Python Dictionary Defaults

05 Dec 2024

Python dictionaries are incredibly versatile and widely used, but there are some handy tricks that can make using them much more streamlined and efficient. In this post, we’ll explore dict.setdefault and collections.defaultdict.

What is dict.setdefault?

The dict.setdefault method is a useful addition when working with dictionaries, particularly when dealing with missing keys. It checks if a key exists, and if it doesn’t, it sets it with a default value. Here’s the syntax:

dict.setdefault(key, default=None)

Why Use dict.setdefault?

1. Less Boilerplate Code

   Instead of writing multiple lines to check for a key and add it if it’s missing, you can simplify it like this:

   Without setdefault:

   if key not in my_dict:
       my_dict[key] = []
   my_dict[key].append(value)
   

   With setdefault:

   my_dict.setdefault(key, []).append(value)
   

   Much tidier, wouldn’t you agree?

2. Easier Data Aggregation

   setdefault is excellent for grouping items or counting occurrences:

   items = [('fruit', 'apple'), ('vegetable', 'carrot'), ('fruit', 'banana')]
   
   categorized = {}
   for category, item in items:
       categorized.setdefault(category, []).append(item)
   
   print(categorised)  # Output: {'fruit': ['apple', 'banana'], 'vegetable': ['carrot']}
   

Meet collections.defaultdict

defaultdict takes things a step further. It automatically sets default values for missing keys using a factory function.

Usage:

from collections import defaultdict

default_dict = defaultdict(default_factory)

Why Use defaultdict?

1. Automatic Default Initialisation

   No need to check for keys manually:

   from collections import defaultdict
   
   words = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
   
   word_count = defaultdict(int)
   for word in words:
       word_count[word] += 1
   
   print(word_count)  # Output: defaultdict(<class 'int'>, {'apple': 3, 'banana': 2, 'orange': 1})
   

2. Cleaner Code

   It enhances readability by reducing repetitive patterns:

   from collections import defaultdict
   
   items = [('fruit', 'apple'), ('vegetable', 'carrot'), ('fruit', 'banana')]
   
   categorized = defaultdict(list)
   for category, item in items:
       categorized[category].append(item)
   
   print(categorised)  # Output: defaultdict(<class 'list'>, {'fruit': ['apple', 'banana'], 'vegetable': ['carrot']})
   

3. Supports Nested Structures

   defaultdict makes creating nested dictionaries a breeze:

   from collections import defaultdict
   
   nested_dict = defaultdict(lambda: defaultdict(int))
   nested_dict['group1']['item1'] += 1
   nested_dict['group1']['item2'] += 2
   
   print(nested_dict)
   

4. Better Performance

   It’s often faster, especially with numerous insertions or lookups.

When to Use Which?

• Use dict.setdefault when you have a standard dict and only occasionally need default handling.

• Use defaultdict for more frequent missing key scenarios, especially when reading and writing to complex data structures.

Conclusion

dict.setdefault and defaultdict are powerful tools that make working with dictionaries in Python less cumbersome. setdefault simplifies the handling of missing keys, while defaultdict takes care of default value management, resulting in clearer, more concise code.

I often notice people getting caught out with standard dictionary methods when they could be using these tools, and just today, defaultdict came in really handy when attempting to solve an Advent of Code puzzle (Day 5, 2024) which was what prompted me to write this post! 🎅

Embrace the full potential of Python dictionaries with setdefault and defaultdict, and you’ll be using the word “Pythonic” to describe your code in no time! 🐍🔑