My Pythonic Code Diary

How to make use of defaultdict(list)?

A defaultdict is a specialized container found in Python’s built-in collections module. It works exactly like a standard dictionary, but with one major advantage: it never raises a KeyError.

When you initialize it as defaultdict(list), the list constructor acts as a default_factory. If a requested key is not found, it automatically creates a new, empty list [] for that key.

Real-World Example: Group Anagrams

This is a classic LeetCode Medium problem (LeetCode 49) often seen in IBM Research coding assessments.

from collections import defaultdict

def group_anagrams(strs):
    # Initialize defaultdict with list factory
    anagram_map = defaultdict(list)
    
    for s in strs:
        # Create a unique key by sorting the string
        key = "".join(sorted(s))
        
        # No need to check if key exists; just append!
        anagram_map[key].append(s)
        
    return list(anagram_map.values())

# Input: ["eat","tea","tan","ate","nat","bat"]
# Output: [['eat', 'tea', 'ate'], ['tan', 'nat'], ['bat']]

---

Accessing Dictionary Values: m[key] vs. m.get(key)

The main difference is how they handle missing keys.

• m[key] (Direct Access):
  • Raises a KeyError if the key is missing.
• m.get(key) (Safe Access):
  • Returns None (or a custom default) if the key is missing.

---

In Python, slicing ([:]) is specifically designed to be “forgiving” and will not raise an IndexError even if the range exceeds the list’s length.

# Scenario: 100 images, batch_size = 30
for i in range(0, 100, 30):
    # i will be 0, 30, 60, 90
    batch = data[i : i + 30] 
    
    # During the final iteration (i = 90):
    # data[90 : 120] is called.
    # Python sees the list ends at 99, so it safely 
    # returns the final 10 items (90-99) as a batch.

s