Concise idiom to initialize dictionaries

[Steven Bethard]

Caleb Hattingh <caleb1 <at> telkomsa.net> writes:
> 
> For interest sake, how would such a thing look with new-style classes?  My  
> (likely misinformed) impression is that __getattr__ for example, doesn't  
> behave in quite the same way?

Just the same[1] =)

>>> class AllDicts(object):
...     def __getattr__(self, name):
...         d = {}
...         setattr(self, name, d)
...         return d
...     def __repr__(self):
...         items = self.__dict__.items()
...         items.sort()
...         return '\n'.join(['%s -> %r' % item for item in items])
...        
>>> ad = AllDicts()
>>> ad.a[1] = 99
>>> ad.b[2] = 42
>>> ad.b[3] = 11
>>> ad
a -> {1: 99}
b -> {2: 42, 3: 11}
>>> 

I believe that __getattr__ works just the same (but to check for yourself, see
http://docs.python.org/ref/attribute-access.html).  I think what you're thinking
of is __getattribute__ which new-style classes offer *in addition* to
__getattr__.  While __getattr__ is called only if an attribute is not found,
__getattribute__ is called unconditionally for every attribute access.

Steve

[1] modulo my preference for list comprehensions/generator expressions instead
of map