fix: raise ImproperlyConfigured for all unguessable storages

collectfast/strategies/base.py

@@ -150,9 +150,9 @@ def load_strategy(klass: Union[str, type, object]) -> Type[Strategy]:
 
 
 def _resolves_to_subclass(subclass_ref: str, superclass_ref: str) -> bool:
-    subclass = locate(subclass_ref)
-    assert isinstance(subclass, type)
     try:
+        subclass = locate(subclass_ref)
+        assert isinstance(subclass, type)
         superclass = locate(superclass_ref)
         assert isinstance(superclass, type)
     except (ImportError, AssertionError) as e:
@@ -176,5 +176,6 @@ def guess_strategy(storage: str) -> str:
     if _resolves_to_subclass(storage, _BOTO3_STORAGE):
         return _BOTO3_STRATEGY
     raise ImproperlyConfigured(
-        "No strategy configured, please make sure COLLECTFAST_STRATEGY is set."
+        "Collectfast failed to guess strategy, please make sure "
+        "COLLECTFAST_STRATEGY is set."
     )

collectfast/tests/strategies/test_guess_strategy.py

@@ -1,5 +1,7 @@
 from unittest import TestCase
 
+from django.core.exceptions import ImproperlyConfigured
+
 from collectfast.strategies.base import _BOTO3_STORAGE
 from collectfast.strategies.base import _BOTO3_STRATEGY
 from collectfast.strategies.base import _BOTO_STORAGE
@@ -24,14 +26,29 @@ def test_guesses_boto3_from_exact(case):
 def test_guesses_boto_from_subclass(case):
     # type: (TestCase) -> None
     case.assertEqual(
-        guess_strategy("collectfast.tests.boto_subclass.CustomStorage"), _BOTO_STRATEGY
+        guess_strategy("collectfast.tests.test_storages.boto_subclass.CustomStorage"),
+        _BOTO_STRATEGY,
     )
 
 
 @test
 def test_guesses_boto3_from_subclass(case):
     # type: (TestCase) -> None
     case.assertEqual(
-        guess_strategy("collectfast.tests.boto3_subclass.CustomStorage"),
+        guess_strategy("collectfast.tests.test_storages.boto3_subclass.CustomStorage"),
         _BOTO3_STRATEGY,
     )
+
+
+@test
+def test_raises_improperly_configured_for_unguessable_class(case):
+    # type: (TestCase) -> None
+    with case.assertRaises(ImproperlyConfigured):
+        guess_strategy("collectfast.tests.test_storages.unguessable.UnguessableStorage")
+
+
+@test
+def test_raises_improperly_configured_for_invalid_type(case):
+    # type: (TestCase) -> None
+    with case.assertRaises(ImproperlyConfigured):
+        guess_strategy("collectfast.tests.test_storages.unguessable.NotAType")

collectfast/tests/test_storages/unguessable.py

@@ -0,0 +1,5 @@
+class UnguessableStorage:
+    pass
+
+
+NotAType = 1

setup.cfg

@@ -37,6 +37,10 @@ source = collectfast
 
 [coverage:report]
 omit = */tests/*
+exclude_lines =
+    pragma: no cover
+    # Don't complain if tests don't hit defensive assertion code:
+    \.\.\.
 
 [metadata]
 license_file = LICENSE