Inheriting dictionary

[Pavel Panchekha]

I want a dictionary that will transparently "inherit" from a parent
dictionary. So, for example:

"""
a = InheritDict({1: "one", 2: "two", 4: "four"})
b = InheritDict({3: "three", 4: "foobar"}, inherit_from=a)

a[1] # "one"
a[4] # "four"
b[1] # "one"
b[3] # "three"
b[4] # "foobar"
"""

I've written something like this in Python already, but I'm wondering
if something like this already exists, preferably written in C, for
speed.