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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
It sure looks weird, but don't worry, the stack is behaving just fine, and Python is also operating as expected. You created a mutable default argument. Instead, you need to create a new object per function call.