Why call `tolist()` when constructing pandas MultiIndex?

[y4n9squared]

What is your issue?

I have a workload that is calling ds.to_dataframe() which takes several seconds because the output DataFrame has 10M+ rows. On my machine, the vast majority of the time (>99%) spent in xr.Dataset.to_dataframe() is constructing the pd.MultiIndex and within that, >80% of the time is spent calling tolist() and forcing the constructor of pd.MultiIndex to iterate through a list rather than an ndarray.

On line L180:

https://github.com/pydata/xarray/blob/main/xarray/core/coordinates.py#L180

is there a reason to call .tolist() rather than just keeping the object as an ndarray? Removing .tolist() results in a significant performance improvement for me.

[welcome]

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[dcherian]

We'd gladly take a PR for that. AFAICT as long as pandas accepts arrays, we should be good