Re-implementing DefaultDict (from Python standard library) using __missing__ dunder method

default_dict_implementation.py

"""Using dict __missing__() to implement DefaultDict."""
from typing import Any, Callable

class DefaultDict(dict):
    """Implementing typing.DefaultDict.
    
    A dict that returns a default value (instead of raising KeyError) when a key is missing.
    
    >>> d = DefaultDict(list, {'a': 1, 'b': 2})
    >>> d['a']  
    1
    >>> d['c'].append(0)
    >>> d        # accessing 'c' key creates a new key-value pair, just like stdlib typing.DefaultDict
    DefaultDict(<class 'list'>, {'a': 1, 'b': 2, 'c': [0]})
    >>> DefaultDict(0, {'a': 1, 'b': 2}) 
    Traceback (most recent call last):
     ... 
    TypeError: Expected callable as default_func, but got <class 'int'>
    """
    def __init__(self, default_func: Callable[[], Any], *args, **kwargs):
        super().__init__(*args, **kwargs)
        if not callable(default_func):
            raise TypeError(f"Expected callable as default_func, but got {type(default_func)}")
        self._default_func = default_func

    def __missing__(self, key):
        self[key] = val = self._default_func()
        return val

    def __repr__(self):
        return f"DefaultDict({self._default_func}, {super().__repr__()})"

if __name__ == '__main__':
    import doctest
    doctest.testmod()

Reddit post about this: https://www.reddit.com/r/learnpython/comments/1ixporv/example_of_using_missing_dunder_method_to/