Best way to handle a keyerror in a dict

Question

I would like to know the best way to handle a keyerror, when I try to get a value from a dict.

I need this because my dict holds the counts of some events. And whenever an event occurs I take the count from the dict and increment it and put it back.

I found some solutions online, but they were for some other languages. Any help is appreciated.

I am handling the keyerror exception now. Would like to know the best approach to handle a keyerror in a dictionary.

Note: This is not about counting items in a list but about handling exception when retrieving a value(that does not exist) from a dict.

@MukundGandlur Try [`defaultdict`](https://docs.python.org/2/library/collections.html#collections.defaultdict) — Selcuk, Apr 13 '16 at 12:14
yes there is exactly [that get you're talking about](https://docs.python.org/2/library/stdtypes.html#dict.get) — MSeifert, Apr 13 '16 at 12:15

Muhammad Tahir · Accepted Answer · 2018-12-18T06:52:39.147

19

You can use dict.get if you want to use dict

mydict[key] = mydict.get(key, 0) + 1

Or you can handle KeyError

try:
    mydict[key] += 1
except KeyError:
    mydict[key] = 1

Or you can use defaultdict

from collections import defaultdict
mydict = defaultdict(int)
mydict[key] += 1

edited Dec 18 '18 at 06:52

answered Apr 13 '16 at 12:13

Muhammad Tahir

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`.get` would be fine, `setdefault` for mutable in-place values – Russia Must Remove Putin Apr 13 '16 at 12:16
.get is exactly what I was looking for. It's nice not to have try except everywhere in the code. – jimh Mar 25 '19 at 00:38

simleo · Answer 2 · 2018-12-17T16:07:53.863

4

The most appropriate data structure for what you want to do is collections.Counter, where missing keys have an implicit value of 0:

from collections import Counter
events = Counter()
for e in "foo", "bar", "foo", "tar":
    events[e] += 1

edited Dec 17 '18 at 16:07

answered Apr 13 '16 at 12:17

simleo

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score 2 · Answer 3 · answered Apr 13 '16 at 12:16

2

collections.defaultdict could help to build a pythonic code:

count = collections.defaultdict(int) # => default value is 0
...
count[event] += 1 # will end to 1 on first hit and will increment later

answered Apr 13 '16 at 12:16

Serge Ballesta

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No Counter? ... that would be more ... Pythonic. – Russia Must Remove Putin Apr 13 '16 at 12:17

Pedru · Answer 4 · 2016-04-13T12:24:42.290

0

Exceptions in Python are not very expensive.

Plus, if you consider that most of the time the key you are looking for (the count) is there, then it is totally fine.

d = {}
while True:
    try:
        d['count'] += 1
    except KeyError:
        d['count'] = 1  # happens only once

edited Apr 13 '16 at 12:24

answered Apr 13 '16 at 12:14

Pedru

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Best way to handle a keyerror in a dict

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