Emit prominent MigrationWarning instead of breaking InferenceData import (#2575)

* Emit prominent `MigrationWarning` instead of breaking `InferenceData` imports

* Subclass `FutureWarning`

* Subclass `MigrationWarning(UserWarning, FutureWarning)`

Author: michaelosthege

src/arviz/__init__.py

@@ -27,8 +27,8 @@
 info = ""
 
 
-class MigrationError(RuntimeError):
-    """Error raised when a legacy name is accessed on the ``arviz`` namespace."""
+class MigrationWarning(UserWarning, FutureWarning):
+    """Warning raised when a legacy name is accessed on the ``arviz`` namespace."""
 
 
 try:
@@ -104,11 +104,17 @@ class MigrationError(RuntimeError):
 def __getattr__(name):
     """Guide users who expect legacy names on the ``arviz`` namespace."""
     if name == "InferenceData":
-        raise MigrationError(
+        import warnings
+        from xarray import DataTree
+
+        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
 
@@ -33,10 +34,10 @@ def test_info_attr():
 
 def test_aliases():
     # These are xarray aliases exposed for user convenience
-    xarray_aliases = {"from_netcdf", "from_zarr"}
+    xarray_aliases = {"from_netcdf", "from_zarr", "InferenceData"}
 
     for obj_name in dir(az):
-        if not obj_name.startswith("_") and obj_name not in ["info", "MigrationError"]:
+        if not obj_name.startswith("_") and obj_name not in ["info", "MigrationWarning"]:
             obj = getattr(az, obj_name)
 
             if obj_name in xarray_aliases:
@@ -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.MigrationWarning, match="https://python.arviz.org/.*/migration_guide.*"):
         az.InferenceData