Partition column comparison is an assertion rather than if block with raise exception

[joe-sharman]

Environment

Delta-rs version:
0.16.0
Binding:
python
Environment:
MacOS

---

Bug

What happened:
An assertion is done between partition columns from incoming data and target table. If this fails an AssertionError is raised.

delta-rs/python/deltalake/writer.py

Line 340 in 15538b9

assert (

What you expected to happen:
This should be an if block, and raise an exception if the two values aren't the same. ValueError would be fine.

How to reproduce it:
Try to write a table with the wrong partition columns specified in the incoming data.

More details:
This exception should also be changed, maybe something like FileExistsError

delta-rs/python/deltalake/writer.py

Line 333 in 15538b9

raise AssertionError("DeltaTable already exists.")

Edit:
assert statements are ignored when running in optimise mode, so this could cause issues for a user running in optimised mode

[ion-elgreco]

@joe-sharman do you want to make a contribution to address this?

[joe-sharman]

I'll put something together :)

[joe-sharman]

Hey @ion-elgreco I've had a go making the change. I've tried to follow the commit conventions in the repo and have used built-in exceptions rather than custom as that seems to be the convention here.