ordereddefaultdictwithkey.py

from collections import OrderedDict, MutableMapping, ItemsView


class ordereddefaultdictwithkey(MutableMapping, ItemsView):
    def __init__(self, factory_func):
        self.__factory_func = factory_func
        self.__dict = OrderedDict()

    def __len__(self):
        return len(self.__dict)

    def __delitem__(self, key):
        del self.__dict[key]

    def __setitem__(self, key, value):
        self.__dict[key] = value

    def __iter__(self):
        return iter(self.__dict)

    def __getitem__(self, key):
        if key not in self.__dict:
            val = self.__factory_func(key)
            self.__dict[key] = val
            return val
        else:
            return self.__dict[key]

    def __contains__(self, key):
        return key in self.__dict

    def __repr__(self):
        return '{}({},{})'.format(self.__class__.__name__, repr(self.__factory_func), repr(list(self.iteritems())))


## Example
import time

def long_computation(name):
    time.sleep(1)
    return len(name)

def another_computation(length):
    time.sleep(1)
    return '*' * length

a = ordereddefaultdictwithkey(lambda key: long_computation(key))
b = ordereddefaultdictwithkey(lambda key: another_computation(a[key]))

b['test']
b['foobarbaz']

print 'a is'
print a
print 'b is'
print b