1. 首先说一下私有变量 (private variable)

    a name prefixed with an underscore (e.g. _spam) should be treated as a non-public part of the API (whether it is a function, a method or a data member).

    __getitem__ is used to implement calls like self[key]

    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 :

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) )

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] )

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.

https://stackoverflow.com/questions/43627405/understanding-getitem-method