Python dict of dicts with default value

Question

In python 2.7 I've got a dictionary of dictionaries, and I'm trying to get values from this in a speedy way. However, sometimes one of the keys (could be either one) doesn't exist in my dictionary, in that case I would like to get a default value.

My dictionary looks like this:

values = { '1A' : { '2A' : 'valAA', '2B' : 'valAB'},
           '1B' : { '2A' : 'valBA', '2B' : 'valBB'} }

which works great when I query it with existing keys:

>>> values['1A']['2A']
'valAA'
>>> values.get('1B').get('2B')
'valBB'

How do I get it to do this:

>>> values.get('not a key').get('not a key')
'not present'

Sorry, missed that one, thought I figured out something new but it's a duplicate — Swier, Mar 10 '16 at 14:43

score 1 · Accepted Answer · answered Mar 10 '16 at 14:37

1

This works like a charm:

values.get(key1, {}).get(key2, defaultValue)

In case the second key is not present in the dictionary, the default value of the second .get() is returned. In case the first key is not present in the dictionary, the default value is an empty dictionary, which ensures the second key will not be present in it. The default value for the second .get() will then be returned as well.

For example:

>>> defaultValue = 'these are not the values you are looking for'
>>> key1, key2 = '1C', '2C'
>>> values.get(key1, {}).get(key2, defaultValue)
'these are not the values you are looking for'
>>> key1, key2 = '1A', '2B'
>>> values.get(key1, {}).get(key2, defaultValue)
'valAB'

answered Mar 10 '16 at 14:37

Swier

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This won't work if you try key2 first – rafaelc Mar 10 '16 at 14:40
1

@RafaelCardoso: why would you try key2 first? This works just fine. – Martijn Pieters Mar 10 '16 at 14:53
@MartijnPieters I've seen no guarantee you know which key comes first. Maybe in this specific example where keys are "ordered", but it is certainly not a generic solution. – rafaelc Mar 10 '16 at 14:57
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@RafaelCardoso: but `key1` is applied to the outer dictionary, `key2` to the inner. There is an order here. I'm not sure what you are getting at. – Martijn Pieters Mar 10 '16 at 14:58
I've just read it again and you're correct. Not sure where the hell I took that from lol – rafaelc Mar 10 '16 at 15:00

Hossain Muctadir · Answer 2 · 2016-03-10T14:42:57.420

1

Create a function to the get the value.

values = { '1A' : { '2A' : 'valAA', '2B' : 'valAB'},
           '1B' : { '2A' : 'valBA', '2B' : 'valBB'} }

def get_value(dict, k1, k2):
    try:
        return dict[k1][k2]
    except KeyError as ex:
        return 'does not exist'

print get_value(values, '1A', '2A')
print get_value(values, '1A', '4A')

edited Mar 10 '16 at 14:42

answered Mar 10 '16 at 14:40

Hossain Muctadir

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Thanks for the suggestion – Hossain Muctadir Mar 10 '16 at 14:43

Python dict of dicts with default value

2 Answers2