kushtrimh kushtrimh - 2 months ago 18x
Python Question

Save Matplotlib plot image into Django model

For learning purposes I'm making a webapp that make plots with matplotlib, and I want to save the image of that plot into the

field of the Plot model but when I make a plot all it does it saves a blank image into the
directory. I did this the way an other post suggested but it's not working.

More Info
When I make the plot, the plot image gets saved in the main directory of my django project with a name like
<_io.StringIO object at 0xb1ac69ec>
, but as I said the plot image that gets saved in the model is blank. Also im using Python 2.7 if that matters.

def simple_plot(request):
if request.method == "POST":
name = request.POST.get("name")
xvalues = request.POST.get("xvalues")
yvalues = request.POST.get("yvalues")
plots.simple_plot(request, name, xvalues, yvalues)
messages.success(request, "Plot created!")
return render(request, 'plots/simple.html')

The file which plots are made and plot instances created.

def simple_plot(request ,name, xvalues, yvalues):
file_name = name+".png"
xvalues = [int(x.replace(" ","")) for x in xvalues.split(",")]
yvalues = [int(y.replace(" ","")) for y in yvalues.split(",")]

figure = io.StringIO()
plot = plt.plot(xvalues, yvalues)
plt.savefig(u'%s' % figure, format="png")

content_file = ContentFile(figure.getvalue())
plot_instance = Plot(name=name, user=request.user)
plot_instance.figure.save(file_name, content_file)
return True

Plot Model

class Plot(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
name = models.CharField(max_length=200)
figure = models.ImageField(upload_to='figures/')
timestamp = models.DateTimeField(auto_now_add=True)


There are several problems with your code:

  • You should not save the figure to a StringIO but instead, a io.BytesIO(). This is because the contents of the PNG file isn't human-readable text but binary data.

  • Another problem is how you handle the BytesIO (StringIO in your code) when passing it to savefig. A BytesIO isn't associated with a file (that's the whole point of having an in-memory file-like object), so it doesn't have a file name – which is what I suppose you want to get at by that u'%s' % figure expression. Instead, just write to the file-like object itself.

  • Third, use a django.core.files.images.ImageFile instead of ContentFile. Also, initialise it with the BytesIO object itself, not its bytes value.

The relevant portion of your code then becomes:

figure = io.BytesIO()
plt.plot(xvalues, yvalues)
plt.savefig(figure, format="png")
content_file = ImageFile(figure)