Understanding __getitem__ method

Question

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.

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

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"]) 

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?

Can someone please explain this in more detail?

Answer 1

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.

Answer 2

The [] syntax for getting item by key or index is just syntax sugar.

When you evaluate a[i] Python calls a.__getitem__(i) (or type(a).__getitem__(a, i), but this distinction is about inheritance models and is not important here). Even if the class of a may not explicitly define this method, it is usually inherited from an ancestor class.

All the (Python 2.7) special method names and their semantics are listed here: https://docs.python.org/2.7/reference/datamodel.html#special-method-names