Inconsistent behavior when passing empty Points to MultiPoint

The documentation on MultiPoint says that I can pass to it a sequence of Points. But it doesn’t warn us against passing empty Points. Interestingly enough, if I pass an empty Point as the first element, it will throw an AssertionError:

>>> from shapely.geometry import Point, MultiPoint
>>> MultiPoint([Point()])

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-30-8942eaebd9a2> in <module>
----> 1 MultiPoint([Point()])

~\Miniconda3\lib\site-packages\shapely\geometry\multipoint.py in __init__(self, points)
     56             pass
     57         else:
---> 58             self._geom, self._ndim = geos_multipoint_from_py(points)
     59 
     60     def shape_factory(self, *args):

~\Miniconda3\lib\site-packages\shapely\geometry\multipoint.py in geos_multipoint_from_py(ob)
    162     except TypeError:
    163         n = ob[0]._ndim
--> 164     assert n == 2 or n == 3
    165 
    166     # Array of pointers to point geometries

AssertionError: 

>>> MultiPoint([Point(), Point(1, 2)])

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-31-08a0b83c889a> in <module>
----> 1 MultiPoint([Point(), Point(1, 2)])

~\Miniconda3\lib\site-packages\shapely\geometry\multipoint.py in __init__(self, points)
     56             pass
     57         else:
---> 58             self._geom, self._ndim = geos_multipoint_from_py(points)
     59 
     60     def shape_factory(self, *args):

~\Miniconda3\lib\site-packages\shapely\geometry\multipoint.py in geos_multipoint_from_py(ob)
    162     except TypeError:
    163         n = ob[0]._ndim
--> 164     assert n == 2 or n == 3
    165 
    166     # Array of pointers to point geometries

AssertionError: 

But if the empty Point goes after a nonempty Point, then it works fine:

>>> MultiPoint([Point(1, 2), Point()])
>>> # no error here

But the resulting object will throw an exception after trying to get its wkt:

>>> MultiPoint([Point(1, 2), Point()]).wkt

---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-38-c645b8b312e6> in <module>
----> 1 MultiPoint([Point(1, 2), Point()]).wkt

~\Miniconda3\lib\site-packages\shapely\geometry\base.py in wkt(self, **kw)
    365     def wkt(self, **kw):
    366         """WKT representation of the geometry"""
--> 367         return WKTWriter(lgeos, **kw).write(self)
    368 
    369     @property

~\Miniconda3\lib\site-packages\shapely\geos.py in write(self, geom)
    361         if geom is None or geom._geom is None:
    362             raise ValueError("Null geometry supports no operations")
--> 363         result = self._lgeos.GEOSWKTWriter_write(self._writer, geom._geom)
    364         text = string_at(result)
    365         lgeos.GEOSFree(result)

OSError: exception: access violation reading 0x0000000000000000

I assume there needs to be a guard against a presence of empty points in the passed sequence, and the docs should be updated too.

---

Shapely version: 1.6.4.post1, installed from conda.

[sgillies]

I’d like to avoid too much validation, because it’s expensive, but in this case I think filtering out empty parts and warning that this has been done is a good approach. Warn instead of log, because user code could handle the warning and change its behavior if desired.

[jorisvandenbossche]

I reported it in the meantime: https://github.com/libgeos/geos/issues/305

I still think it warrants some preventative action from Shapely, though.

For sure! The question might be if Shapely should raise an error upon creation, or when converting to WKT.