Hi - I discussed this a bit with @phofl , as I'm aiming to have zero-cost-abstraction around Dask DataFrame in Narwhals

One thing I'd like to check is how to check that two Dask Series have the same index. Or, rather, that concatenating them would not result in any index alignment

Patrick pointed me to dask_expr._expr.are_co_aligned, which seemed to work great for me until I tried using __getitem__. Here's an example:

In [21]: df = dd.from_pandas(pd.DataFrame({'a': [1,2,3], 'b': [4,5,6]}))

In [22]: dask_expr._expr.are_co_aligned(df._expr, df['a'][[1,2,0]]._expr)
Out[22]: True

This isn't quite what I was expecting - if I compute df['a'][[1,2,0]], then the index has indeed been shuffled with respect to df

Is this a bug in are_co_aligned? If not, is there another way to check that index alignment does not happen?

Thanks 🙏