fix(python): DataFrame descending sorting by single list element

[khalidmammadov]

When calling sort with single str column and list of single descending parameter it fails with error:

ldf = pl.DataFrame({
    "age": [2, 5, 1],
    "names": ["Alice", "Cob", "Bob"]
}).lazy()

sorted_df = ldf.sort("age", descending=[False])
print(sorted_df)

  File "..../site-packages/polars/lazyframe/frame.py", line 1378, in sort
    self._ldf.sort(
TypeError: argument 'descending': 'list' object cannot be converted to 'PyBool'

It checks for types and avoids path that does not use list.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.96%. Comparing base (d52e13e) to head (e7ee26f).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19233      +/-   ##
==========================================
- Coverage   80.00%   79.96%   -0.04%     
==========================================
  Files        1527     1529       +2     
  Lines      209203   209820     +617     
  Branches     2415     2416       +1     
==========================================
+ Hits       167371   167783     +412     
- Misses      41284    41488     +204     
- Partials      548      549       +1     

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[nameexhaustion]

We should check and raise an error if the list length is not 1

[ritchie46]

I am not sure here. This will be a huge combinatorial nightmare checking this for the whole API. If you pass a single &str value for column, you should also pass a single bool value for sortedness.

So it's either all lists or all scalars.

[orlp]

Can't we fix this in some central location?

[khalidmammadov]

The way I see it, there are two branches in the body of the func and signature does not convey which permutations are allowed and which not.
PR aims to help user to be more permissive and honor the signature.
I have simplified the code to avoid isinstances to follow more Pythonic principles. Please take a look and let me know what you think.

[ritchie46]

I think we should just not hit the fast path if the user passes a list:

if isinstance(by, str) and isintance(descending, bool) and not more_by:

[ritchie46]

This isn't per se correct as the lenght is not checked.

[khalidmammadov]

fixed

[eitsupi]

I think the title should be fix(python): ...