BUG-Fix: AssertionError when slicing MultiIndex and setting value of …
- [x] closes #34870 
- [x] tests added / passed (1 / 1)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

@dsaxton, please, can you watch it. I looked in more detail. In variables, `start_slice` and `end_slice` are lists with coordinates. Therefore, we get an error when checking for a scalar. Having studied the code, I did not find the reasons why we cannot process them. > @dsaxton, please, can you watch it. I looked in more detail. In variables, `start_slice` and `end_slice` are lists with coordinates. Therefore, we get an error when checking for a scalar. Having studied the code, I did not find the reasons why we cannot process them.

I'm not very familiar with this part of the code base so take my advice with a grain of salt, but as far as I can tell start_slice and end_slice _should_ be scalars so we don't want to just remove these assertions as you're doing here.

I would instead look upstream of this to figure out why this is happening (it does seem that the Index is mangled by the time we get here and is no longer a MultiIndex, if that's any clue). @dsaxton, I rolled back the changes and took your advice into account. You can view? @LuckyDenis can you address comments and add a release note @LuckyDenis can you address @jreback comment https://github.com/pandas-dev/pandas/pull/35018#discussion_r468680783 Closing as stale