Matplotlib Can't Render Multiple Contour Plots On Django

Whenever (at least) 2 people try to generate a contour plot in my application, at least one of them will receive a random error depending on how far the first person managed to dra

Solution 1:

First, let me start by saying that this is much more easy to reproduce by calling _test_chart in a couple threads

from threading import Thread
for i in xrange(2):
    Thread(target=_test_chart).start()

Doing the above, one will work as desired whilst the second one will crash.

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The simple reason for this is that the pyplot module is not designed for multithreading and therefore the two charts are getting their data mixed up as they attempt to draw.

This can be better explained by mdboom

...pyplot is used for convenient plotting at the commandline and keeps around global state. For example, when you say plt.figure() it adds the figure to a global list and then sets the "current figure" pointer to the most recently created figure. Then subsequent plotting commands automatically write to that figure. Obviously, that's not threadsafe...

There are two ways to fix this issue,

1. Force these charts to be drawn in different processes.

for i in xrange(2):
    pool =Pool(processes=1)
    pool.apply(_test_chart)   

Whilst this will work you will find that there is a noticable drop in performance since it will often take just as long to create the process as it will to generate the chart (which I didn't think was acceptable!)

2. The real solution is to use the OO interface modules of Matplotlib which will then allow you to work with the correct objects - essentially this works down to working with subplots rather than plots. For the given example in the question, this would look like the following

def _test_chart2():

    delta = 0.5

    x = arange(-3.0, 4.001, delta)
    y = arange(-4.0, 3.001, delta)
    X, Y = np.meshgrid(x, y)
    Z1 = bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
    Z2 = bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
    Z = (Z1 - Z2) * 10

    fig = figure(figsize=(10, 5))

    ax1 = fig.add_subplot(111)
    extents = [x.min(), x.max(), y.min(), y.max()]
    im = ax1.imshow(Z,
                    interpolation='spline36',
                    extent=extents,
                    origin='lower',
                    aspect='auto',
                    cmap=cm.jet)
    ax1.contour(X, Y, Z, 10, colors='k')

    jpg_image_buffer = cStringIO.StringIO()
    fig.savefig(jpg_image_buffer)

    array = base64.b64encode(jpg_image_buffer.getvalue())
    jpg_image_buffer.close()

    return array