Indexing ObsCollection always returns "obs" column about hydropandas

[martinvonk]

This is the reason why #209 fails

[martinvonk]

Happened to me after updating pandas to v.2.2.2
Updating pandas (2.2.1 -> 2.2.2)

[martinvonk]

Not sure if we need to do something with this. #209 is already fixed.
Maybe if behavior keeps existing after v2.2.2 we need to adress this.

[OnnoEbbens]

It has become an issue in the bro_bronhouder notebook now as well. I think it has something to do with this error: pandas-dev/pandas#57032

[OnnoEbbens]

Geopandas had the same problem: geopandas/geopandas#3060

[OnnoEbbens]

For now I will pin the pandas version to 2.2.1 or lower because this also gives errors in nlmod.

[OnnoEbbens]

It seems like a change in pandas behavior that caused this issue can actually help to improve hydropandas.

Behavior pandas <=2.2.1:

oc.loc[:, ["x", "y"]]

returned an ObsCollection. Just like in geopandas:

gdf.loc[:, ['id']]

would return a GeoDataframe

Recent changes to pandas (=2.2.2) changed this behavior in such a way that:

oc.loc[:, ["x", "y"]]

returns a DataFrame. Due to this behavior the ObsCollection constructor would attach an Obs column to it. I was quite surprised that the loc function calls the constructor when it returns an object but apparently it does.

I added an extra check to the constructor to see if it was called using a DataFrame and an obs_list/ObsClass. Only if one of the obs_list/ObsClass is given the 'obs' column is attached.

We should improve hydropandas further by following the behavior of Geopandas. If you slice an ObsCollection without the 'obs' column a DataFrame should be returned. Now an ObsCollection without the 'obs' column is returned which is an invalid ObsCollection.

We could even think about supporting an ObsSeries. Atm when you obtain a single column from an ObsCollection it will always return a pandas Series. For now I don't really like to do this. I think this adds more headache than hooray.