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

j1642 · j1642 · commit c029f1d45c0f · 2022-11-21T14:22:43.000-05:00
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2506,6 +2506,10 @@ def _check_layout_engines_compat(self, old, new):
         """
         if old is None or new is None:
             return True
+        if isinstance(old, ConstrainedLayoutEngine) \
+                and isinstance(new, TightLayoutEngine):
+            _api.warn_external("The layout manager has been changed to "
+                               "'tight'.")
         if old.colorbar_gridspec == new.colorbar_gridspec:
             return True
         # colorbar layout different, so check if any colorbars are on the
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)
@@ -581,7 +582,8 @@ def test_invalid_layouts():
     pc = ax.pcolormesh(np.random.randn(2, 2))
     fig.colorbar(pc)
     with pytest.raises(RuntimeError, match='Colorbar layout of new layout'):
-        fig.set_layout_engine("tight")
+        with pytest.warns(UserWarning):
+            fig.set_layout_engine("tight")
     fig.set_layout_engine("none")
     with pytest.raises(RuntimeError, match='Colorbar layout of new layout'):
         fig.set_layout_engine("tight")
@@ -598,6 +600,14 @@ 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
