advanced python autovivification

Question

This question is about implementing the full Perl autovivification in python. I know similar questions were asked before and so far the best answre is http://stackoverflow.com/questions/635483/what-is-the-best-way-to-implement-nested-dictionaries-in-python/652284#652284. However, I'm looking to do this:

a['x']['y'].append('z')

without declaring a['x']['y'] = [] first, or rather, not declaring a['x'] = {} either. (Note in Perl you can do push @{$a->{x}{y}}, 'z';.) I know dict and list classes sorta don't mix so this is hard, but I'm interested in seeing if someone has an ingenius solution probably involving creating an inherited class from dict but defined a new append method on it? I also know this might throw off some python purists who will ask me to stick with Perl. But even just for a challenge, I'd like to see something. thx!

Answer 1

a = collections.defaultdict(lambda: collections.defaultdict(list))

Answer 2

Perhaps this solves your need for any number of “dimensions” in your dictionary:

a= collections.defaultdict(list)

the only change in your code being:

a['x', 'y'].append('z')

Of course, this solution being the one you want depends on two conditions:

1. whether you need to easily access all lists with e.g. 'x' in the “first dimension“
2. whether you are stuck to the way Perl magically pleases you more :)

If either of these two conditions is true, my solution won't help you.