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

Question

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 else:   my_dict[key] += 1 

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

Answer 1

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 else:     my_dict[key] = 1 

Answer 2

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.