python - Check if a given key already exists in a dictionary

Thursday 2 January 2020

Answer

I wanted to test if a key exists in a dictionary before updating the value for the key.

I wrote the following code:

if 'key1' in dict.keys():
  print "blah"
else:
  print "boo"

I think this is not the best way to accomplish this task. Is there a better way to test for a key in the dictionary?

Answer

in is the intended way to test for the existence of a key in a dict.

d = dict()

for i in range(100):
    key = i % 10
    if key in d:
        d[key] += 1
    else:
        d[key] = 1

If you wanted a default, you can always use dict.get():

d = dict()

for i in range(100):
    key = i % 10
    d[key] = d.get(key, 0) + 1

... and if you wanted to always ensure a default value for any key you can use defaultdict from the collections module, like so:

from collections import defaultdict

d = defaultdict(int)

for i in range(100):
    d[i % 10] += 1

... but in general, the in keyword is the best way to do it.