105 – Dunder method missing

105 – Dunder method `missing`#

The dunder method __missing__ is part of the dict data model. If you look up a key that isn’t in the dictionary, __missing__ is called to let you handle the missing key.

class Test(dict):
    def __missing__(self, key):
        print(f"In __missing__ with {key = }.")

d = Test()
d[42]  # Output: In __missing__ with key = 42.

This can be used, for example, to implement collections.defaultdict yourself:

class defaultdict_(dict):
    def __init__(self, factory, **kwargs):
        super().__init__(**kwargs)
        self.default_factory = factory

    def __missing__(self, key):
        """Populate the missing key and
        return its value."""
        self[key] = self.default_factory()
        return self[key]

# int() gives 0.
olympic_medals = defaultdict_(int)
olympic_medals["Phelps"] = 28

print(olympic_medals["Phelps"])  # 28
print(olympic_medals["me"])  # 0 :(

Further reading:

Module collections overview