向现有对象实例添加方法 [英] Adding a Method to an Existing Object Instance

问题描述

我已经阅读到可以在Python中向现有对象添加方法(即，不在类定义中).

I've read that it is possible to add a method to an existing object (i.e., not in the class definition) in Python.

我知道这样做并不总是一件好事.但是怎么可能呢?

I understand that it's not always good to do so. But how might one do this?

推荐答案

在Python中，函数和绑定方法之间存在差异.

In Python, there is a difference between functions and bound methods.

>>> def foo():
...     print "foo"
...
>>> class A:
...     def bar( self ):
...         print "bar"
...
>>> a = A()
>>> foo
<function foo at 0x00A98D70>
>>> a.bar
<bound method A.bar of <__main__.A instance at 0x00A9BC88>>
>>>

绑定方法已绑定"(描述性强)到一个实例，并且无论何时调用该方法，该实例都将作为第一个参数传递.

Bound methods have been "bound" (how descriptive) to an instance, and that instance will be passed as the first argument whenever the method is called.

作为类(而不是实例)的属性的可调用对象仍未绑定，因此您可以在需要时随时修改类定义:

Callables that are attributes of a class (as opposed to an instance) are still unbound, though, so you can modify the class definition whenever you want:

>>> def fooFighters( self ):
...     print "fooFighters"
...
>>> A.fooFighters = fooFighters
>>> a2 = A()
>>> a2.fooFighters
<bound method A.fooFighters of <__main__.A instance at 0x00A9BEB8>>
>>> a2.fooFighters()
fooFighters

先前定义的实例也会被更新(只要它们本身没有覆盖属性):

Previously defined instances are updated as well (as long as they haven't overridden the attribute themselves):

>>> a.fooFighters()
fooFighters

当您要将方法附加到单个实例时出现问题:

The problem comes when you want to attach a method to a single instance:

>>> def barFighters( self ):
...     print "barFighters"
...
>>> a.barFighters = barFighters
>>> a.barFighters()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: barFighters() takes exactly 1 argument (0 given)

该函数直接附加到实例时不会自动绑定:

The function is not automatically bound when it's attached directly to an instance:

>>> a.barFighters
<function barFighters at 0x00A98EF0>

要绑定它，我们可以在 MethodType函数中使用类型模块:

To bind it, we can use the MethodType function in the types module:

>>> import types
>>> a.barFighters = types.MethodType( barFighters, a )
>>> a.barFighters
<bound method ?.barFighters of <__main__.A instance at 0x00A9BC88>>
>>> a.barFighters()
barFighters

这一次该类的其他实例未受影响:

This time other instances of the class have not been affected:

>>> a2.barFighters()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'barFighters'

有关更多信息，请阅读有关描述符和元类

More information can be found by reading about descriptors and metaclass programming.

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