ben@tigger:~/Documents/InteractiveMPL$ python chp3/track_tails.py Traceback (most recent call last):  File "chp3/track_tails.py", line 65, in <module>    stormcells['frame_index'].max() + 1, fargs=(stormcells,))  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1128, in __init__    TimedAnimation.__init__(self, fig, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 973, in __init__    *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 594, in __init__    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1156, in _init_draw    self._draw_frame(next(self.new_frame_seq()))  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1173, in _draw_frame    for a in self._drawn_artists:TypeError: 'NoneType' object is not iterable

   <...snipped...>  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1140, in draw    self.canvas.draw_event(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backend_bases.py", line 1808, in draw_event    self.callbacks.process(s, event)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 562, in process    proxy(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/cbook.py", line 429, in __call__    return mtd(*args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 616, in _start    self._init_draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/animation.py", line 1044, in _init_draw    fig.canvas.draw()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_qt5agg.py", line 149, in draw    FigureCanvasAgg.draw(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/backends/backend_agg.py", line 474, in draw    self.figure.draw(self.renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/figure.py", line 1134, in draw    func(*args)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/axes/_base.py", line 2132, in draw    self.patch.draw(renderer)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/artist.py", line 61, in draw_wrapper    draw(artist, renderer, *args, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/patches.py", line 514, in draw    transform = self.get_transform()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/patches.py", line 195, in get_transform    return self.get_patch_transform() + artist.Artist.get_transform(self)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/patches.py", line 670, in get_patch_transform    self._update_patch_transform()  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/patches.py", line 663, in _update_patch_transform    bbox = transforms.Bbox.from_bounds(x, y, width, height)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/transforms.py", line 834, in from_bounds    return Bbox.from_extents(x0, y0, x0 + width, y0 + height)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/transforms.py", line 845, in from_extents    return Bbox(points)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/transforms.py", line 787, in __init__    BboxBase.__init__(self, **kwargs)  File "/home/ben/miniconda/lib/python2.7/site-packages/matplotlib-1.5.dev1-py2.7-linux-x86_64.egg/matplotlib/transforms.py", line 98, in __init__    self._parents = WeakValueDictionary()  File "/home/ben/miniconda/lib/python2.7/weakref.py", line 59, in __init__    UserDict.UserDict.__init__(self, *args, **kw)RuntimeError: maximum recursion depth exceeded while calling a Python object