[Python] What is the difference between deep copy and shallow copy in Python, and when should...

I am trying to understand the difference between shallow and deep copying in Python, particularly for nested lists or dictionaries. I was noticing some unexpected behavior by using copy.copy() and copy.deepcopy() on nested elements when modified. For example, in the code snippet below, sometimes modifications to the shallow copy impact the original, while modifications to a deep copy do not. Would someone be able to explain why this is and under what circumstances to use shallow vs. deep copies?

I attempted both copy.copy() and copy.deepcopy() to make copies of a nested list. I reasoned that changing either copy wouldn't affect the original list. Actually, when I fiddled with the shallow copy, I was surprised to see changes in the original list for the nested stuff; but when I did it on the deep copy nothing shows on the original list. I'm trying to get my head around why this is happening and how to choose which copy method to roll with in different situations.

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