René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Reusing cleared figure with suptitle and constrained layout raises an exception","articleBody":"### Bug report\r\n\r\n**Bug summary**\r\n\r\nSuptitle and constrained layout raises an exception whenclearing-and-reusing  an existing figure. Originally noticed in #11035 .\r\n\r\nAttn @jklymak aka. our layout manager expert\r\n\r\n**Code for reproduction**\r\n\r\n```python\r\nimport matplotlib.pyplot as plt\r\n\r\n# Use-case: at the beginning of a session\r\nfig, ax = plt.subplots(constrained_layout=True, num='boo')\r\nfig.suptitle(\"Dummy\")\r\nplt.show()\r\n\r\n# Use-case: later in the same session\r\nfig, ax = plt.subplots(constrained_layout=True, num='boo', clear=True)\r\nax.plot(range(30))\r\nplt.show()\r\n\r\n#=\u003e boom, AttributeError\r\n```\r\n\r\n**Actual outcome**\r\n\r\n```python\r\n---------------------------------------------------------------------------\r\nAttributeError                            Traceback (most recent call last)\r\n\u003cipython-input-6-a852cdf04f60\u003e in \u003cmodule\u003e()\r\n      5 plt.show()\r\n      6 \r\n----\u003e 7 fig, ax = plt.subplots(constrained_layout=True, num='boo', clear=True)\r\n      8 ax.plot(range(30))\r\n      9 plt.show()\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/pyplot.py in subplots(nrows, ncols, sharex, sharey, squeeze, subplot_kw, gridspec_kw, **fig_kw)\r\n   1196     subplot\r\n   1197     \"\"\"\r\n-\u003e 1198     fig = figure(**fig_kw)\r\n   1199     axs = fig.subplots(nrows=nrows, ncols=ncols, sharex=sharex, sharey=sharey,\r\n   1200                        squeeze=squeeze, subplot_kw=subplot_kw,\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/pyplot.py in figure(num, figsize, dpi, facecolor, edgecolor, frameon, FigureClass, clear, **kwargs)\r\n    574 \r\n    575     if clear:\r\n--\u003e 576         figManager.canvas.figure.clear()\r\n    577 \r\n    578     return figManager.canvas.figure\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/figure.py in clear(self, keep_observers)\r\n   1434         Clear the figure -- synonym for :meth:`clf`.\r\n   1435         \"\"\"\r\n-\u003e 1436         self.clf(keep_observers=keep_observers)\r\n   1437 \r\n   1438     @allow_rasterization\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/figure.py in clf(self, keep_observers)\r\n   1427         self._suptitle = None\r\n   1428         if self.get_constrained_layout():\r\n-\u003e 1429             layoutbox.nonetree(self._layoutbox)\r\n   1430         self.stale = True\r\n   1431 \r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonetree(lb)\r\n    672             # Clear the solver.  Hopefully this garbage collects.\r\n    673             lb.solver.reset()\r\n--\u003e 674             nonechildren(lb)\r\n    675         else:\r\n    676             nonetree(lb.parent)\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonechildren(lb)\r\n    679 def nonechildren(lb):\r\n    680     for child in lb.children:\r\n--\u003e 681         nonechildren(child)\r\n    682     lb.artist._layoutbox = None\r\n    683     lb = None\r\n\r\n~/anaconda3/lib/python3.6/site-packages/matplotlib/_layoutbox.py in nonechildren(lb)\r\n    680     for child in lb.children:\r\n    681         nonechildren(child)\r\n--\u003e 682     lb.artist._layoutbox = None\r\n    683     lb = None\r\n    684 \r\n\r\nAttributeError: 'NoneType' object has no attribute '_layoutbox'\r\n\r\n```\r\n\r\n**Expected outcome**\r\n\r\nNo Exception, but a clear Figure named 'boo'.\r\n\r\n\r\n**Matplotlib version**\r\n  * Operating system: Fedora 27\r\n  * Matplotlib version: master ('2.2.2.post856+g506520f30') and 2.2.2\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): Qt5Agg\r\n  * Python version: 3.6 (Anaconda)\r\n\r\n**Being there**\r\n\r\nSome test snippet to put in `test_constrainedlayout.py`\r\n```python\r\ndef test_constrained_layout23():\r\n    '''\r\n    Comment in #11035: suptitle used to cause an exception when\r\n    reusing a figure w/ CL with ``clear=True``.\r\n    '''\r\n\r\n    for i in range(2):\r\n        fig, ax = plt.subplots(num=\"123\", constrained_layout=True, clear=True)\r\n        fig.suptitle(f\"Suptitle {i}\".format(i))\r\n\r\n```\r\n","author":{"url":"https://github.com/afvincent","@type":"Person","name":"afvincent"},"datePublished":"2018-04-24T04:25:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/11115/matplotlib/issues/11115"}