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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Bug in AxesStack.add","articleBody":"I think I've found a bug where the format of data actually stored in Stack is different from that expected in Stack.remove. It seems that the line below needs to be:\nStack.remove(self, self._entry_from_axes(a_existing))\n\nI don't have a minimal repro case (probably something to do with multi-threading in a tensorflow py_func), but I've been seeing exceptions in the .remove method. Adding some more useful exception messaging shows me a missing self._ind 0th tuple element.\n\n```\n\nAxesStack.add(((1, 1, 1), ()),..): Axes(0.125,0.1;0.775x0.8) vs Axes(0.125,0.1;0.775x0.8)\n                         ^key                ^ a                     ^a_existing\n\nmatplotlib/cbook.py\", line 1410, in remove\n    raise ValueError('Unknown element o: %s vs %s' % (o, self._elements))\n\nUnknown element o: (((1, 1, 1), ()), \u003cmatplotlib.axes._subplots.AxesSubplot object at 0\nx7fd416508550\u003e) vs [(((1, 1, 1), ()), (1, \u003cmatplotlib.axes._subplots.AxesSubplot object at 0x7fd416\n508550\u003e))]\n\n    def add(self, key, a):\n        \"\"\"\n        Add Axes *a*, with key *key*, to the stack, and return the stack.\n        If *a* is already on the stack, don't add it again, but\n        return *None*.\n        \"\"\"\n        # All the error checking may be unnecessary; but this method\n        # is called so seldom that the overhead is negligible.\n        if not isinstance(a, Axes):\n            raise ValueError(\"second argument, %s, is not an Axes\" % a)\n        try:\n            hash(key)\n        except TypeError:\n            raise ValueError(\"first argument, %s, is not a valid key\" % key)\n\n        a_existing = self.get(key)\n        if a_existing is not None:\nHERE--\u003e            Stack.remove(self, (key, a_existing))\n            warnings.warn(\n                \"key %s already existed; Axes is being replaced\" % key)\n            # I don't think the above should ever happen.\n\n        if a in self:\n            return None\n        self._ind += 1\n        return Stack.push(self, (key, (self._ind, a)))\n\n\n```\n\nTo help us understand and resolve your issue please check that you have provided\nthe information below.\n- [ ] Matplotlib version, Python version and Platform (Windows, OSX, Linux ...)\n- [ ] How did you install Matplotlib and Python (pip, anaconda, from source ...)\n- [ ] If possible please supply a [Short, Self Contained, Correct, Example](http://sscce.org/)\n    that demonstrates the issue i.e a small piece of code which reproduces the issue\n    and can be run with out any other (or as few as possible) external dependencies.\n- [ ] If this is an image generation bug attach a screenshot demonstrating the issue.\n- [ ] If this is a regression (Used to work in an earlier version of Matplotlib), please \n    note where it used to work.\n","author":{"url":"https://github.com/brianwa84","@type":"Person","name":"brianwa84"},"datePublished":"2016-09-28T17:04:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/7194/matplotlib/issues/7194"}