If the side effect of defaultdict is unpleasant, the correct way is combining the translation mapping with the custom mapping by ChainMap. But this example is too complex for the documentation of str.translate(). On other side, it is trivial for more experience users and don't need special mentioning.

I think other resources (ActiveState Code Reciepes [1] or books) are better places for this example.

[1] http://code.activestate.com/recipes/popular/