defaultdict

The defaultdict data type is one of my favorites. It allows you to avoid checking if a key is in a dictionary by simply inserting it for you on your first access attempt, with a default value whose type you pass on creation. In some cases, this tool can be very handy and shorten your code a little. Let's see a quick example. Say we are updating the value of age, by adding one year. If age is not there, we assume it was 0 and we update it to 1:

>>> d = {}
>>> d['age'] = d.get('age', 0) + 1  # age not there, we get 0 + 1
>>> d
{'age': 1}
>>> d = {'age': 39}
>>> d['age'] = d.get('age', 0) + 1  # age is there, we get 40
>>> d
{'age': 40}

Now let's see how it would work with a defaultdict data type. The second line is actually the short version of a four-lines-long if clause that we would have to write if dictionaries didn't have the get method (we'll see all about if clauses in Chapter 3, Iterating and Making Decisions):

>>> from collections import defaultdict
>>> dd = defaultdict(int)  # int is the default type (0 the value)
>>> dd['age'] += 1  # short for dd['age'] = dd['age'] + 1
>>> dd
defaultdict(<class 'int'>, {'age': 1})  # 1, as expected

Notice how we just need to instruct the defaultdict factory that we want an int number to be used in case the key is missing (we'll get 0, which is the default for the int type). Also, notice that even though in this example there is no gain on the number of lines, there is definitely a gain in readability, which is very important. You can also use a different technique to instantiate a defaultdict data type, which involves creating a factory object. To dig deeper, please refer to the official documentation.