BUG: Warn when an existing layout manager changes to tight layout

j1642 · j1642 · commit 4bbd8229938f · 2022-11-21T15:25:55.000-05:00
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2576,6 +2576,10 @@ def set_layout_engine(self, layout=None, **kwargs):
 
         if self._check_layout_engines_compat(self._layout_engine,
                                              new_layout_engine):
+            if isinstance(self._layout_engine, ConstrainedLayoutEngine) \
+                    and isinstance(new_layout_engine, TightLayoutEngine):
+                _api.warn_external("The layout manager has been changed to "
+                                   "'tight'.")
             self._layout_engine = new_layout_engine
         else:
             raise RuntimeError('Colorbar layout of new layout engine not '
diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -571,7 +571,8 @@ def test_invalid_layouts():
 
     # test that layouts can be swapped if no colorbar:
     fig, ax = plt.subplots(layout="constrained")
-    fig.set_layout_engine("tight")
+    with pytest.warns(UserWarning):
+        fig.set_layout_engine("tight")
     assert isinstance(fig.get_layout_engine(), TightLayoutEngine)
     fig.set_layout_engine("constrained")
     assert isinstance(fig.get_layout_engine(), ConstrainedLayoutEngine)
@@ -598,6 +599,14 @@ def test_invalid_layouts():
         fig.set_layout_engine("constrained")
 
 
+@pytest.mark.parametrize('layout', ['constrained', 'compressed'])
+def test_layout_change_warning(layout):
+    """Raise a warning when an existing layout manager is changed to tight."""
+    fig, ax = plt.subplots(layout=layout)
+    with pytest.warns(UserWarning, match='The layout manager has been'):
+        plt.tight_layout()
+
+
 @check_figures_equal(extensions=["png", "pdf"])
 def test_add_artist(fig_test, fig_ref):
     fig_test.dpi = 100
