ENH: add ability to remove layout engine

This also adds a "place holder" layout engine to ensure that users can not "go
through zero" and change to an incompatible layout engine.

Co-authored-by: Jody Klymak <[email protected]>

Author: tacaswell

lib/matplotlib/figure.py

@@ -35,8 +35,10 @@
 
 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,
+    PlaceHolderLayoutEngine
+)
 import matplotlib.legend as mlegend
 from matplotlib.patches import Rectangle
 from matplotlib.text import Text
@@ -2382,7 +2384,9 @@ def _check_layout_engines_compat(self, old, new):
         If the figure has used the old engine and added a colorbar then the
         value of colorbar_gridspec must be the same on the new engine.
         """
-        if old is None or old.colorbar_gridspec == new.colorbar_gridspec:
+        if old is None or new is None:
+            return True
+        if old.colorbar_gridspec == new.colorbar_gridspec:
             return True
         # colorbar layout different, so check if any colorbars are on the
         # figure...
@@ -2398,15 +2402,29 @@ def set_layout_engine(self, layout=None, **kwargs):
 
         Parameters
         ----------
-        layout: {'constrained', 'compressed', 'tight'} or `~.LayoutEngine`
-            'constrained' will use `~.ConstrainedLayoutEngine`,
-            'compressed' will also use ConstrainedLayoutEngine, but with a
-            correction that attempts to make a good layout for fixed-aspect
-            ratio Axes. 'tight' uses `~.TightLayoutEngine`.  Users and
-            libraries can define their own layout engines as well.
+        layout: {'constrained', 'compressed', 'tight', 'none'} or \
+`LayoutEngine` or None
+
+            - 'constrained' will use `~.ConstrainedLayoutEngine`
+            - 'compressed' will also use `~.ConstrainedLayoutEngine`, but with
+              a correction that attempts to make a good layout for fixed-aspect
+              ratio Axes.
+            - 'tight' uses `~.TightLayoutEngine`
+            - 'none' removes layout engine.
+
+            If `None`, the behavior is controlled by :rc:`figure.autolayout`
+            (which if `True` behaves as if 'tight' were passed) and
+            :rc:`figure.constrained_layout.use` (which if true behaves as if
+            'constrained' were passed).  If both are true,
+            :rc:`figure.autolayout` takes priority.
+
+            Users and libraries can define their own layout engines and pass
+            the instance directly as well.
+
         kwargs: dict
             The keyword arguments are passed to the layout engine to set things
             like padding and margin sizes.  Only used if *layout* is a string.
+
         """
         if layout is None:
             if mpl.rcParams['figure.autolayout']:
@@ -2423,6 +2441,14 @@ def set_layout_engine(self, layout=None, **kwargs):
         elif layout == 'compressed':
             new_layout_engine = ConstrainedLayoutEngine(compress=True,
                                                         **kwargs)
+        elif layout == 'none':
+            if self._layout_engine is not None:
+                new_layout_engine = PlaceHolderLayoutEngine(
+                    self._layout_engine.adjust_compatible,
+                    self._layout_engine.colorbar_gridspec
+                )
+            else:
+                new_layout_engine = None
         elif isinstance(layout, LayoutEngine):
             new_layout_engine = layout
         else:

lib/matplotlib/layout_engine.py

@@ -100,6 +100,30 @@ def execute(self, fig):
         raise NotImplementedError
 
 
+class PlaceHolderLayoutEngine(LayoutEngine):
+    """
+    This layout engine does not adjust the figure layout at all.
+
+    The purpose of this `.LayoutEngine` is to act as a place holder when the
+    user removes a layout engine to ensure an incompatible `.LayoutEngine` can
+    not be set later.
+
+    Parameters
+    ----------
+    adjust_compatible, colorbar_gridspec : bool
+        Allow the PlaceHolderLayoutEngine 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

lib/matplotlib/tests/test_figure.py

@@ -17,7 +17,8 @@
 from matplotlib.axes import Axes
 from matplotlib.figure import Figure, FigureBase
 from matplotlib.layout_engine import (ConstrainedLayoutEngine,
-                                      TightLayoutEngine)
+                                      TightLayoutEngine,
+                                      PlaceHolderLayoutEngine)
 from matplotlib.ticker import AutoMinorLocator, FixedFormatter, ScalarFormatter
 import matplotlib.pyplot as plt
 import matplotlib.dates as mdates
@@ -578,6 +579,9 @@ 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")
 
@@ -586,6 +590,12 @@ def test_invalid_layouts():
     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 isinstance(fig.get_layout_engine(), PlaceHolderLayoutEngine)
 
 
 @check_figures_equal(extensions=["png", "pdf"])