python-3.xpython-2.7migration

How do I avoid "'zip' object is not reversible" errors when migrating to Python 3?

In my recently migrated from 2 to 3 Python code I have

list(reversed(zip(*positions)))

which generates the error

TypeError: 'zip' object is not reversible

I can fix this by changing the problematic code to

list(reversed(list(zip(*positions))))

but this seems like the wrong way to go about it.

What is the correct way to revers a zip in Python 3?

Solution

  • reversed is used to iterate on the list. It doesn't create a list on purpose, because it's often used just to iterate backwards on elements, not to create lists.

    That's why you have to use list on it to create a list. And it needs a sequence to be able to get to the last element directly so you have to do list(zip()) in python 3.

    Maybe you could shorten 

    list(reversed(list(zip(*positions))))

    to

    list(zip(*positions))[::-1]

    it creates a list directly without the need for reverse so it's probably slightly faster too.