scikit-learn · glemaitre · Dec 28, 2022 · Dec 29, 2021 · Dec 29, 2021 · Dec 29, 2021
diff --git a/doc/model_persistence.rst b/doc/model_persistence.rst
@@ -55,6 +55,9 @@ with::
    available `here
    <https://joblib.readthedocs.io/en/latest/persistence.html>`_.
 
+Note that you can access to the attribute `__sklearn_pickle_version__` to check the
+version of scikit-learn used to pickle the model.
+
 .. _persistence_limitations:
 
 Security & maintainability limitations

diff --git a/doc/whats_new/v1.3.rst b/doc/whats_new/v1.3.rst
@@ -27,6 +27,12 @@ random sampling procedures.
 Changes impacting all modules
 -----------------------------
 
+- |Enhancement| All scikit-learn estimators now save the scikit-learn version
+  with which they have been pickled into a private attribute,
+  `__sklearn_pickle_version__`. This allows access to this information without
+  having to parse the warning message.
+  :pr:`22094` by :user:`Gawein Le Goff <Edern76>`.
+
 Changelog
 ---------
 

diff --git a/sklearn/base.py b/sklearn/base.py
@@ -288,13 +288,21 @@ def __getstate__(self):
             state = self.__dict__.copy()
 
         if type(self).__module__.startswith("sklearn."):
-            return dict(state.items(), _sklearn_version=__version__)
+            return dict(
+                state.items(),
+                __sklearn_pickle_version__=__version__,
+            )
         else:
             return state
 
     def __setstate__(self, state):
         if type(self).__module__.startswith("sklearn."):
+            # Before 1.3, `_sklearn_version` was used to store the sklearn version
+            # when the estimator was pickled
             pickle_version = state.pop("_sklearn_version", "pre-0.18")
+            pickle_version = state.setdefault(
+                "__sklearn_pickle_version__", pickle_version
+            )
             if pickle_version != __version__:
                 warnings.warn(
                     "Trying to unpickle estimator {0} from version {1} when "

diff --git a/sklearn/tests/test_base.py b/sklearn/tests/test_base.py
@@ -376,15 +376,18 @@ def test_pickle_version_warning_is_not_raised_with_matching_version():
 
 class TreeBadVersion(DecisionTreeClassifier):
     def __getstate__(self):
-        return dict(self.__dict__.items(), _sklearn_version="something")
+        return dict(self.__dict__.items(), __sklearn_pickle_version__="something")
 
 
 pickle_error_message = (
     "Trying to unpickle estimator {estimator} from "
     "version {old_version} when using version "
     "{current_version}. This might "
     "lead to breaking code or invalid results. "
-    "Use at your own risk."
+    "Use at your own risk. "
+    "For more info please refer to:\n"
+    "https://scikit-learn.org/stable/model_persistence.html"
+    "#security-maintainability-limitations"
 )
 
 
@@ -397,7 +400,7 @@ def test_pickle_version_warning_is_issued_upon_different_version():
         old_version="something",
         current_version=sklearn.__version__,
     )
-    with pytest.warns(UserWarning, match=message):
+    with pytest.warns(UserWarning, match=re.escape(message)):
         pickle.loads(tree_pickle_other)
 
 
@@ -419,7 +422,7 @@ def test_pickle_version_warning_is_issued_when_no_version_info_in_pickle():
         current_version=sklearn.__version__,
     )
     # check we got the warning about using pre-0.18 pickle
-    with pytest.warns(UserWarning, match=message):
+    with pytest.warns(UserWarning, match=re.escape(message)):
         pickle.loads(tree_pickle_noversion)
 
 
@@ -666,6 +669,29 @@ def transform(self, X):
         trans.transform(df_mixed)
 
 
+def test_base_estimator_pickle_version(monkeypatch):
+    """Check the version is embedded when pickled and checked when unpickled."""
+    old_version = "0.21.3"
+    monkeypatch.setattr(sklearn.base, "__version__", old_version)
+    original_estimator = MyEstimator()
+    old_pickle = pickle.dumps(original_estimator)
+    loaded_estimator = pickle.loads(old_pickle)
+    assert loaded_estimator.__sklearn_pickle_version__ == old_version
+    assert not hasattr(original_estimator, "__sklearn_pickle_version__")
+
+    # Loading pickle with newer version will raise a warning
+    new_version = "1.3.0"
+    monkeypatch.setattr(sklearn.base, "__version__", new_version)
+    message = pickle_error_message.format(
+        estimator="MyEstimator",
+        old_version=old_version,
+        current_version=new_version,
+    )
+    with pytest.warns(UserWarning, match=re.escape(message)):
+        reloaded_estimator = pickle.loads(old_pickle)
+        assert reloaded_estimator.__sklearn_pickle_version__ == old_version
+
+
 def test_clone_keeps_output_config():
     """Check that clone keeps the set_output config."""
 
@@ -694,7 +720,7 @@ def test_estimator_empty_instance_dict(estimator):
     ``AttributeError``. Non-regression test for gh-25188.
     """
     state = estimator.__getstate__()
-    expected = {"_sklearn_version": sklearn.__version__}
+    expected = {"__sklearn_pickle_version__": sklearn.__version__}
     assert state == expected
 
     # this should not raise