👀 Views: 265 💬 Answers: 1 📅 Created: 2025-08-08

python defaultdict dictionary nested collections Python

I'm integrating two systems and I'm trying to implement I'm relatively new to this, so bear with me. I'm working with a `defaultdict` from the `collections` module in Python 3.9, and I'm trying to build a nested dictionary structure for storing user activity logs. I expected that using `defaultdict` would handle missing keys gracefully, but I'm running into a `KeyError` when I try to access a nested key that should be created automatically. Here's a simplified version of my code: ```python from collections import defaultdict activity_log = defaultdict(lambda: defaultdict(list)) # Simulating user activities activity_log['user1']['login'].append('2023-10-01 10:00') activity_log['user1']['logout'].append('2023-10-01 15:00') # This works fine, but when I try to access a key that hasn't been set: try: print(activity_log['user1']['view']) # This should create the key, but it raises KeyError except KeyError as e: print(f"KeyError: {e}") ``` I was under the impression that accessing a key in a `defaultdict` would automatically initialize it, yet it seems that nested keys don't behave the same way. I also tried initializing the nested dictionary separately, but that added unnecessary complexity. Am I missing something in how `defaultdict` works, or is there a different approach I should use for creating the nested structure without working with `KeyError`? Any insights or best practices would be greatly appreciated! For context: I'm using Python on macOS. Am I missing something obvious? I'm coming from a different tech stack and learning Python. Any feedback is welcome! Thanks for any help you can provide! My development environment is CentOS. Am I missing something obvious?