MAINT: unify spatial index error handling

Currently, there is no unified approach to error handling for empty spatial indexes or lack of a spatail index backend:

• sjoin does it’s own check for rtree, this means you need rtree even if you have pygeos!
• overlay and clip do not do any checks, they would fail with something like None does not have any attribute query_bulk, which is not very helpful.

Further, when you create a spatial index, if:

• The spatial index is empty, GeoDataFrame.sindex gets set to None
• There is not spatial index backend, GeoDataFrame.sindex gets set to None and a warning is raised.

I don’t particularly like that handling. There is no way to tell what happened, aside from a warning.

I would like to propose the following:

• No modules outside of sindex shall check for a spatial index backend. They should just blindly try to access the sindex property.
• The sindex backends should be instantiated even with no geometries, and should have a __len__ property that reports 0 when empty (this preserves the if gdf.sindex: syntax used in some modules).
• In the case of no spatial index backend, a clear error should be raised by `sindex.*

* Currently, the error is raised only when a property of the spatial index is accessed (sindex.intersection). There are two options with this:

1. Preserve the current behavior, but make the error more useful. This would require setting GeoDataFrame.sindex to some non-None value, perhaps a dummy class that raises a user friendly error if any of its methods (query, query_bulk, etc.) are accessed and always reports a len of 0. The big con of this approach is complexity. We’d have to make a dummy class, etc.
2. Raise an error when GeoDataFrame.sindex is accessed. This is the lower complexity approach, we just replace what is currently a warning with an error. It would however require either a try/except block or an import of geopandas.sindex.has_sindex by other modules to handle this error if they want to operate without a spatial index. Currently, all modules that use a spatial index cannot operate without a spatial index, so they would actually not need to do anything.

[adriangb]

A deprecation warning seems fine to me. So:

• No backend raises an error (currently a warning, which is followed by some downstream error).
• Empty spatial index raises a deprecation warning and gets set to None, in the future will be set to a spatial index instance with len == 0.

[brendan-ward]

👍

In the case of no spatial index backend, a clear error should be raised by sindex.*

Conflating None as an indicator of an empty spatial index with a missing backend is not at all ideal here. Unmet dependencies should raise errors.

A deprecation period seems fine for this, in which case it would be good to get the deprecation warning into 0.8.0.