import matplotlib.pyplot as plt

# Use-case: at the beginning of a session
fig, ax = plt.subplots(constrained_layout=True, num='boo')
fig.suptitle("Dummy")
plt.show()

# Use-case: later in the same session
fig, ax = plt.subplots(constrained_layout=True, num='boo', clear=True)
ax.plot(range(30))
plt.show()

#=> boom, AttributeError

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-a852cdf04f60> in <module>()
      5 plt.show()
      6 
----> 7 fig, ax = plt.subplots(constrained_layout=True, num='boo', clear=True)
      8 ax.plot(range(30))
      9 plt.show()

~/anaconda3/lib/python3.6/site-packages/matplotlib/pyplot.py in subplots(nrows, ncols, sharex, sharey, squeeze, subplot_kw, gridspec_kw, **fig_kw)
   1196     subplot
   1197     """
-> 1198     fig = figure(**fig_kw)
   1199     axs = fig.subplots(nrows=nrows, ncols=ncols, sharex=sharex, sharey=sharey,
   1200                        squeeze=squeeze, subplot_kw=subplot_kw,

~/anaconda3/lib/python3.6/site-packages/matplotlib/pyplot.py in figure(num, figsize, dpi, facecolor, edgecolor, frameon, FigureClass, clear, **kwargs)
    574 
    575     if clear:
--> 576         figManager.canvas.figure.clear()
    577 
    578     return figManager.canvas.figure

~/anaconda3/lib/python3.6/site-packages/matplotlib/figure.py in clear(self, keep_observers)
   1434         Clear the figure -- synonym for :meth:`clf`.
   1435         """
-> 1436         self.clf(keep_observers=keep_observers)
   1437 
   1438     @allow_rasterization

~/anaconda3/lib/python3.6/site-packages/matplotlib/figure.py in clf(self, keep_observers)
   1427         self._suptitle = None
   1428         if self.get_constrained_layout():
-> 1429             layoutbox.nonetree(self._layoutbox)
   1430         self.stale = True
   1431 

~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonetree(lb)
    672             # Clear the solver.  Hopefully this garbage collects.
    673             lb.solver.reset()
--> 674             nonechildren(lb)
    675         else:
    676             nonetree(lb.parent)

~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonechildren(lb)
    679 def nonechildren(lb):
    680     for child in lb.children:
--> 681         nonechildren(child)
    682     lb.artist._layoutbox = None
    683     lb = None

~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonechildren(lb)
    680     for child in lb.children:
    681         nonechildren(child)
--> 682     lb.artist._layoutbox = None
    683     lb = None
    684 

AttributeError: 'NoneType' object has no attribute '_layoutbox'

def test_constrained_layout23():
    '''
    Comment in #11035: suptitle used to cause an exception when
    reusing a figure w/ CL with ``clear=True``.
    '''

    for i in range(2):
        fig, ax = plt.subplots(num="123", constrained_layout=True, clear=True)
        fig.suptitle(f"Suptitle {i}".format(i))