👀 Views: 1060 💬 Answers: 1 📅 Created: 2025-06-21

python defaultdict collections nested-dictionary Python

I'm migrating some code and I'm getting frustrated with I'm working on a project and hit a roadblock... I am working with unexpected behavior when using a dictionary with a default factory in Python 3.11. I want to create a nested dictionary where the default value for each key is another dictionary. However, when I try to append to the inner dictionaries, it seems to overwrite the existing values instead of adding to them. Here’s the code I’m using: ```python from collections import defaultdict nested_dict = defaultdict(lambda: defaultdict(int)) nested_dict['first']['key'] += 1 nested_dict['first']['key'] += 2 nested_dict['second']['key'] += 3 print(nested_dict) ``` I expected the output to reflect that 'first' has a total of 3 for 'key' and 'second' has 3 as well. Instead, I see: ``` defaultdict(<function <lambda> at 0x7f8c125e5e50>, {'first': defaultdict(<class 'int'>, {'key': 1}), 'second': defaultdict(<class 'int'>, {'key': 3})}) ``` It looks like the value for `first['key']` is only 1 instead of 3. I suspect it might be related to how I’m using the `defaultdict`, but I don’t quite understand why this is happening. Can anyone explain what’s going wrong here and how I can fix it? I’ve tried changing the way I access the keys but the behavior remains unchanged. Thanks in advance! I'm working with Python in a Docker container on Windows 10. For reference, this is a production web app.