问题描述
由于讨论了的评论主题,所以我提出这个问题。我是90%的方式让我的头部轮到它。
由 def 语句创建,或 lambda 。在Python 2下,当一个函数出现在类语句的正文中(或者传递给类型类构造调用),它将转换为未绑定的方法。 (Python 3没有未绑定的方法,参见下面的内容。)当一个函数在类实例上被访问时,它被转换成一个绑定方法,自动将实例作为第一个方法提供给方法 self 参数。
def f1(self):
通过
这里 f1 是一个函数。
class C(object):
f1 = f1
现在 C.f1 是一个未绑定的方法。
>>> C.f1
>>> C.f1.im_func是f1
True
我们也可以使用<$ c $
>>>类型类构造函数: C2 = type('C2',(object,),{'f1':f1})
>>> C2.f1
我们可以将 f1 转换为未绑定的方法手动:
>>>导入类型
>>> types.MethodType(f1,None,C)
未绑定的方法受到类实例的访问限制:
>>> C()。f1
< __ main __的边界方法C.f1。0x2abeecf87250处的C对象>>
访问通过描述符协议转换为调用:
>>> C.f1 .__获得__(C(),C)
合并这些:
>>> types.MethodType(f1,None,C).__ get __(C(),C)
或直接:
>>>类型。方法类型(f1,C(),C)
函数和未绑定方法的主要区别在于后者知道绑定哪个类;调用或绑定未绑定的方法需要其类类型的实例:
>>> f1(无)
>>> C.f1(None)
TypeError:必须使用C实例作为第一个参数调用f1()的非绑定方法(改为获得NoneType实例)
>>> D类(对象):pass
>>> f1 .__得到__(D(),D)
>>> C.f1 .__ get __(D(),D)
由于函数和非绑定方法之间的差异非常小,因此Python 3摆脱了这种差别;在Python 3下访问类实例上的函数只会给你自己的函数:
>>> C.f1
>
>>> C.f1是f1
True
在Python 2和Python 3中,这些三个是等价的:
f1(C())
C.f1(C())
C()。f1()
将函数绑定到实例会影响修复其第一个参数(通常称为 self )到实例。因此,绑定方法 C()。f1 相当于以下任一项:
(lamdba * args,** kwargs:f1(C(),* args,** kwargs))
functools.partial(f1,C())
I'm asking this question because of a discussion on the comment thread of this answer. I'm 90% of the way to getting my head round it.
In [1]: class A(object): # class named 'A' ...: def f1(self): pass ...: In [2]: a = A() # an instancef1 exists in three different forms:
In [3]: a.f1 # a bound method Out[3]: <bound method a.f1 of <__main__.A object at 0x039BE870>> In [4]: A.f1 # an unbound method Out[4]: <unbound method A.f1> In [5]: a.__dict__['f1'] # doesn't exist KeyError: 'f1' In [6]: A.__dict__['f1'] # a function Out[6]: <function __main__.f1>What is the difference between the bound method, unbound method and function objects, all of which are described by f1? How does one call these three objects? How can they be transformed into each other? The documentation on this stuff is quite hard to understand.
解决方案A function is created by the def statement, or by lambda. Under Python 2, when a function appears within the body of a class statement (or is passed to a type class construction call), it is transformed into an unbound method. (Python 3 doesn't have unbound methods; see below.) When a function is accessed on a class instance, it is transformed into a bound method, that automatically supplies the instance to the method as the first self parameter.
def f1(self): passHere f1 is a function.
class C(object): f1 = f1Now C.f1 is an unbound method.
>>> C.f1 <unbound method C.f1> >>> C.f1.im_func is f1 TrueWe can also use the type class constructor:
>>> C2 = type('C2', (object,), {'f1': f1}) >>> C2.f1 <unbound method C2.f1>We can convert f1 to an unbound method manually:
>>> import types >>> types.MethodType(f1, None, C) <unbound method C.f1>Unbound methods are bound by access on a class instance:
>>> C().f1 <bound method C.f1 of <__main__.C object at 0x2abeecf87250>>Access is translated into calling through the descriptor protocol:
>>> C.f1.__get__(C(), C) <bound method C.f1 of <__main__.C object at 0x2abeecf871d0>>Combining these:
>>> types.MethodType(f1, None, C).__get__(C(), C) <bound method C.f1 of <__main__.C object at 0x2abeecf87310>>Or directly:
>>> types.MethodType(f1, C(), C) <bound method C.f1 of <__main__.C object at 0x2abeecf871d0>>The main difference between a function and an unbound method is that the latter knows which class it is bound to; calling or binding an unbound method requires an instance of its class type:
>>> f1(None) >>> C.f1(None) TypeError: unbound method f1() must be called with C instance as first argument (got NoneType instance instead) >>> class D(object): pass >>> f1.__get__(D(), D) <bound method D.f1 of <__main__.D object at 0x7f6c98cfe290>> >>> C.f1.__get__(D(), D) <unbound method C.f1>Since the difference between a function and an unbound method is pretty minimal, Python 3 gets rid of the distinction; under Python 3 accessing a function on a class instance just gives you the function itself:
>>> C.f1 <function f1 at 0x7fdd06c4cd40> >>> C.f1 is f1 TrueIn both Python 2 and Python 3, then, these three are equivalent:
f1(C()) C.f1(C()) C().f1()Binding a function to an instance has the effect of fixing its first parameter (conventionally called self) to the instance. Thus the bound method C().f1 is equivalent to either of:
(lamdba *args, **kwargs: f1(C(), *args, **kwargs)) functools.partial(f1, C())
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