added: ignore facet args with empty dataset

[AaronStiff]

Closes #3984

Code PR

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Being brand new to plotly, I'm unsure of where to put a test for this, as the original issue relates to a pandas.DataFrame.

[nicolaskruchten]

This seems like a reasonable fix, but probably belongs near the bottom of the infer_config function, just so it's in roughly the same place as most of the other similar mutations of args.

While we're on this topic of empty data frames though, are there any other args that fail when the df length is 0? would be good to understand why these specifically fail and if it impacts other categorical-type attributes like symbol or somethinng. Maybe there could be a more generic fix?

[nicolaskruchten]

PS: thanks for the PR! :)

[AaronStiff]

Ah yes, that makes sense. And you're very welcome. :)

I've moved it into infer_config for now, but I'll look into whether there could be a more generic solution.

[AaronStiff]

So it looks like facet_col and facet_row both fail with an empty dataset because they are used in the computation of ncols and nrows:

plotly.py/packages/python/plotly/plotly/express/_core.py

Lines 2014 to 2021 in 2b6ec1e

	if m.facet == "col":
	prefix = get_label(args, args["facet_col"]) + "="
	col_labels = [prefix + str(s) for s in sorted_values]
	ncols = len(col_labels)
	if m.facet == "row":
	prefix = get_label(args, args["facet_row"]) + "="
	row_labels = [prefix + str(s) for s in sorted_values]
	nrows = len(row_labels)

Neither ncols nor nrows can equal 0, hence the failure. As far as I can tell, nothing else in args effects either of the two in any other way.

To me, this seems like a fairly specific failure requiring a specific fix, but I'm still far from familiar with the codebase, so I could easily be wrong. :)

[alexcjohnson]

💃 Thanks @AaronStiff !

[AaronStiff]

You're most welcome! 👍