Question

My tags and title quite clearly state my problem. I want to use matplotlib to create real-time plots in Google App Engine. I’ve read the documentation and searched on SO and Google. I found a post, pointing to this working demo. But when I try it on my own, it doesn’t work for me.

I created a simple application, consisting only of a handler-script hello_world.py

import numpy as np
import os
import sys
import cStringIO

print "Content-type: image/png\n"

os.environ["MATPLOTLIBDATA"] = os.getcwdu()  # own matplotlib data
os.environ["MPLCONFIGDIR"] = os.getcwdu()    # own matplotlibrc
import matplotlib.pyplot as plt

plt.plot(np.random.random((20))) #imshow(np.random.randint((10,10)))

sio = cStringIO.StringIO()
plt.savefig(sio, format="png")
sys.stdout.write(sio.getvalue())

and a a configuration file app.yaml

application: helloworldtak
version: 1
runtime: python27
api_version: 1
threadsafe: no

handlers:
- url: /.*
  script: hello_world.py

libraries:
- name: numpy
  version: "latest"
- name: matplotlib
  version: "latest"

I want to plot something and then return the content as png-image. This procedure works fine for a normal web-server like Apache or IIS, I did this a million times.

The problem is rather: when I run my script locally within the development server, I get an error that is probably due to my MPL version 1.1.1, which is only “experimental” in GAE. But when I deploy my app to GAE, I get a completely different, uncorrelated error.

Looking at the looks, the traceback is:

Traceback (most recent call last):
  File "/base/data/home/apps/s~helloworldtak/1.364765672279579252/hello_world.py", line 16, in <module>
    import matplotlib.pyplot as plt
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/pyplot.py", line 23, in <module>
    from matplotlib.figure import Figure, figaspect
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/figure.py", line 18, in <module>
    from axes import Axes, SubplotBase, subplot_class_factory
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/axes.py", line 14, in <module>
    import matplotlib.axis as maxis
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/axis.py", line 10, in <module>
    import matplotlib.font_manager as font_manager
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/font_manager.py", line 1324, in <module>
    _rebuild()
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/font_manager.py", line 1278, in _rebuild
    fontManager = FontManager()
  File "/python27_runtime/python27_lib/versions/third_party/matplotlib-1.1.1/matplotlib/font_manager.py", line 995, in __init__
    self.defaultFont['ttf'] = self.ttffiles[0]
IndexError: list index out of range

It seems to have to do something with the fonts-cache of MPL. I read in the docs that caching and file-access is one of the problems with MPL in GAE, but obviously, the import works for others.

What am I doing wrong?

Edit
Based on the answer below, I changed my code to be

import numpy as np
import cStringIO
import matplotlib.pyplot as plt

import webapp2

class MainPage(webapp2.RequestHandler):
    def get(self):
        plt.plot(np.random.random((20)),"r-")
        sio = cStringIO.StringIO()
        plt.savefig(sio, format="png")
        self.response.headers['Content-Type'] = 'image/png'

        self.response.out.write(sio.getvalue())

app = webapp2.WSGIApplication([('/', MainPage)],
                              debug=True)

and like this, it’s working.

Answer 1

I’m not familiar with sys module. To give an answer to the question I prefer using webapp2. This is a working handler:

import webapp2
import StringIO
import numpy as np
import matplotlib.pyplot as plt


class MainPage(webapp2.RequestHandler):
    def get(self):
        plt.plot(np.random.random((20)))
        sio = StringIO.StringIO()
        plt.savefig(sio, format="png")
        img_b64 = sio.getvalue().encode("base64").strip()
        plt.clf()
        sio.close()
        self.response.write("""<html><body>""")
        self.response.write("<img src='data:image/png;base64,%s'/>" % img_b64)
        self.response.write("""</body> </html>""")

app = webapp2.WSGIApplication([('/', MainPage)], debug=True)

Alternatively, you could write the sio.getvalue() in the blobstore with files api and use the method get_serving_url() of images api for avoid to encode in base64.