Emit prominent MigrationWarning instead of breaking InferenceData imports

Author: michaelosthege

src/arviz/__init__.py

@@ -17,8 +17,9 @@
 import functools
 import logging
 import re
+import warnings
 
-from xarray import open_datatree as from_netcdf
+from xarray import open_datatree as from_netcdf, DataTree
 
 from_zarr = functools.partial(from_netcdf, engine="zarr")
 
@@ -27,8 +28,8 @@
 info = ""
 
 
-class MigrationError(RuntimeError):
-    """Error raised when a legacy name is accessed on the ``arviz`` namespace."""
+class MigrationWarning(DeprecationWarning):
+    """Warning raised when a legacy name is accessed on the ``arviz`` namespace."""
 
 
 try:
@@ -104,11 +105,13 @@ class MigrationError(RuntimeError):
 def __getattr__(name):
     """Guide users who expect legacy names on the ``arviz`` namespace."""
     if name == "InferenceData":
-        raise MigrationError(
+        warnings.warn(
             "arviz.InferenceData is no longer available on the "
             "arviz package; ArviZ now uses xarray's DataTree for the same "
-            f"role. See the migration guide: {_MIGRATION_GUIDE_URL}"
+            f"role. See the migration guide: {_MIGRATION_GUIDE_URL}",
+            MigrationWarning,
         )
+        return DataTree
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 

tests/test_namespace.py

@@ -18,6 +18,7 @@
 import arviz_plots
 import arviz_stats
 import pytest
+from xarray import DataTree
 
 import arviz as az
 
@@ -88,15 +89,15 @@ def test_incompatible_package_versions(monkeypatch):
 
 def test_inference_data_import_points_to_migration_guide():
     """Legacy arviz.InferenceData should error with a link to the migration guide."""
-    with pytest.raises(az.MigrationError, match="https://python.arviz.org/.*/migration_guide.*"):
+    with pytest.warns(az.MigrationWarning, match="https://python.arviz.org/.*/migration_guide.*"):
         from arviz import InferenceData
 
-        InferenceData
+        assert InferenceData is DataTree
 
 
 def test_inference_data_getattr_points_to_migration_guide():
     """Legacy arviz.InferenceData should error with a link to the migration guide."""
-    with pytest.raises(az.MigrationError, match="https://python.arviz.org/.*/migration_guide.*"):
+    with pytest.warns(az.InferenceData, match="https://python.arviz.org/.*/migration_guide.*"):
         az.InferenceData