Rather than using if statements or for loops to initialize the keys of a dict, use the Python Standard Library built-in method defaultdict() to automatically initialize the value of every key in the dict. Using defaultdict() is more concise and less error-prone, as the programmer no longer has to remember to manually initialize the value of every key.

When a dict is created using defaultdict(), the value for each key in the dict will default to the value provided as the first argument of defaultdict(). This is more concise and less error-prone than manually setting the value of each key.

The module below defines an empty dict and then manually initializes the keys of the dict. Although there is nothing wrong with this code, there is a more concise and less error-prone way to achieve the same idea, as explained in the solution below.

d = {}

if not "k" in d:
    d["k"] = 6

d["k"] += 1

print d["k"]  # 7

The modified module below uses defaultdict to initialize the dict. Whenever a new key is created, the default value for that key is 6. This module is functionally equivalent to the previous module, but this one is more concise and less error-prone, because every key automatically initializes to 6 with no work on the part of the programmer.

from collections import defaultdict

d = defaultdict(lambda : 6)
d["k"] += 1

print d["k"]  # 7

• Python Standard Library - collections.defaultdict