Subset fields from an ordereddict?

Question

So, I have an ordered dictionary, and its has a pile of key/value pairs. I can extract ALL them with the items() method. But what if I only want to select SOME of them ?

>>> import collections
>>> d = collections.OrderedDict({'banana': 3, 'apple': 4, 'pear': 1,'orange': 2})
>>> d.items()
[('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)]

What if I just want apple and banana ?

Is there a way I can specify the keys I want ?

>>> d['apple','banana'] <-- does not work

I was considering using a list comprehension at the end to filter the results, but seems messy and I hope there is a better way.

Answer 1

You'll most likely wind up using a list comprehension one way or another. Here is a way to bury the list comprehension in a method implementation:

from collections import OrderedDict

class MultOrderedDict(OrderedDict):
    def items(self, *items):
        all = super(MultOrderedDict, self).items()
        if not items:
            return all
        return type(self)((key, value) for (key, value) in all if key in items).items()

With that class definition (assuming it's in multorddict.py) you can do something like this:

>>> from multorddict import MultOrderedDict
>>> mod = MultOrderedDict(banana=3, apple=4, pear=1, orange=2)
>>> mod.items()
[('orange', 2), ('pear', 1), ('apple', 4), ('banana', 3)]
>>> mod.items('apple','banana')
[('apple', 4), ('banana', 3)]
>>> 

This approach changes the semantics of the items method, which might not be such a good thing.