Fun Extending Dict

I’ve been curious about and disappointed with the limited ability to update, combine, split, and otherwise have my way with the built-in Python dict, so I set out to have some fun and try to enhance the dict object.

Have a look at builtout: builtout/dictb.py.

>>> from builtout import dictb
>>> map = dictb(a=1, b=2, c=3)

dictb is just a dict subclass and it behaves pretty much just like any other dict:

>>> map == {'a': 1, 'b': 2, 'c': 3}
True

>>> isinstance(map, dict)
True

But, there are so many things I want to do with dict that I can’t …

Getting, Setting & Deleting Multiple Items

Items in a dict are accessed by passing a key in square brackets like so:

>>> map['a']
1

Now, what if I want the values for keys 'a' and 'c'? Just pass a list of keys!

>>> keys = ['a', 'c']

>>> tuple( map[keys] )
(1, 3)
>>> tuple( map[keys + keys] )
(1, 3, 1, 3)

That’s awesome! Instances of list are not hashable, so they fail for use as dict keys which gives us the opportunity to use them for another purpose here.

But, actually, I want a subset of map containing only those keys (not just the values). Just pass a set of keys!

>>> map[set(keys)] == {'a': 1, 'c': 3}
True

And what about the subset of map minus those keys?

>>> map - keys == {'b': 2}
True

So, you can get multiple values from, or a subset of, map … what about setting and deleting values and keys?

>>> map[keys] = 11, 33
>>> map == {'a': 11, 'b': 2, 'c': 33}
True

# or set all keys to the same value
>>> map[set(keys)] = 0
>>> map == {'a': 0, 'b': 2, 'c': 0}
True

Nice. And deleting keys?

>>> del map[keys]
>>> map == {'b': 2}
True

I’m starting to get excited. But this is only the beginning. Moving on …

Subsetting & Combining Dictionaries

Wait. I thought we just saw how to get a subset of map up above? Sure, but item access must fail if the key is missing. Remember that map is currently just {'b': 2}:

>>> map[keys]
Traceback (most recent call last):
    ...
KeyError: ['a', 'c']

Clearly that only works if we know ahead of time that the keys we’re requesting are present. But, what if I don’t konw?

>>> map & keys == {}
True

Ah, that’s more like it. How about a logical or?

>>> map | keys == {'a': None, 'b': 2, 'c': None}
True

Sweet! What happens if we or with two maps?

>>> map = map | {'a': 1, 'b': 'in left map so ignored', 'c': 3}
>>> map == {'a': 1, 'b': 2, 'c': 3}
True

Now map is back where it started. Let’s try the logical and again:

>>> map & keys == {'a': 1, 'c': 3}
True

That works swimmingly. Let’s try xor.

>>> map ^ {'b': 2, 'c': 3, 'd': 4} == {'a': 1, 'd': 4}
True

If you’re not excited at this point, I’m not sure what to tell you. But I’ll let you know what I think …

Thoughts

• This is not a completely original idea, but it is an original implementation (afaik). And I believe Raymond’s comment on that post is only true for dict keys.

• Subclassing dict is an ugly (but simple) way to accomplish this. I might rather have a set of functions which do the same thing but accept built-in dicts, something like: xor(map, map2) == {'a': 1, 'd': 4}.

• Please don’t go use dictb on production code just yet. I need to write a suite of tests and the implementation will probably change somewhat. Help (and criticism) welcomed.

What do you think?