[ENH] safer get_fitted_params default functionality to avoid except…

…ion on `getattr` (#353)

This PR makes the `get_fitted_params` core functionality and defaults
safer against exceptions on `getattr`.

In rare cases, `getattr` can cause an exception, namely if a property is
being accessed in a way that generates an exception.

Examples are some fitted parameter arguments in `sklearn` that are
decorated as property in newer versions, e.g.,
`RandomForestRegressor.estimator_` in unfitted state.

In most use cases, there will be no change in behaviour; in the
described case where an exception would be raised, this is now caught
and suppressed, and the corresponding parameter is considered as not
present.

Changes occur only in cases that would have previously raised genuine
exceptions, so no working code is affected, and no deprecation is
necessary despite this being a change to a core interface element.

skbase/base/_base.py

@@ -1292,10 +1292,47 @@ def _get_fitted_params_default(self, obj=None):
         fitted_params = [
             attr for attr in dir(obj) if attr.endswith("_") and not attr.startswith("_")
         ]
-        # remove the "_" at the end
-        fitted_param_dict = {
-            p[:-1]: getattr(obj, p) for p in fitted_params if hasattr(obj, p)
-        }
+
+        def getattr_safe(obj, attr):
+            """Get attribute of object, safely.
+
+            Safe version of getattr, that returns None if attribute does not exist,
+            or if an exception is raised during getattr.
+            Also returns a boolean indicating whether the attribute was successfully
+            retrieved, to distinguish between None value and non-existent attribute,
+            or exception during getattr.
+
+            Parameters
+            ----------
+            obj : any object
+                object to get attribute from
+            attr : str
+                attribute name to get from obj
+
+            Returns
+            -------
+            attr : Any
+                attribute of obj, if it exists and does not raise on getattr;
+                otherwise None
+            success : bool
+                whether the attribute was successfully retrieved
+            """
+            try:
+                if hasattr(obj, attr):
+                    attr = getattr(obj, attr)
+                    return attr, True
+                else:
+                    return None, False
+            except Exception:
+                return None, False
+
+        fitted_param_dict = {}
+
+        for p in fitted_params:
+            attr, success = getattr_safe(obj, p)
+            if success:
+                p_name = p[:-1]  # remove the "_" at the end to get the parameter name
+                fitted_param_dict[p_name] = attr
 
         return fitted_param_dict