FIX Adjusts html_repr based on configuration (scikit-learn#17093)

* ENH Adjusts html_repr based on configuration

* CLN Returns None instead

* CLN Uses property hack

* CLN Address comments

Author: thomasjpfan

sklearn/base.py

@@ -437,8 +437,25 @@ def _validate_data(self, X, y=None, reset=True,
 
         return out
 
+    @property
     def _repr_html_(self):
-        """HTML representation of estimator"""
+        """HTML representation of estimator.
+
+        This is redundant with the logic of `_repr_mimebundle_`. The latter
+        should be favorted in the long term, `_repr_html_` is only
+        implemented for consumers who do not interpret `_repr_mimbundle_`.
+        """
+        if get_config()["display"] != 'diagram':
+            raise AttributeError("_repr_html_ is only defined when the "
+                                 "'display' configuration option is set to "
+                                 "'diagram'")
+        return self._repr_html_inner
+
+    def _repr_html_inner(self):
+        """This function is returned by the @property `_repr_html_` to make
+        `hasattr(estimator, "_repr_html_") return `True` or `False` depending
+        on `get_config()["display"]`.
+        """
         return estimator_html_repr(self)
 
     def _repr_mimebundle_(self, **kwargs):

sklearn/tests/test_base.py

@@ -525,3 +525,15 @@ def test_repr_mimebundle_():
         output = tree._repr_mimebundle_()
         assert "text/plain" in output
         assert "text/html" in output
+
+
+def test_repr_html_wraps():
+    # Checks the display configuration flag controls the html output
+    tree = DecisionTreeClassifier()
+    msg = "_repr_html_ is only defined when"
+    with pytest.raises(AttributeError, match=msg):
+        output = tree._repr_html_()
+
+    with config_context(display='diagram'):
+        output = tree._repr_html_()
+        assert "<style>" in output