Python __getitem__() Magic Method

Python’s magic method __getitem__(self, key) to evaluate the expression self[key]. So, if you call my_obj[key], Python will call my_obj.__getitem__(key).

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Syntax and Minimal Example

object.__getitem__(self, key)

Let’s have a look at an example where you override the __getitem__ magic method of a custom class HelloWorld.

class HelloWorld:
    def __getitem__(self, key):
        print('hello world', key)

hi = HelloWorld()

hi[42]
# hello world 42

hi['wide web']
# hello world wide web

This code shows several interesting things:

  • You use obj[key] to call obj.__getitem__(key).
  • You can pass arbitrary keys into it.
  • You don’t even need to return anything from __getitem__ (although it’s of course not recommended). In this case, None is returned.

However, this is just a dummy example, let’s have a look at a full example implementing all methods __getitem__, __setitem__, and __delitem__ that together play together beautifully to implement a collection-like custom data type.

Full Example

In the following example, you create a custom list-like type that overrides the __getitem__, __setitem__, and __delitem__ methods.

class MyList:
    def __init__(self, lst):
        self.lst = lst

    def __getitem__(self, key):
        print('__getitem__', key)
        return self.lst[key]

    def __setitem__(self, key, value):
        print('__setitem__', key, value)
        self.lst[key] = value

    def __delitem__(self, key):
        print('__delitem__', key)
        del self.lst[key]

Each time you add a new element, get an element, or delete an element from your custom list, it prints the respective information so you see what happens.

Now, let’s use this to create a simple list wrapper and print the second element with index 1 (zero-based indexing).

my = MyList(['alice', 'bob', 'carl'])

print(my[1])
# __getitem__ 1
# bob

Note how Python first calls the print statement within the __getitem__ output and key 1 and then returns the element my[1] itself, puts it into the print(...) statement which results in the output bob.

Next, you overwrite the value of the second list element with index 1:

my[1] = '42'
print(my[1])
# __setitem__ 1 42
# __getitem__ 1
# 42

You can see from the print output __setitem__ 1 42 that __setitem__ was called with key 1 and value 42.

Next, you delete an element with the expression del my[1]:

del my[1]
print(my[1])
# carl

References:

Where to Go From Here?

Enough theory. Let’s get some practice!

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