id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28160	Prevent hiding GDAL errors if it's not installed	Tom Kazimiers	nobody	"Under some circumstances `django.contrib.gis.gdal.HAS_GDAL` is `False`. This can be due to not having GDAL installed in the first place or by overriding one's OS name (and thereby causing the test in `django.contrib.gis.gdal.__init__.py` to result in an exception which in turns causes `HAS_GDAL = False`).¹


Now with `HAS_GDAL = False`, I can't start Django anymore (neither the shell or any other command), because I get the following error:
{{{
django.core.exceptions.ImproperlyConfigured: 'postgresql' isn't an available database backend.
Try using 'django.db.backends.XXX', where XXX is one of:
    'mysql', 'oracle', 'postgresql', 'sqlite3'
Error was: No module named postgresql.base
}}}

Digging a bit deeper into this reveals this is actually an ImportError: `Error was: cannot import name GDALRaster`. Using other `django.contrib.gis` functionality should also work without GDAL (and did so in the past).

The problem seems to be that in commit `bbfad84dd980a97174c3b061a3d1b5f1373c380d` ([https://github.com/django/django/commit/bbfad84dd980a97174c3b061a3d1b5f1373c380d#diff-9501b5a77c1985430f52d03aedb3e434R7 here]) a new import has been added to `django/contrib/gis/db/backends/postgis/operations.py`, which tries to import `GDALRaster`: `from django.contrib.gis.gdal import GDALRaster`. This of course fails with `HAS_GDAL = False`, because `GDALRaster` has never been defined when `HAS_GDAL` was set false due to an (expected) exception in `django/django/contrib/gis/gdal/libgdal.py`, which is imported before `from django.contrib.gis.gdal.raster.source import GDALRaster` in `django/django/contrib/gis/gdal/__init__.py`.

So maybe the import in `django/contrib/gis/db/backends/postgis/operations.py` and the use of `GDALRaster` should be conditional and only run if `HAS_GDAL = True`. I can provide a patch if wanted.

This is a problem in both Django 1.10 and 1.11.

A workaround for this is to run this code before the (postgis) database driver is loaded (e.g. through an own custom database driver that just sub-classes the original):

{{{
import django.contrib.gis.gdal
class GDALRasterMock(object):
    pass
django.contrib.gis.gdal.GDALRaster = GDALRasterMock
}}}

¹ In my project I need to do the latter to disable GDAL support completely, because loading the library conflicts with `pgmagick` on Ubuntu and there is no other way to disable GDAL, but this shouldn't matter here. It would be nice though to have a way to force `HAS_GDAL = False`, because unlike said in `django/django/contrib/gis/gdal/__init__.py`, setting `GDAL_LIBRARY_PATH='/null/path'` doesn't work (the automatic OS checks override this)."	Bug	closed	GIS	1.10	Normal	fixed	GIS, GDAL, Contrib, Bug		Accepted	1	0	0	0	0	0