Disallow creation of non-CRS (eg pipeline) in CRS class

[jorisvandenbossche]

From discussion with @snowman2

Currently, you can create a CRS object from eg a proj pipeline:

In [4]: pyproj.CRS("+proj=pipeline +ellps=GRS80 +step +proj=merc +step +proj=axisswap +order=2,1")                                                            
Out[4]: 
<CRS: +proj=pipeline +ellps=GRS80 +step +proj=merc +step ...>
Name: PROJ-based coordinate operation
Ellipsoid:
- UNDEFINED
Area of Use:
- UNDEFINED
Prime Meridian:
- UNDEFINED
Axis Info:
- UNDEFINED

which does not give you an actual CRS, but generic coordinate transformation. So it is probably a good idea to disallow this in the construction of a CRS object.

For this specific case we could check for proj=pipeline in the string. Or more in general we could check the proj_type and raise an error if it is not one of the CRS types (we could still do the first pipeline check as well to have a better error message for that specific case).

[snowman2]

I think just need to check that proj_type == PJ_TYPE_CRS

[jorisvandenbossche]

I think just need to check that proj_type == PJ_TYPE_CRS

Not sure if that is enough, as you have also PJ_TYPE_PROJECTED_CRS etc (don't know how such an equality check works, but I assume it is like an enum, and not aware that PJ_TYPE_PROJECTED_CRS is a subset of PJ_TYPE_CRS) ?

There is also proj_is_crs that can be probably used for this

[snowman2]

The proj_is_crs looks perfect. I am glad you found that.

[snowman2]

I have a less than optimal implementation in #263.

[snowman2]

I am going to close this as the solution is merged into master. Thanks for bringing up the idea!