Python for Programmers: Defaultdict

Welcome to the Defaultdict lesson!

This lesson is shown as static text below. However, it's designed to be used interactively. Click the button below to start!

Suppose that we want to count how many times each word appears in a string. We might build a dictionary where the keys are words and the values are word counts.

def word_count(s):
  counts = {}

  for word in s.split(" "):
    if word not in counts:
      counts[word] = 0
    counts[word] += 1

  return counts

Note: this code example reuses elements (variables, etc.) defined in earlier examples.
>
word_count("a cat is a cat")
Result:
Dicts raise KeyError when we access keys that don't exist. That's why our code had to check if word not in counts.

counts = {}
counts["cat"]

Result:

KeyError: 'cat'

With Python's defaultdict, we can skip the if word not in counts check. It's a dictionary-like object that takes a default value function. When we access a key that doesn't exist, the defaultdict calls the function and gives us its return value.

from collections import defaultdict

counts = defaultdict(lambda: 0)
counts["a key that doesn't exist"]

Result:

With defaultdict, we can simplify our original word_count function.
Here's a code problem:
The word_count function below has a bug: it tries to modify dict keys that don't exist, which raises an exception. Fix the function by using a defaultdict instead of a regular dict. The default value should be 0.

(Only one line of code is editable. You don't need to change any other lines!)
from collections import defaultdict

def word_count(s):
counts = defaultdict(lambda: 0)
for word in s.split(" "):
counts[word] += 1
return counts

# We convert back to a dict for printing, since it's more readable.
dict(word_count("a cat is a cat"))
Goal:
{'a': 2, 'cat': 2, 'is': 1}
Yours:
{'a': 2, 'cat': 2, 'is': 1}
Show Author's Answer
After we access a key that doesn't exist, does that key now exist in the defaultdict? Or does it return the default value to us without actually adding the key to the dictionary? We can write some code to check.

from collections import defaultdict

counts = defaultdict(lambda: 0)

counts["some key"]
"some key" in counts

Result:

Merely reading a defaultdict key ensures that it exists, even if we never write to that key.
You might wonder why we provide the default value in the form of a function, like defaultdict(lambda: 0), rather than directly providing the value, like defaultdict(0).
One important reason is that we might want the default value to change over time. For example, imagine that we're tracking integer ID numbers for users. When we try to get a user's ID for the first time, we want to assign them a new, unique ID. Then, we should always get that same ID for that user.

from collections import defaultdict

next_id = 1

def get_next_id():
  global next_id
  id = next_id
  next_id += 1
  return id

user_ids = defaultdict(get_next_id)

first_amir_id = user_ids["Amir"]
betty_id = user_ids["Betty"]
second_amir_id = user_ids["Amir"]

(first_amir_id, betty_id, second_amir_id)

Result:

(1, 2, 1)

That code is already pretty terse. However, in a future lesson we'll see a way to make it much shorter!