Merge pull request #10657 from matplotlib/auto-backport-of-pr-10629

jklymak · web-flow · commit b67df2df6d5e · 2018-03-03T09:49:19.000-08:00
Backport PR #10629 on branch v2.2.x
diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
@@ -1126,6 +1126,7 @@ def make_axes(parents, location=None, orientation=None, fraction=0.15,
     anchor = kw.pop('anchor', loc_settings['anchor'])
     parent_anchor = kw.pop('panchor', loc_settings['panchor'])
 
+    parents_iterable = cbook.iterable(parents)
     # turn parents into a list if it is not already. We do this w/ np
     # because `plt.subplots` can return an ndarray and is natural to
     # pass to `colorbar`.
@@ -1193,7 +1194,7 @@ def make_axes(parents, location=None, orientation=None, fraction=0.15,
         # and we need to set the aspect ratio by hand...
         cax.set_aspect(aspect, anchor=anchor, adjustable='box')
     else:
-        if len(parents) == 1:
+        if not parents_iterable:
             # this is a single axis...
             ax = parents[0]
             lb, lbpos = constrained_layout.layoutcolorbarsingle(
diff --git a/tutorials/intermediate/constrainedlayout_guide.py b/tutorials/intermediate/constrainedlayout_guide.py
@@ -105,15 +105,39 @@ def example_plot(ax, fontsize=12, nodec=False):
 fig.colorbar(im, ax=ax, shrink=0.6)
 
 ############################################################################
-# If you specify multiple axes to the ``ax`` argument of ``colorbar``,
-# constrained_layout will take space from all axes that share the same
-# gridspec.
+# If you specify a list of axes (or other iterable container) to the
+# ``ax`` argument of ``colorbar``, constrained_layout will take space from all # axes that share the same gridspec.
 
 fig, axs = plt.subplots(2, 2, figsize=(4, 4), constrained_layout=True)
 for ax in axs.flatten():
     im = ax.pcolormesh(arr, rasterized=True)
 fig.colorbar(im, ax=axs, shrink=0.6)
 
+############################################################################
+# Note that there is a bit of a subtlety when specifying a single axes
+# as the parent.  In the following, it might be desirable and expected
+# for the colorbars to line up, but they don't because the colorbar paired
+# with the bottom axes is tied to the subplotspec of the axes, and hence
+# shrinks when the gridspec-level colorbar is added.
+
+fig, axs = plt.subplots(3, 1, figsize=(4, 4), constrained_layout=True)
+for ax in axs[:2]:
+    im = ax.pcolormesh(arr, rasterized=True)
+fig.colorbar(im, ax=axs[:2], shrink=0.6)
+im = axs[2].pcolormesh(arr, rasterized=True)
+fig.colorbar(im, ax=axs[2], shrink=0.6)
+
+############################################################################
+# The API to make a single-axes behave like a list of axes is to specify
+# it as a list (or other iterable container), as below:
+
+fig, axs = plt.subplots(3, 1, figsize=(4, 4), constrained_layout=True)
+for ax in axs[:2]:
+    im = ax.pcolormesh(arr, rasterized=True)
+fig.colorbar(im, ax=axs[:2], shrink=0.6)
+im = axs[2].pcolormesh(arr, rasterized=True)
+fig.colorbar(im, ax=[axs[2]], shrink=0.6)
+
 ####################################################
 # Suptitle
 # =========
