The following code prints HTML with responsive images support. It's ready to use and defines three levels of resolution: 400px, 800px and original image.

Adapt and start using the following code in Python 2 (we use Picturefill as polyfill):

image_url = "http://images.rethumb.com/image_coimbra_999x999.jpg";

small_width = 400; # Image width for small resolutions (less than 400px). 
large_width = 800; # Image width for medium resolutions (less than 800px).
image_width = 999; # Image original width (used for resolutions larger than 800px).

print("""

<script src="https://cdnjs.cloudflare.com/ajax/libs/picturefill/2.3.1/picturefill.min.js"></script>

<img src="http://api.rethumb.com/v1/width/{small_width}/{image_url}"

     srcset="http://api.rethumb.com/v1/width/{small_width}/{image_url} {small_width}w,
             http://api.rethumb.com/v1/width/{large_width}/{image_url} {large_width}w,
             {image_url} {image_width}w"

     sizes="100vw" />
""".format(image_url = image_url, small_width = small_width, large_width = large_width, image_width = image_width))

Use the following commands to get started:

$ git clone https://github.com/rethumb/rethumb-python2-examples.git
$ cd rethumb-python2-examples
$ python responsive-images.js