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.