For reference, this is called autovivification. http://en.wikipedia.org/wiki/Autovivification#Python

Oops, messed this up (too late to edit). Thanks choochootrain for the correction:

  >>> NestedDD = lambda: collections.defaultdict(NestedDD)
  >>> nested_dd = NestedDD()
  >>> nested_dd['a']['b']['c']['d'] = 'hello'

does the proper thing.

I don't think that's what they wanted:

    >>> nested_dd['d']
    'hello'

  from collections import defaultdict

  defaultobj = lambda: type('defaultobj', (defaultdict,), {
      '__getattr__': lambda self, x:    self.__getitem__(x),
      '__setattr__': lambda self, x, v: self.__setitem__(x, v)
      })(defaultobj)

  names = defaultobj()
  names.mammalia.primates.homo['H. Sapiens'] = 'Human being'

  print(names)

The same but also allows dot notation.