[ENH] merge sktime BaseEstimator into skbase BaseEstimator (#370

)

This merges some minor improvements from `sktime` `BaseEstimator` into
`skbase` `BaseEstimator`:

* docstring formatting
* clearer error messages
* deduplicating logic with `check_is_fitted` calls

Accompanies sktime/sktime#7213, see there for
explanation of merge and deduplication.

skbase/base/_base.py

@@ -1445,13 +1445,13 @@ class BaseEstimator(BaseObject):
     def __init__(self):
         """Construct BaseEstimator."""
         self._is_fitted = False
-        super(BaseEstimator, self).__init__()
+        super().__init__()
 
     @property
     def is_fitted(self):
-        """Whether `fit` has been called.
+        """Whether ``fit`` has been called.
 
-        Inspects object's `_is_fitted` attribute that should initialize to False
+        Inspects object's ``_is_fitted` attribute that should initialize to ``False``
         during object construction, and be set to True in calls to an object's
         `fit` method.
 
@@ -1460,25 +1460,43 @@ def is_fitted(self):
         bool
             Whether the estimator has been `fit`.
         """
-        return self._is_fitted
+        if hasattr(self, "_is_fitted"):
+            return self._is_fitted
+        else:
+            return False
 
-    def check_is_fitted(self):
+    def check_is_fitted(self, method_name=None):
         """Check if the estimator has been fitted.
 
-        Inspects object's `_is_fitted` attribute that should initialize to False
-        during object construction, and be set to True in calls to an object's
-        `fit` method.
+        Check if ``_is_fitted`` attribute is present and ``True``.
+        The ``is_fitted``
+        attribute should be set to ``True`` in calls to an object's ``fit`` method.
+
+        If not, raises a ``NotFittedError``.
+
+        Parameters
+        ----------
+        method_name : str, optional
+            Name of the method that called this function. If provided, the error
+            message will include this information.
 
         Raises
         ------
         NotFittedError
             If the estimator has not been fitted yet.
         """
         if not self.is_fitted:
-            raise NotFittedError(
-                f"This instance of {self.__class__.__name__} has not been fitted yet. "
-                f"Please call `fit` first."
-            )
+            if method_name is None:
+                msg = (
+                    f"This instance of {self.__class__.__name__} has not been fitted "
+                    f"yet. Please call `fit` first."
+                )
+            else:
+                msg = (
+                    f"This instance of {self.__class__.__name__} has not been fitted "
+                    f"yet. Please call `fit` before calling `{method_name}`."
+                )
+            raise NotFittedError(msg)
 
     def get_fitted_params(self, deep=True):
         """Get fitted parameters.
@@ -1503,19 +1521,15 @@ def get_fitted_params(self, deep=True):
             Dictionary of fitted parameters, paramname : paramvalue
             keys-value pairs include:
 
-            * always: all fitted parameters of this object, as via `get_param_names`
+            * always: all fitted parameters of this object, as via ``get_param_names``
               values are fitted parameter value for that key, of this object
-            * if `deep=True`, also contains keys/value pairs of component parameters
-              parameters of components are indexed as `[componentname]__[paramname]`
-              all parameters of `componentname` appear as `paramname` with its value
-            * if `deep=True`, also contains arbitrary levels of component recursion,
-              e.g., `[componentname]__[componentcomponentname]__[paramname]`, etc
+            * if ``deep=True``, also contains keys/value pairs of component parameters
+              parameters of components are indexed as ``[componentname]__[paramname]``
+              all parameters of ``componentname`` appear as ``paramname`` with its value
+            * if ``deep=True``, also contains arbitrary levels of component recursion,
+              e.g., ``[componentname]__[componentcomponentname]__[paramname]``, etc
         """
-        if not self.is_fitted:
-            raise NotFittedError(
-                f"estimator of type {type(self).__name__} has not been "
-                "fitted yet, please call fit on data before get_fitted_params"
-            )
+        self.check_is_fitted(method_name="get_fitted_params")
 
         # collect non-nested fitted params of self
         fitted_params = self._get_fitted_params()