[Python] Add tests for recursive imports

experimental/python/databricks/bundles/core/_load.py

@@ -7,7 +7,7 @@
 from types import ModuleType
 from typing import Any, Iterable
 
-from databricks.bundles.core import Diagnostics
+from databricks.bundles.core._diagnostics import Diagnostics
 from databricks.bundles.core._location import Location
 from databricks.bundles.core._resource import Resource
 from databricks.bundles.core._resources import Resources

experimental/python/databricks_tests/core/test_import.py

@@ -0,0 +1,62 @@
+import sys
+from types import ModuleType
+
+import pytest
+
+
+@pytest.fixture(scope="function")
+def reset_sys_modules():
+    """
+    Reset sys.module before test and restore it after test.
+    """
+
+    original_modules = sys.modules.copy()
+    try:
+        import databricks.bundles
+
+        for module in original_modules.keys():
+            if is_subpackage(module, databricks.bundles):
+                del sys.modules[module]
+
+        assert databricks.bundles.__package__
+        del sys.modules[databricks.bundles.__package__]
+
+        yield
+    finally:
+        # Clear any modifications and restore the original modules
+        sys.modules.clear()
+        sys.modules.update(original_modules)
+
+
+def test_import_regression(reset_sys_modules):
+    """
+    We don't want importing databricks.bundles.core to import any of resources.
+
+    We refer to resources in type signatures (e.g. in Resources or @job_mutator),
+    unless we use forward references (e.g. "Job" instead of Job) we can create a
+    cycling dependency resulting in a runtime error.
+
+    We want every package with resources (e.g., databricks.bundles.jobs) to be loaded
+    only when needed.
+    """
+
+    import databricks.bundles.core
+
+    assert databricks.bundles.core
+
+    import databricks.bundles
+
+    for module in sys.modules.keys():
+        if is_subpackage(module, databricks.bundles):
+            is_core = module == databricks.bundles.core.__package__
+            is_core_subpackage = is_subpackage(module, databricks.bundles.core)
+
+            assert is_core or is_core_subpackage, (
+                f"Unexpected loaded module AFTER loading core: {module}"
+            )
+
+
+def is_subpackage(module: str, parent: ModuleType) -> bool:
+    assert parent.__package__
+
+    return module.startswith(parent.__package__ + ".")