Check if a given key already exists in the dictionary

Question

I want to test whether the key exists in the dictionary before updating the value of the key. I wrote the following code:

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

I don't think this is the best way to accomplish this task. Is there a better way to test keys in a dictionary?

Answer 1

Use key in my_dict directly instead of key in my_dict.keys():

if 'key1' in my_dict:
    print("blah")
else:
    print("boo")

This will be faster because it uses an O(1) hash of the dictionary instead of performing an O(n) linear search of the list of keys.

Answer 2

in Test whether the key exists in dict:

d = {"key1": 10, "key2": 23}

if "key1" in d:
    print("this will execute")

if "nonexistent key" in d:
    print("this will not")

---

Using dict.get() Provides a default value when the key does not exist:

d = {}

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

---

To provide a default value for each key, use dict.setdefault() on each job:

d = {}

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

...or better yet, use defaultdict from the collections module:

from collections import defaultdict

d = defaultdict(int)

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