理解 __getitem__ 方法 [英] Understanding __getitem__ method

问题描述

Python docs中__getitem__的大部分文档我都看过了，但还是没能理解其中的含义.

I have gone through most of the documentation of __getitem__ in the Python docs, but I am still unable to grasp the meaning of it.

所以我能理解的是，__getitem__ 用于实现像 self[key] 这样的调用.但是有什么用呢?

So all I can understand is that __getitem__ is used to implement calls like self[key]. But what is the use of it?

假设我有一个以这种方式定义的 python 类:

Lets say I have a python class defined in this way:

class Person:
    def __init__(self,name,age):
        self.name = name
        self.age = age

    def __getitem__(self,key):
        print ("Inside `__getitem__` method!")
        return getattr(self,key)

p = Person("Subhayan",32)
print (p["age"])

这将按预期返回结果.但是为什么首先要使用 __getitem__ 呢?我还听说 Python 在内部调用了 __getitem__.但它为什么要这样做?

This returns the results as expected. But why use __getitem__ in the first place? I have also heard that Python calls __getitem__ internally. But why does it do it?

有人可以更详细地解释一下吗?

Can someone please explain this in more detail?

推荐答案

Cong Ma 很好地解释了 __getitem__ 的用途 - 但我想给你一个可能有用的例子.想象一个对建筑物建模的类.在建筑物的数据中，它包括许多属性，包括对占据每一层楼的公司的描述:

Cong Ma does a good job of explaining what __getitem__ is used for - but I want to give you an example which might be useful. Imagine a class which models a building. Within the data for the building it includes a number of attributes, including descriptions of the companies that occupy each floor :

如果不使用 __getitem__ 我们会有这样的类:

Without using __getitem__ we would have a class like this :

class Building(object):
     def __init__(self, floors):
         self._floors = [None]*floors
     def occupy(self, floor_number, data):
          self._floors[floor_number] = data
     def get_floor_data(self, floor_number):
          return self._floors[floor_number]

building1 = Building(4) # Construct a building with 4 floors
building1.occupy(0, 'Reception')
building1.occupy(1, 'ABC Corp')
building1.occupy(2, 'DEF Inc')
print( building1.get_floor_data(2) )

然而，我们可以使用 __getitem__(及其对应的 __setitem__)来使 Building 类的使用更好".

We could however use __getitem__ (and its counterpart __setitem__) to make the usage of the Building class 'nicer'.

class Building(object):
     def __init__(self, floors):
         self._floors = [None]*floors
     def __setitem__(self, floor_number, data):
          self._floors[floor_number] = data
     def __getitem__(self, floor_number):
          return self._floors[floor_number]

building1 = Building(4) # Construct a building with 4 floors
building1[0] = 'Reception'
building1[1] = 'ABC Corp'
building1[2] = 'DEF Inc'
print( building1[2] )

您是否像这样使用 __setitem__ 实际上取决于您计划如何抽象数据 - 在这种情况下，我们决定将建筑物视为楼层的容器(并且您也可以实现迭代器对于建筑物，甚至可能具有切片能力 - 即一次获取多个楼层的数据 - 这取决于您的需求.

Whether you use __setitem__ like this really depends on how you plan to abstract your data - in this case we have decided to treat a building as a container of floors (and you could also implement an iterator for the Building, and maybe even the ability to slice - i.e. get more than one floor's data at a time - it depends on what you need.

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