Flask send variable data to python through url_for()

Question

I want to send a variable value to python from flask html/js through url_for().

Python Code :

@app.route('/video_feed/<device>')
def video_feed(device):
    # return the response generated along with the specific media
    # type (mime type)
    print(device)
    try:
        return Response(gen(int(device)),mimetype = "multipart/x-mixed-replace; boundary=frame")
    except Exception as e:
        return Response(gen(device),mimetype = "multipart/x-mixed-replace; boundary=frame")

Desired in flask html file:

data.value = 0;
image.src = "{{ url_for('video_feed', device=data.value)}}";

But it does not work, I get the error:

jinja2.exceptions.UndefinedError: 'data' is undefined

This does work, however I can't use the variable data.value in url_for() :

image.src = "{{ url_for('video_feed', device=0)}}";

I have tried several different things like:

image.src = "{{ url_for('video_feed', device=${data.value})}}";
image.src = "{{ url_for('video_feed', device=$data.value)}}";
image.src = "{{ url_for('video_feed', device=%s)}}", data.value;

But nothing seems to work. My javascript is a bit rusty.

Any help would be appreciated!

Cheers!

Answer 1

url_for is expanded on the server side, before the page gets sent to the browser and well before JavaScript runs.

If you know the number (for device/data.value) at page generation time, pass it in to the template via render_template().

If, however, you don't know the value until after the page is rendered, you're going to need to construct the img element from JavaScript.