Python __missing__() Magic Method

Syntax

object.__missing__(self, key)

The __missing__(self, key) method defines the behavior of a dictionary subclass if you access a non-existent key. More specifically, Python’s __getitem__() dictionary method internally calls the __missing__() method if the key doesn’t exist. The return value of __missing__() is the value to be returned when trying to access a non-existent 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.

Example

In the following example, you create a dictionary subclass My_Dict_Subclass and overwrite the __missing__() method that simply returns a dummy string 'finxter'.

When trying to access a key that doesn’t exist, Python’s __getitem__() dictionary method internally calls the __missing__() method if the key doesn’t exist.

In our case, the key 'David' doesn’t exist in the dict subclass, so Python calls __missing__() which returns the string 'finxter' instead of raising the KeyError (which would be the default behavior).

class My_Dict_Subclass(dict):
    def __missing__(self, key):
        return 'finxter'



x = {'Alice': 23, 'Bob': 24, 'Carl': 25}
my_dict = My_Dict_Subclass(x)

# Try accessing existing key:
print(my_dict['Bob'])
# 24


# Try accessing missing key:
print(my_dict['David'])
# finxter

If you hadn’t defined the __missing__() method, Python would’ve raised a KeyError.

How to Resolve KeyError on a Dictionary Subclass

Consider the following code snippet where you try to access a non-existent key on a dict subclass.

class My_Dict_Subclass(dict):
    pass


x = {'Alice': 23, 'Bob': 24, 'Carl': 25}
my_dict = My_Dict_Subclass(x)

# Try accessing missing key:
print(my_dict['David'])
# finxter

This results in the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\...\code.py", line 10, in <module>
    print(my_dict['David'])
KeyError: 'David'

You can fix this by defining the __missing__() method as outlined above—of course, override it with your own desired behavior!

class My_Dict_Subclass(dict):
    def __missing__(self, key):
        return 'finxter'

References:

Related Video

For some additional context, feel free to check out our related video on the dict.get() method that internally calls the dict.__getitem__() method which calls the dict.__missing__() method. Deep shit! 🙂

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