Check if a given key already exists in a dictionary and increment it


Given a dictionary, how can I find out if a given key in that dictionary has already been set to a non-None value?

I.e., I want to do this:

my_dict = {}

if (my_dict[key] != None):
  my_dict[key] = 1
  my_dict[key] += 1

I.e., I want to increment the value if there's already one there, or set it to 1 otherwise.

4/16/2015 11:55:44 AM

Accepted Answer

You are looking for collections.defaultdict (available for Python 2.5+). This

from collections import defaultdict

my_dict = defaultdict(int)
my_dict[key] += 1

will do what you want.

For regular Python dicts, if there is no value for a given key, you will not get None when accessing the dict -- a KeyError will be raised. So if you want to use a regular dict, instead of your code you would use

if key in my_dict:
    my_dict[key] += 1
    my_dict[key] = 1
2/2/2019 8:26:10 AM

I prefer to do this in one line of code.

my_dict = {}

my_dict[some_key] = my_dict.get(some_key, 0) + 1

Dictionaries have a function, get, which takes two parameters - the key you want, and a default value if it doesn't exist. I prefer this method to defaultdict as you only want to handle the case where the key doesn't exist in this one line of code, not everywhere.

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