ENH: add a NullLayoutEngine

tacaswell · tacaswell · commit 8640cebc9abc · 2022-03-24T23:32:29.000-04:00
It can mirror what ever was there to make sure things stay compatible.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -35,8 +35,9 @@
 
 from matplotlib.axes import Axes, SubplotBase, subplot_class_factory
 from matplotlib.gridspec import GridSpec
-from matplotlib.layout_engine import (ConstrainedLayoutEngine,
-                                      TightLayoutEngine, LayoutEngine)
+from matplotlib.layout_engine import (
+    ConstrainedLayoutEngine, TightLayoutEngine, LayoutEngine, NullLayoutEnigne
+)
 import matplotlib.legend as mlegend
 from matplotlib.patches import Rectangle
 from matplotlib.text import Text
@@ -2356,7 +2357,13 @@ def set_layout_engine(self, layout=None, **kwargs):
         elif layout == 'constrained':
             new_layout_engine = ConstrainedLayoutEngine(**kwargs)
         elif layout == 'none':
-            new_layout_engine = None
+            if self._layout_engine is not None:
+                new_layout_engine = NullLayoutEnigne(
+                    self._layout_engine.adjust_compatible,
+                    self._layout_engine.colorbar_gridspec
+                )
+            else:
+                new_layout_engine = None
         elif isinstance(layout, LayoutEngine):
             new_layout_engine = layout
         else:
diff --git a/lib/matplotlib/layout_engine.py b/lib/matplotlib/layout_engine.py
@@ -101,6 +101,28 @@ def execute(self, fig):
         raise NotImplementedError
 
 
+class NullLayoutEnigne(LayoutEngine):
+    """
+    A no-op LayoutEngine.
+
+    This is primarily to act as a place holder if we remove a layout engine.
+
+    Parameter
+    ---------
+    adjust_compatible, colorbar_gridspec : bool
+        Allow the NullLayoutEnigne to mirror the behavior of whatever
+        layout engine it is replacing.
+
+    """
+    def __init__(self, adjust_compatible, colorbar_gridspec, **kwargs):
+        self._adjust_compatible = adjust_compatible
+        self._colorbar_gridspec = colorbar_gridspec
+        super().__init__(**kwargs)
+
+    def execute(self, fig):
+        return
+
+
 class TightLayoutEngine(LayoutEngine):
     """
     Implements the ``tight_layout`` geometry management.  See
diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -18,7 +18,8 @@
 from matplotlib.axes import Axes
 from matplotlib.figure import Figure
 from matplotlib.layout_engine import (ConstrainedLayoutEngine,
-                                      TightLayoutEngine)
+                                      TightLayoutEngine,
+                                      NullLayoutEnigne)
 from matplotlib.ticker import AutoMinorLocator, FixedFormatter, ScalarFormatter
 import matplotlib.pyplot as plt
 import matplotlib.dates as mdates
@@ -610,17 +611,23 @@ def test_invalid_layouts():
     fig, ax = plt.subplots(layout="constrained")
     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")
+    fig.set_layout_engine("none")
     with pytest.raises(RuntimeError, match='Colorbar layout of new layout'):
         fig.set_layout_engine("tight")
 
     fig, ax = plt.subplots(layout="tight")
     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("constrained")
+    fig.set_layout_engine("none")
     with pytest.raises(RuntimeError, match='Colorbar layout of new layout'):
         fig.set_layout_engine("constrained")
 
     fig.set_layout_engine("none")
-    assert fig.get_layout_engine() is None
+    assert isinstance(fig.get_layout_engine(), NullLayoutEnigne)
 
 
 @check_figures_equal(extensions=["png", "pdf"])
