Add explanatory error messages to parameter checks

timokau · timokau · commit 5aeae49e6476 · 2020-07-16T16:58:11.000+02:00
This can make it much easier to work with this test while making a
pre-existing estimator compatible.
diff --git a/sklearn/utils/estimator_checks.py b/sklearn/utils/estimator_checks.py
@@ -2594,21 +2594,26 @@ def param_filter(p):
             allowed_types = {str, int, float, bool, tuple, type(None), type, types.FunctionType, joblib.Memory}
             # Any numpy numeric such as np.int32.
             allowed_types.update({np.dtype(type_name) for type_name in np.core.numeric_types.genericTypeRank})
-            assert type(init_param.default) in allowed_types
+            assert type(init_param.default) in allowed_types, (
+                    f"Parameter '{init_param.name}' of estimator '{Estimator.__name__}' is of type {type(init_param.default).__name__} which is not allowed. "
+                    f"All init parameters have to be immutable to make cloning possible. "
+                    f"Therefore we restrict the set of legal types to {set(type.__name__ for type in allowed_types)}."
+            )
             if init_param.name not in params.keys():
                 # deprecated parameter, not in get_params
-                assert init_param.default is None
+                assert init_param.default is None, f"Estimator parameter '{init_param.name}' of estimator '{Estimator.__name__}' is not returned by get_params. If it is deprecated, set its default value to None."
                 continue
 
             param_value = params[init_param.name]
             if isinstance(param_value, np.ndarray):
                 assert_array_equal(param_value, init_param.default)
             else:
+                failure_text = f"Parameter {init_param.name} was mutated on init. All parameters must be stored unchanged."
                 if is_scalar_nan(param_value):
                     # Allows to set default parameters to np.nan
-                    assert param_value is init_param.default, init_param.name
+                    assert param_value is init_param.default, failure_text
                 else:
-                    assert param_value == init_param.default, init_param.name
+                    assert param_value == init_param.default, failure_text
 
 
 def _enforce_estimator_tags_y(estimator, y):
