Raise ImportError on circular import (#2040)

Co-authored-by: Michael Jones <4087139+mikeedjones@users.noreply.github.com>

tests/importer/circular_import_a.py

@@ -0,0 +1,4 @@
+# Used by test_importer.py
+from .circular_import_b import foo # noqa
+
+bar = 123

tests/importer/circular_import_b.py

@@ -0,0 +1,4 @@
+# Used by test_importer.py
+from .circular_import_a import bar # noqa
+
+foo = 123

tests/importer/test_importer.py

@@ -41,3 +41,13 @@ def test_no_import_needed() -> None:
 
  instance = import_from_string(TemporaryFile)
  assert instance == TemporaryFile
+
+
+def test_circular_import_error() -> None:
+ with pytest.raises(ImportError) as exc_info:
+ import_from_string("tests.importer.circular_import_a:bar")
+ expected = (
+ "cannot import name 'bar' from partially initialized module "
+ "'tests.importer.circular_import_a' (most likely due to a circular import)"
+ )
+ assert expected in str(exc_info.value)

uvicorn/importer.py

@@ -19,7 +19,7 @@ def import_from_string(import_str: Any) -> Any:
 
  try:
  module = importlib.import_module(module_str)
- except ImportError as exc:
+ except ModuleNotFoundError as exc:
  if exc.name != module_str:
  raise exc from None
  message = 'Could not import module "{module_str}".'