创建有序计数器 [英] Creating an Ordered Counter

问题描述

我一直在研究super()的工作方式.我遇到了此食谱，它演示了如何创建订购柜台:

from collections import Counter, OrderedDict

class OrderedCounter(Counter, OrderedDict):
     'Counter that remembers the order elements are first seen'
     def __repr__(self):
         return '%s(%r)' % (self.__class__.__name__,
                            OrderedDict(self))
     def __reduce__(self):
         return self.__class__, (OrderedDict(self),)

例如:

oc = OrderedCounter('adddddbracadabra')

print(oc)

OrderedCounter(OrderedDict([('a', 5), ('d', 6), ('b', 2), ('r', 2), ('c', 1)]))

有人能够解释这是如何神奇地工作的吗?

这也出现在 Python文档中.

在__mro__:

中

OrderedCounter.__mro__

(<class '__main__.OrderedCounter'>, <class 'collections.Counter'>, <class 'collections.OrderedDict'>, <class 'dict'>, <class 'object'>)

当OrderedDict的实例调用__setitem__()时，它将按以下顺序搜索类:OrderedCounter，Counter，OrderedDict(在找到位置).因此，像oc['a'] = 0这样的语句最终会调用OrderedDict.__setitem__().

相反，__getitem__不会被mro中的任何子类覆盖，因此count = oc['a']由dict.__getitem__()处理.

oc = OrderedCounter()    
oc['a'] = 1             # this call uses OrderedDict.__setitem__
count = oc['a']         # this call uses dict.__getitem__

对于类似oc.update('foobar').的语句，会发生一个更有趣的调用序列.首先，调用Counter.update(). Counter.update()的代码使用self [elem]，它变成对OrderedDict.__setitem__()的调用.还有 调用dict.__setitem__()的代码.

如果基类被反转，它将不再起作用.因为mro不同，所以调用了错误的方法.

class OrderedCounter(OrderedDict, Counter):   # <<<== doesn't work
    pass

有关mro的更多信息，请参见Python 2.3 文档.

I've been reading into how super() works. I came across this recipe that demonstrates how to create an Ordered Counter:

from collections import Counter, OrderedDict

class OrderedCounter(Counter, OrderedDict):
     'Counter that remembers the order elements are first seen'
     def __repr__(self):
         return '%s(%r)' % (self.__class__.__name__,
                            OrderedDict(self))
     def __reduce__(self):
         return self.__class__, (OrderedDict(self),)

For example:

oc = OrderedCounter('adddddbracadabra')

print(oc)

OrderedCounter(OrderedDict([('a', 5), ('d', 6), ('b', 2), ('r', 2), ('c', 1)]))

Is someone able to explain how this magically works?

This also appears in the Python documentation.

解决方案

OrderedCounter is given as an example in the OrderedDict documentation, and works without needing to override any methods:

class OrderedCounter(Counter, OrderedDict):
    pass

When a class method is called, Python has to find the correct method to execute. There is a defined order in which it searches the class hierarchy called the "method resolution order" or mro. The mro is stored in the attribute __mro__:

OrderedCounter.__mro__

(<class '__main__.OrderedCounter'>, <class 'collections.Counter'>, <class 'collections.OrderedDict'>, <class 'dict'>, <class 'object'>)

When an instance of an OrderedDict is calling __setitem__(), it searches the classes in order: OrderedCounter, Counter, OrderedDict (where it is found). So an statement like oc['a'] = 0 ends up calling OrderedDict.__setitem__().

In contrast, __getitem__ is not overridden by any of the subclasses in the mro, so count = oc['a'] is handled by dict.__getitem__().

oc = OrderedCounter()    
oc['a'] = 1             # this call uses OrderedDict.__setitem__
count = oc['a']         # this call uses dict.__getitem__

A more interesting call sequence occurs for a statement like oc.update('foobar'). First, Counter.update() gets called. The code for Counter.update() uses self[elem], which gets turned into a call to OrderedDict.__setitem__(). And the code for that calls dict.__setitem__().

If the base classes are reversed, it no longer works. Because the mro is different and the wrong methods get called.

class OrderedCounter(OrderedDict, Counter):   # <<<== doesn't work
    pass

More info on mro can be found in the Python 2.3 documentation.

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