I need a dict that inherits its mappings

[samwyse]

I need a dict-like object that, if it doesn't contain a key, will
return the value from a "parent" object.  Is there an easy way to do
this so I don't have to define __getitem__ and __contains__ and others
that I haven't even thought of yet?  Here's a use case, if you're
confused:

en_GB=mydict()
en_US=mydict(en_GB)

en_GB['bonnet']='part of your car'
print en_US['bonnet']  # prints 'part of your car'

en_US['bonnet']='a type of hat'
print en_US['bonnet']  # prints 'a type of hat'
print en_GB['bonnet']  # prints 'part of your car'