Better traceback for bad imports in custom path

[dset0x]

No description provided.

[RonnyPfannschmidt]

i suspect a potential use case lingering in lazy code loaders that inject modules into sys.modules at their own import

on closer examination the internal logic is grown a bit strange and should probably do a different loop altogether

[davidism]

We can preserve the "not set up by parent" case by saving the exception and raising it in the AttributeError block. However, this just adds more complexity to a code path no one can reproduce. And it makes the chained traceback in Python 3 look like it's coming from the wrong place. I'm inclined to merge this PR as-is, leaving this diff as a note in case someone comes up with a failing test case.

diff --git a/werkzeug/utils.py b/werkzeug/utils.py
index 5c8f5125..b3e859f7 100644
--- a/werkzeug/utils.py
+++ b/werkzeug/utils.py
@@ -423,10 +423,17 @@ def import_string(import_name, silent=False):
             return sys.modules[import_name]
 
         module_name, obj_name = import_name.rsplit('.', 1)
-        module = __import__(module_name, globals(), locals(), [obj_name])
+        preserved_import_error = None
+        try:
+            module = __import__(module_name, globals(), locals(), [obj_name])
+        except ImportError as e:
+            preserved_import_error = e
+            module = import_string(module_name)
         try:
             return getattr(module, obj_name)
         except AttributeError as e:
+            if preserved_import_error is not None:
+                raise preserved_import_error
             raise ImportError(e)
 
     except ImportError as e:

[dset0x]

Nearly three and a half years later, I still vividly recall the dozens of hours of pain that this was helping solve for the (now-deprecated) referenced module. Thanks for the merge. I hope it helps people get useful tracebacks.

[davidism]

Sorry it took so long 😞 It came up in a much more recent issue, so it will still help. Thanks for working on it.