Error in Django when using matplotlib examples

Question

I am testing several cases of Django and matplotlib such as this question or in french.

Each time, it works on my mac, but does not on my server, where I receive the following error:

Internal Server Error: /mj/charts/mplimage.png
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/exception.py", line 35, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 128, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 126, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/root/src/jm/majority_judgment/views.py", line 39, in mplimage
    canvas.print_png(response)
  File "/usr/local/lib/python3.6/dist-packages/matplotlib/backends/backend_agg.py", line 526, in print_png
    with cbook.open_file_cm(filename_or_obj, "wb") as fh:
  File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.6/dist-packages/matplotlib/cbook/__init__.py", line 624, in open_file_cm
    fh, opened = to_filehandle(path_or_file, mode, True, encoding)
  File "/usr/local/lib/python3.6/dist-packages/matplotlib/cbook/__init__.py", line 615, in to_filehandle
    raise ValueError('fname must be a PathLike or file handle')
ValueError: fname must be a PathLike or file handle
[28/Mar/2018 19:09:11] "GET /mj/charts/mplimage.png HTTP/1.1" 500 82804

Here is a minimal snippet:

def mplimage(request):
    f = matplotlib.figure.Figure()
    canvas = FigureCanvasAgg(f)
    response = HttpResponse(content_type='image/png')
    canvas.print_png(response)
    plt.close(f)
    return response

I have tried to update matplotlib, django and so on, but it did nothing...

Use print_jpg and 'image/jpg' for Content-Type, and it should work as expected. — jredd
– jredd, Commented Dec 20, 2018 at 18:47

phihag · Accepted Answer · 2018-11-04 19:37:23Z

19

At the moment, matplotlib's writing functions require the seek ducktype to use the response at a file. You can write to a buffer, like this:

import io

def mplimage(request):
    f = matplotlib.figure.Figure()

    # Code that sets up figure goes here; in the question, that's ...
    FigureCanvasAgg(f)

    buf = io.BytesIO()
    plt.savefig(buf, format='png')
    plt.close(f)
    response = HttpResponse(buf.getvalue(), content_type='image/png')
    return response

edited Nov 4, 2018 at 19:37

answered Mar 28, 2018 at 21:06

phihag

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Comments

JRM · Accepted Answer · 2018-08-09 12:11:22Z

2

You can just replace the response with a buffer and then add the buffer to the response. This will give an appropriate object to canvas.print_png() and keep code changes to a minimum.

def mplimage(request):
    f = matplotlib.figure.Figure()
    buf = io.BytesIO()
    canvas = FigureCanvasAgg(f)
    canvas.print_png(buf)
    response=HttpResponse(buf.getvalue(),content_type='image/png')
    # if required clear the figure for reuse 
    f.clear()
    # I recommend to add Content-Length for Django
    response['Content-Length'] = str(len(response.content))
    #
    return response

answered Aug 9, 2018 at 12:11

JRM

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