docs: Add note on return type of columns property

[FBruzzesi]

Description

1. Do you like the phrasing?
2. Should we add the same note for the Schema spec? For the same reason we cannot guarantee it to be a Mapping from str to DType.

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes [Bug]: narwhals.DataFrame.columns does not always return list of strings #3571

[EdAbati]

I think the phrasing looks good!

I tried to find alternatives.

For the 2nd sentence we can use something like:
This is discouraged but supported, so the return type is not guaranteed to be list[str].

Or have it all one sentence:

Non-string column names (e.g. integers or booleans) are discouraged but supported by pandas-like and Dask backends, so the return type is not guaranteed to be list[str].

But again, I think your version looks good :)

[EdAbati]

Should we add the same note for the Schema spec? For the same reason we cannot guarantee it to be a Mapping from str to DType.

Do you mean in schema/collect_schema? I think think it'd be helpful 👍

We could also update the column_names.md to mention Dask too

[FBruzzesi]

Thanks for the feedback @EdAbati

Do you mean in schema/collect_schema? I think think it'd be helpful 👍

Both schema and collect_schema return a Schema object. I added similar notes in such class in beb053a

We could also update the column_names.md to mention Dask too

Updated the doc page in 934d15d