[1.8.x] Fixed #24802 -- Delayed GDAL check for OSMGeoAdmin

Backport of a37dcfd0 from master

django/contrib/gis/admin/__init__.py

 # Getting the normal admin routines, classes, and `site` instance.
-from django.contrib.admin import (  # NOQA: flake8 detects only the last __all__
+from django.contrib.admin import (
     autodiscover, site, AdminSite, ModelAdmin, StackedInline, TabularInline,
     HORIZONTAL, VERTICAL,
 )
 # Geographic admin options classes and widgets.
-from django.contrib.gis.admin.options import GeoModelAdmin      # NOQA
-from django.contrib.gis.admin.widgets import OpenLayersWidget   # NOQA
+from django.contrib.gis.admin.options import GeoModelAdmin, OSMGeoAdmin
+from django.contrib.gis.admin.widgets import OpenLayersWidget
 
 __all__ = [
-    "autodiscover", "site", "AdminSite", "ModelAdmin", "StackedInline",
-    "TabularInline", "HORIZONTAL", "VERTICAL",
-    "GeoModelAdmin", "OpenLayersWidget", "HAS_OSM",
+    'autodiscover', 'site', 'AdminSite', 'ModelAdmin', 'StackedInline',
+    'TabularInline', 'HORIZONTAL', 'VERTICAL', 'GeoModelAdmin', 'OSMGeoAdmin',
+    'OpenLayersWidget',
 ]
-
-try:
-    from django.contrib.gis.admin.options import OSMGeoAdmin
-    HAS_OSM = True
-    __all__ += ['OSMGeoAdmin']
-except ImportError:
-    HAS_OSM = False

django/contrib/gis/admin/options.py

@@ -2,6 +2,9 @@ from django.contrib.admin import ModelAdmin
 from django.contrib.gis.admin.widgets import OpenLayersWidget
 from django.contrib.gis.db import models
 from django.contrib.gis.gdal import HAS_GDAL, OGRGeomType
+from django.core.exceptions import ImproperlyConfigured
+
+spherical_mercator_srid = 3857
 
 
 class GeoModelAdmin(ModelAdmin):
@@ -123,14 +126,17 @@ class GeoModelAdmin(ModelAdmin):
                       }
         return OLMap
 
-if HAS_GDAL:
-    spherical_mercator_srid = 3857
 
-    class OSMGeoAdmin(GeoModelAdmin):
-        map_template = 'gis/admin/osm.html'
-        num_zoom = 20
-        map_srid = spherical_mercator_srid
-        max_extent = '-20037508,-20037508,20037508,20037508'
-        max_resolution = '156543.0339'
-        point_zoom = num_zoom - 6
-        units = 'm'
+class OSMGeoAdmin(GeoModelAdmin):
+    map_template = 'gis/admin/osm.html'
+    num_zoom = 20
+    map_srid = spherical_mercator_srid
+    max_extent = '-20037508,-20037508,20037508,20037508'
+    max_resolution = '156543.0339'
+    point_zoom = num_zoom - 6
+    units = 'm'
+
+    def __init__(self, *args):
+        if not HAS_GDAL:
+            raise ImproperlyConfigured("OSMGeoAdmin is not usable without GDAL libs installed")
+        super(OSMGeoAdmin, self).__init__(*args)

tests/gis_tests/geoadmin/models.py

 from django.contrib.gis import admin
 from django.contrib.gis.db import models
+from django.contrib.gis.gdal import HAS_GDAL
 from django.utils.encoding import python_2_unicode_compatible
 
 
@@ -16,4 +17,5 @@ class City(models.Model):
     def __str__(self):
         return self.name
 
-admin.site.register(City, admin.OSMGeoAdmin)
+if HAS_GDAL:
+    admin.site.register(City, admin.OSMGeoAdmin)