Practical dict Patterns for Interviews

Practical dict patterns: frequency, index map, memo for DP, graph adjacency, memoization of recursion, and dedup while preserving first wins via dict.fromkeys order.

Why this shows up in the real world

Configuration maps env keys to values; feature flags map users to variant dicts.

Core idea (explained for students)

Choose structure by operation mix: mostly reads → dict; mostly membership → set; ordered uniqueness → dict as ordered set (3.7+).

Try this in Python

keys = ['x', 'y', 'z']
scores = dict.fromkeys(keys, 0)
scores['x'] += 5
print(scores)

Common mistakes

• Using dict when only keys matter—wastes memory.
• Relying on order across different Python versions <3.7 (legacy).

Key takeaways

• dict.fromkeys(keys, 0) for zeroed counters with known keys.
• Read PEP 448 unpacking merges {**a, **b}.