Understanding __getitem__ method

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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 :

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] )

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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