每次访问都会创建一个新对象，在这种情况下是一个新功能对象。

combomethod doesn't create a method object when accessed but a specially wrapped function. Like methods each access creates a new object, in this case a new function object.

class A:
    def __init__(self):
        self.data = 'instance'

    @combomethod 
    def foo(param):
        if isinstance(param, A):
            print("This is an " + param.data + " method.")
        elif param is A:
            print("This is a class method.")

>>> a = A()
>>> A.foo
<function foo at 0x00CFE810>
>>> a.foo
<function foo at 0x00CFE858>

>>> A.foo()
This is a class method.
>>> a.foo()
This is an instance method.

每次访问都是新功能：

>>> A.foo is A.foo
False
>>> a.foo is a.foo
False

foo 实际上是 _wrapper 的伪装：

>>> A.foo.__code__.co_name
'_wrapper'

从类调用闭包时具有obj == None（请注意， self在这里指的是组合方法，它在 self.method 中引用了原始函数对象）：

When called from a class the closure has obj == None (note that 'self' here refers to the combomethod, which has a reference to the original function object in self.method):

>>> print(*zip(A.foo.__code__.co_freevars, A.foo.__closure__), sep='\n')
('obj', <cell at 0x011983F0: NoneType object at 0x1E1DF8F4>)
('self', <cell at 0x01198530: combomethod object at 0x00D29630>)
('objtype', <cell at 0x00D29D10: type object at 0x01196858>)

作为实例的属性调用时，obj是实例：

When called as the attribute of an instance, obj is the instance:

>>> print(*zip(a.foo.__code__.co_freevars, a.foo.__closure__), sep='\n')
('obj', <cell at 0x01198570: A object at 0x00D29FD0>)
('self', <cell at 0x01198530: combomethod object at 0x00D29630>)
('objtype', <cell at 0x00D29D10: type object at 0x01196858>)

以下是存储在comomethod中的原始函数：

Here is the original function stored in the combomethod:

>>> A.foo.__closure__[1].cell_contents.method
<function foo at 0x00D1CB70>
>>> A.foo.__closure__[1].cell_contents.method.__code__.co_name
'foo'

_wrapper 执行 self.method ，将给定obj值的类或实例作为第一个参数： / p>

_wrapper executes self.method with either the class or instance as the first argument given the value of obj:

if obj is not None:
    return self.method(obj, *args, **kwargs)
else:
    return self.method(objtype, *args, **kwargs)

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