Fix #821: Improvements to errors/warnings when dynamic Backend class lookups fail (#834)

* Raise Attribute error from __getattr__ as python docs recommend: https://docs.python.org/3/reference/datamodel.html#module.__getattr__
* Increase warning stacklevel by 1 so warning comes from the code doing the importing.

Author: LincolnPuzey

dramatiq/rate_limits/backends/__init__.py

@@ -37,28 +37,23 @@ def import_memcached():
         from .memcached import MemcachedBackend
 
         return MemcachedBackend
-    except ModuleNotFoundError:
-        warnings.warn(
-            "MemcachedBackend is not available.  Run `pip install dramatiq[memcached]` "
-            "to add support for that backend.",
-            category=ImportWarning,
-            stacklevel=2,
+    except ModuleNotFoundError as e:
+        message = (
+            "MemcachedBackend is not available.  Run `pip install dramatiq[memcached]` to add support for that backend."
         )
-        raise
+        warnings.warn(message, category=ImportWarning, stacklevel=3)
+        raise AttributeError(message) from e
 
 
 def import_redis():
     try:
         from .redis import RedisBackend
 
         return RedisBackend
-    except ModuleNotFoundError:
-        warnings.warn(
-            "RedisBackend is not available.  Run `pip install dramatiq[redis]` to add support for that backend.",
-            category=ImportWarning,
-            stacklevel=2,
-        )
-        raise
+    except ModuleNotFoundError as e:
+        message = "RedisBackend is not available.  Run `pip install dramatiq[redis]` to add support for that backend."
+        warnings.warn(message, category=ImportWarning, stacklevel=3)
+        raise AttributeError(message) from e
 
 
 _module_importers = {

dramatiq/results/backends/__init__.py

@@ -37,28 +37,23 @@ def import_memcached():
         from .memcached import MemcachedBackend
 
         return MemcachedBackend
-    except ModuleNotFoundError:
-        warnings.warn(
-            "MemcachedBackend is not available.  Run `pip install dramatiq[memcached]` "
-            "to add support for that backend.",
-            category=ImportWarning,
-            stacklevel=2,
+    except ModuleNotFoundError as e:
+        message = (
+            "MemcachedBackend is not available.  Run `pip install dramatiq[memcached]` to add support for that backend."
         )
-        raise
+        warnings.warn(message, category=ImportWarning, stacklevel=3)
+        raise AttributeError(message) from e
 
 
 def import_redis():
     try:
         from .redis import RedisBackend
 
         return RedisBackend
-    except ModuleNotFoundError:
-        warnings.warn(
-            "RedisBackend is not available.  Run `pip install dramatiq[redis]` to add support for that backend.",
-            category=ImportWarning,
-            stacklevel=2,
-        )
-        raise
+    except ModuleNotFoundError as e:
+        message = "RedisBackend is not available.  Run `pip install dramatiq[redis]` to add support for that backend."
+        warnings.warn(message, category=ImportWarning, stacklevel=3)
+        raise AttributeError(message) from e
 
 
 _module_importers = {