Trying to import GIS without GEOS installed leads to obscure error message

[Carl Meyer]

The error message is "cannot import name GEOSException."

Given that we already calculate a HAS_GEOS variable, it would not be hard to issue a clearer message.

Here is an example of a new user being led quite a ways down a rabbit-hole by the confusing message: ​https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/django-users/W3IBOBTQMyU/3F8xBVMAwsYJ

[Claude Paroz]

Maybe something like that:

django/contrib/gis/db/models/__init__.py

@@ +1 @@
+from django.core.exceptions import ImproperlyConfigured
+
 # Want to get everything from the 'normal' models package.
 from django.db.models import *  # NOQA
 
+from django.contrib.gis.geos import HAS_GEOS
+
+if not HAS_GEOS:
+    raise ImproperlyConfigured(
+        "GEOS is required and has not been detected. Are you sure it is installed? "
+        "See also https://docs.djangoproject.com/en/stable/ref/contrib/gis/install/geolibs/")
+
 # Geographic aggregate functions
 from django.contrib.gis.db.models.aggregates import *  # NOQA

[Carl Meyer]

Yep, that looks pretty reasonable to me.

[Carl Meyer]

Created a pull request (​https://github.com/django/django/pull/3581) just to get CI verification that this doesn't break anything.

[Carl Meyer <carl@…>]

In d2bcb0598058dd93eac94bdbb7dd2565cff0abea:

Fixed #23873 -- Improved GIS error message when GEOS is not installed.

Thanks Claude for writing the patch.