Check if a given key already exists in a dictionary

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 tests for the existence of a key in a dict:

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

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

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

---

Use dict.get() to provide a default value when the key does not exist:

d = {}

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

---

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

d = {}

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

or use defaultdict from the collections module:

from collections import defaultdict

d = defaultdict(int)

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