Python __missing__

According to the python documentation:

If a subclass of dict defines a method __missing__(), if the key key is not present, the d[key] operation calls that method with the key key as argument. The d[key] operation then returns or raises whatever is returned or raised by the __missing__(key) call if the key is not present. No other operations or methods invoke __missing__(). If __missing__() is not defined, KeyError is raised. __missing__() must be a method; it cannot be an instance variable.

For an example, see collections.defaultdict.

This is, at least, incomplete, since __missing__ must not only return the default value, but also assign it internally. This is made clear in the documentation for collections.defaultdict:

If default_factory is not None, it is called without arguments to provide a default value for the given key, this value is inserted in the dictionary for the key, and returned.

Surprisingly, the __missing__ method is not mentioned in the special method names section of the python documentation.

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class memoize(dict):
    def __init__(self, func):
        self.func = func

    def __call__(self, *args):
        print "__call__"
        return self[args]

    def __missing__(self, key):
        print "__missing__"
        result = self[key] = self.func(*key)
        return result

@memoize
def foo(a, b):
    return a * b

print foo(1,2)
print foo(1,2)
print foo(1,2)
print foo(1,2)
print foo(1,2)
print foo(1,2)

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__call__
__missing__
2
__call__
2
__call__
2
__call__
2
__call__
2
__call__
2

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