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'

 

[Tobias Weber]

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

What's wrong with that?

You can subclass collections.defaultdict and override __missing__

 

[Terry Reedy]

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'

For that specific case:

def lookup(key):
try: return en_US[key]
except KeyError: return en_GB[key]

More generally,

def make_lookup_with_backup(d1, d2):
def _l(key):
try: return d1[key]
except KeyError: return d2[key]
return _

Terry Jan Reedy

 

[Rachel P]

I need a dict-like object that, if it doesn't contain a key, will
return the value from a "parent" object.  

See: http://code.activestate.com/recipes/305268/
Also try subclassing dict and implementing a __missing__() method.


Raymond