Handling Duplicate Values in a Dictionary: How to Update Instead of Overwriting in Python?
I'm maintaining legacy code that I'm optimizing some code but I've been struggling with this for a few days now and could really use some help... I'm working on a Python application where I need to store user preferences in a dictionary, but I want to ensure that when a user updates their preferences, the new value is added to a list rather than overwriting the existing value. For example, if a user selects multiple languages, I want to keep all selections rather than just the latest one. Here's a simplified version of what I've tried so far: ```python user_preferences = {} def update_preferences(user_id, new_preference): if user_id in user_preferences: if isinstance(user_preferences[user_id], list): user_preferences[user_id].append(new_preference) else: user_preferences[user_id] = [user_preferences[user_id], new_preference] else: user_preferences[user_id] = new_preference # Example usage: update_preferences('user1', 'English') update_preferences('user1', 'Spanish') update_preferences('user1', 'French') print(user_preferences) # Expected: {'user1': ['English', 'Spanish', 'French']} ``` However, when I run this code, I get the following output: ``` {'user1': ['English', 'Spanish']} ``` The third update seems to be failing. I suspect it's because the condition to check if the existing value is a list is not working as intended. Can anyone tell me how I can ensure that all preferences are stored correctly? I'm using Python 3.8. Any best practices or design patterns that would help in this scenario would be greatly appreciated! This is part of a larger service I'm building. Am I missing something obvious? I'm working on a application that needs to handle this. Has anyone else encountered this? Is there a simpler solution I'm overlooking? Any ideas what could be causing this?