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.

Ok, I will try that. Is there something like a get("<key>","<default value>") — Mukund Gandlur
– Mukund Gandlur, Commented Apr 13, 2016 at 12:14

Muhammad Tahir · Accepted Answer · 2018-12-18 06:52:39Z

21

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, 2018 at 6:52

answered Apr 13, 2016 at 12:13

Muhammad Tahir

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2 Comments

Aaron Hall Over a year ago

.get would be fine, setdefault for mutable in-place values

jimh Over a year ago

.get is exactly what I was looking for. It's nice not to have try except everywhere in the code.

simleo · Accepted Answer · 2018-12-17 16:07:53Z

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, 2018 at 16:07

answered Apr 13, 2016 at 12:17

simleo

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Comments

Serge Ballesta · Accepted Answer · 2016-04-13 12:16:13Z

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, 2016 at 12:16

Serge Ballesta

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1 Comment

Aaron Hall Over a year ago

No Counter? ... that would be more ... Pythonic.

Pedru · Accepted Answer · 2016-04-13 12:24:42Z

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, 2016 at 12:24

answered Apr 13, 2016 at 12:14

Pedru

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