使用 matplotlib 绘制图形时出现间歇性错误“RuntimeError: main thread is not in main loop”

Question

I am trying to pull data from different end points using my python code and feeding the same data to plot a graph using matplotlib. I dont have any problem in reading the data but when i invoke the methods to plot graph by feeding the data i see intermittent error caused by matplot lib. below are the error details.

Traceback (most recent call last):
  File "C:\Python35\lib\site-packages\slackbot\dispatcher.py", line 55, in _dispatch_msg_handler
    func(Message(self._client, msg), *args)
  File "C:\PycharmProjects\SlackBot\src\plugins\bot_response.py", line 248, in checkmarx
    draw_chart.riskscore_bar(top_riskscore, project_name, "output_files", "riskscore_bar.png")
  File "C:\PycharmProjects\SlackBot\src\drawchart.py", line 111, in riskscore_bar
    fig, ax = plt.subplots()
  File "C:\Python35\lib\site-packages\matplotlib\pyplot.py", line 1202, in subplots
    fig = figure(**fig_kw)
  File "C:\Python35\lib\site-packages\matplotlib\pyplot.py", line 535, in figure
    **kwargs)
  File "C:\Python35\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 81, in new_figure_manager
    return new_figure_manager_given_figure(num, figure)
  File "C:\Python35\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 98, in new_figure_manager_given_figure
    icon_img = Tk.PhotoImage(file=icon_fname)
  File "C:\Python35\lib\tkinter\__init__.py", line 3403, in __init__
    Image.__init__(self, 'photo', name, cnf, master, **kw)
  File "C:\Python35\lib\tkinter\__init__.py", line 3359, in __init__
    self.tk.call(('image', 'create', imgtype, name,) + options)
RuntimeError: main thread is not in main loop

I have tried looking into other cases from stackoverflow with same error messages, but it didnt help me fix this. Below is my code snippet that invokes an error.

def riskscore_bar(self, top_riskscore, project_id, output_folder, output_filename):

logger.debug("Inside method plotgraph in drawchart.py.")

y_pos = np.arange(len(project_id))
width = .4

fig, ax = plt.subplots()
graph = ax.bar(y_pos+1, top_riskscore, width, color='#feb308')
ax.set_ylabel('Risk Score')
ax.set_title('Summary')
ax.set_xticks(y_pos + 1)
ax.set_xticklabels(project_id,fontsize=5, rotation=45 )

def autolabel(rects):
    for rect in rects:
        height = rect.get_height()
        ax.text(rect.get_x() + rect.get_width()/2., 1.001*height,
                '%d' % int(height),
                ha='center', va='bottom')

autolabel(graph)
pylab.savefig(os.path.join(os.path.abspath(os.path.dirname(__file__)), '..', output_folder,output_filename))

The error seems to occur at "fig, ax = plt.subplots()". any ideas on how this can be fixed?

Answer 1

this can be solved by adding plt.switch_backend('agg') below right after you import matplotlib.pyplot as plt. As shown below

import matplotlib.pyplot as plt    
plt.switch_backend('agg')