CODEWITHSUNDEEP
python
Incerteza
Incerteza

Reputation: 34884

Get a value by one of 2 keys

I have a dictionary. Each key is represented in one of 2 ways: short and long. I want to get the values of some of they keys. I can do this:

d = dict(....)
a = d["a"] or d["aaa"]
b = d["b"] or d["bbb"]

But it throws an exception when the key "a" doesn't exist, so it won't call d["aaa"] even it must exist if d["a"] doesn't. It can be solved easily, I know, but how can I do that in an elegant way?

Upvotes: 0

Views: 69

Answers (3)

Chris Charles
Chris Charles

Reputation: 4446

You can use the get method of a dictionary and provide a default value:

d.get("a", None)

This will return d["a"] if "a" is a key or None otherwise

Upvotes: 1

georg
georg

Reputation: 214959

If you often use such a structure, you can have a small dict wrapper similar to this:

class altdict(dict):
    def __getitem__(self, item):
        if isinstance(item, (tuple, list)):
            for p in item:
                try:
                    return self[p]
                except KeyError:
                    pass
            raise KeyError, item
        return dict.__getitem__(self, item)

and then

d = altdict({
    'a'   :'aaa!',
    'bbb' :'bbb!',
})

print d['a', 'aaa'] # aaa!
print d['b', 'bbb'] # bbb!
print d['c', 'ccc'] # KeyError

Note that this works with arbitrary lists of "alternate" keys:

d['B', 'Bill',  'William'] # pretty nice

while a .get solution will turn into a nightmare very quickly:

d.get('a', d.get('b', d.get('c'))) # wtf?

Upvotes: 2

Andre Holzner
Andre Holzner

Reputation: 18685

you can use dict.get:

a = d.get("a", d.get("aaa"))
b = d.get("b", d.get("bbb"))

Note however that this will do a lookup of "aaa" and "bbb" even if "a" and "b" exist.

This works also (because or works e.g. for None and str etc.):

a = d.get("a") or d.get("aaa")

and does not do a second lookup if the first one succeeds.

Note that this does NOT work if None is a possible value associated to a key in your dict.

As @TimPietzcker points out, if you don't have None as a possible value, you can do:

a = d.get("a") or d["aaa"]

in order to get an exception if both keys do not exist.

Upvotes: 2

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