Python 2.7.14: "+=1 to Dictionary Item if Key Exists Else = 1" Syntax Doesn't Work

user_2357647 0
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I'm trying to +=1 to an item in a dictionary if it exists, else create the key in the dictionary and set it to 1. I know I can use more than one line of logic for this, but I really want to see if its possible to keep the logic on one line. This is what I'm trying to do:

myDict = {}
myDict[100] += 1 if 100 in myDict else 1

It throw KeyError though if the key 100 doesn't exist. How can I set the one lined else logic to set key 100 to 1? Is it even possible?

Many thanks

dictionary python
5 Answers

The problem is that this is parsed as

myDict[100] += (1 if 100 in myDict else 1)

which is just the same as writing

myDict[100] += 1

and of course that requires myDict[100] already to exist. You can instead use a regular assignment and the dictionary get method, which lets you specify what is to be returned if the key is not already there:

myDict[100] = myDict.get(100, 0) + 1

Or you can use collections.Counter, which is a special-purpose dictionary designed for exactly this job:

import collections
myDict = collections.Counter()
myDict[100] += 1
user_3885200

Use get:

my_dict = {}
my_dict[100] =  my_dict.get(100, 0) +  1

Output

{100: 1}

Explanation (from the documentation)

Return the value for key if key is in the dictionary, else default

user_40015920

You are indexing already in the value so the error is thrown if is not in the dict This would work

if 100 in mydict:
    mydict[100] += 1
else:
    mydict[100] = 1

However even prettier would be mydict[100] = mydict.get(100, 0) + 1

user_73437210

In addition to collections.Counter there is collections.defaultdict, which allows you to set a default value to be used when a non-initialized key is passed to the dict. It is overkill for counters of course, but can be really wonderful in other situations.

user_105688060 
myDict[100] = myDict[100]+1 if 100 in myDict else 1

or, maybe better:

myDict.setdefault(100, 0)
myDict[100] +=1
user_3892890
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