Question

纯粹的好奇心问题：

class Li(list): pass
m, n= Li([1]), Li([2])
def r(*args, **kwargs): raise Exception('hop')
setattr(m, '__iadd__', r)
m += n
print m # [1, 2]
setattr(Li, '__iadd__', r)
m += n

输出：

[1, 2]
Traceback (most recent call last):
  File "C:\...\test_override.py", line 8, in <module>
    m+=n
  File "C:\...\test_override.py", line 3, in r
    def r(*args, **kwargs): raise Exception('hop')
Exception: hop

如果我使用setattr(m, 'append', r)，则m.append(2)将失败。那么__iadd__是否在类对象上调用了？

我也可以在课程范围内使用settattr吗？一个天真的尝试，如：

def r(*args, **kwargs): raise Exception('hop')
class Li(list):
    for s in {'__iadd__','append'}: setattr(Li, s, r)

因错误而错误 NameError：name＆＃39; Li＆＃39;未定义

Answer 1

在新样式类中，实例__getattr__方法不再拦截内置操作发出的调用。使用内置操作时，搜索从类级别开始，而不是在实例级别。然而，对方法名称的显式调用将起作用：

>>> class Li(list): pass
>>> m, n= Li([1]), Li([2])
>>> def r(*args, **kwargs):
        raise Exception('hop')

>>> setattr(m, '__iadd__', r)
>>> m += n           # no exception raised

>>> m.__iadd__(n)    # explicitly calling the method searches the instance first
Traceback (most recent call last):
  File "<pyshell#76>", line 1, in <module>
    m.__iadd__(n)
  File "<pyshell#73>", line 1, in r
    def r(*args, **kwargs): raise Exception('hop')
Exception: hop

实现第二个目标的一种方法是使用a metaclass，特别是如果您只想创建属性一次（根据您的评论）：

>>> class Meta(type):
    def __new__(meta, name, bases, attrdict):
        print 'adding class attributes!'
        for s in {'__iadd__','append'}:
            attrdict[s] = r
        return super(Meta, meta).__new__(meta, name, bases, attrdict)
    def __init__(cls, name, bases, attrdict):
        super(Meta, cls).__init__(name, bases, attrdict)


>>> class Li(list):
    __metaclass__ = Meta


adding class attributes!          # attributes added here
>>> t = Li()                      # but not on instance creation
>>> Li.__dict__
dict_proxy({'__module__': '__main__', '__metaclass__': <class '__main__.Meta'>, '__iadd__': <function r at 0x02AAA270>, '__dict__': <attribute '__dict__' of 'Li' objects>, '__weakref__': <attribute '__weakref__' of 'Li' objects>, '__doc__': None, 'append': <function r at 0x02AAA270>})

在类范围内动态覆盖列表的内置函数

1 个答案: