Make 'multiple Definitions objects' error message more actionable (#2…

…5453)

## Summary

Ran into this with a user - the "Cannot have more than one Definitions
object defined at module scope." error can be a bit of a rabbithole to
track down. The case we hit in particular was
when one of the `Definitions` objects was being imported unknowingly.

This PR updates the error message to include the names of all the
located `Definitions` objects, so it's easier to track down where
they're coming from and rectify the error.

## How I Tested These Changes

Update unit tests.

## Changelog

> Errors raised from defining more than one `Definitions` object at
module scope now include the object names so that the source is easier
to determine.

python_modules/dagster/dagster/_core/workspace/autodiscovery.py

@@ -46,6 +46,10 @@ def loadable_targets_from_python_package(
     return loadable_targets_from_loaded_module(module)
 
 
+def _format_loadable_def(module: ModuleType, loadable_target: LoadableTarget) -> str:
+    return f"{module.__name__}.{loadable_target.attribute}"
+
+
 def loadable_targets_from_loaded_module(module: ModuleType) -> Sequence[LoadableTarget]:
     from dagster._core.definitions import JobDefinition
     from dagster._utils.test.definitions import LazyDefinitions
@@ -64,8 +68,12 @@ def loadable_targets_from_loaded_module(module: ModuleType) -> Sequence[Loadable
 
     if loadable_defs:
         if len(loadable_defs) > 1:
+            loadable_def_names = ", ".join(
+                _format_loadable_def(module, loadable_def) for loadable_def in loadable_defs
+            )
             raise DagsterInvariantViolationError(
-                "Cannot have more than one Definitions object defined at module scope"
+                "Cannot have more than one Definitions object defined at module scope."
+                f" Found Definitions objects: {loadable_def_names}"
             )
 
         return loadable_defs

python_modules/dagster/dagster_tests/cli_tests/workspace_tests/autodiscovery_tests/autodiscover_in_module_multiple/__init__.py

@@ -0,0 +1,9 @@
+from dagster import Definitions
+
+from .defs1 import defs1  # noqa: TID252
+from .defs2 import defs2  # noqa: TID252
+
+defs = Definitions.merge(
+    defs1,
+    defs2,
+)

python_modules/dagster/dagster_tests/cli_tests/workspace_tests/autodiscovery_tests/autodiscover_in_module_multiple/defs1.py

@@ -0,0 +1,3 @@
+from dagster import Definitions
+
+defs1 = Definitions()

python_modules/dagster/dagster_tests/cli_tests/workspace_tests/autodiscovery_tests/autodiscover_in_module_multiple/defs2.py

@@ -0,0 +1,3 @@
+from dagster import Definitions
+
+defs2 = Definitions()

python_modules/dagster/dagster_tests/cli_tests/workspace_tests/autodiscovery_tests/test_autodiscovery.py

@@ -202,11 +202,34 @@ def test_double_defs_in_file() -> None:
     dagster_defs_path = file_relative_path(__file__, "double_defs.py")
     with pytest.raises(
         DagsterInvariantViolationError,
-        match="Cannot have more than one Definitions object defined at module scope",
+        match="Cannot have more than one Definitions object defined at module scope. Found Definitions objects: double_defs.defs, double_defs.double_defs",
     ):
         loadable_targets_from_python_file(dagster_defs_path)
 
 
+def test_local_directory_module_multiple_defs() -> None:
+    # this is testing that modules that *can* be resolved using the current working directory
+    # for local python modules instead of installed packages will succeed.
+
+    # pytest will insert the current directory onto the path when the current directory does is not
+    # a module
+    assert not os.path.exists(file_relative_path(__file__, "__init__.py"))
+    assert os.path.dirname(__file__) in sys.path
+
+    if "autodiscover_in_module_multiple" in sys.modules:
+        del sys.modules["autodiscover_in_module_multiple"]
+
+    with pytest.raises(
+        DagsterInvariantViolationError,
+        match="Cannot have more than one Definitions object defined at module scope. Found Definitions objects: autodiscover_in_module_multiple.defs, autodiscover_in_module_multiple.defs1, autodiscover_in_module_multiple.defs2",
+    ):
+        loadable_targets_from_python_module(
+            "autodiscover_in_module_multiple",
+            working_directory=os.path.dirname(__file__),
+            remove_from_path_fn=_current_test_directory_paths,
+        )
+
+
 def _current_test_directory_paths():
     # need to remove the current test directory when checking for module/package resolution.  we
     # have to manually remove the appropriate path from sys.path because pytest does a lot of