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.

I must have missed it then, thanks kind stranger

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.

Hello, I am facing an issue, some python tests are expecting an output that shouldn't be possible,

on test number 'Add message 7':
the expected output is 8@6 : 3@30
but there are no buy or sell orders with this much quantity
the output should be 6@6 : 1@30

on test case number 'Cancel message 8'
the expected output is again something thats not possible with the given input.

Good!

Finally did it after like a day, my code is as ugly, long and unreadable as it could get, but finally. Great kata g964!

are you perchance modifying the input?