I think arr1 = arr.copy() is more pythonic.

Yes, this mutates the input. arr1 = arr does not produce a new list, it just makes a reference to it. So, when you empty arr1, you are emptying arr. Instead, use arr1 = arr[:], which will make a new list.

In the instructions it states When various orders have the same price, their quantities are aggregated and all of those orders appear at the same level in the orderbook.

Good!

are you perchance modifying the input?