Deployment – How to Use pail

pail provides a WSGI middleware component

from pail.wsgi import ImageAdaptingMiddleware

that does all the work. It requires a list of supported resolutions (string with a comma-separated list of integers like "480, 922, 1322"). Also a factory for this middleware is available:

from pail.wsgi import filter_app

that returns instances of the middleware. See the API for details.

With Paste

pail provides a Paste-compatible WSGI filter app named main (this means that Paste, once pail is installed, can find the middleware as egg:pail automatically). It is meant to be used as a WSGI application wrapper (a ‘filter app’ in Paste terms), so that incoming requests are parsed, then passed to the wrapped application and the result is scanned for image data before it is passed on to the client.

A simple Paste config that makes use of pail might look like this:

use = egg:Paste#http
host =
port = 8000

use = egg:pail
resolutions = 1024, 480
next = static

use = egg:Paste#static
document_root = %(here)s/static-dir/

Here the first section [server:main] defines a server listening on port 8000.

The [app:static] section tells where the real content comes from: it’s simple static content read from local dir static-dir. We use the Paste app static for that purpose.

The relevant part, however, is the second section [filter-app:main], where we tell Paste to filter the static content through the pail filter app: use = egg:pail.

Then we tell the pail filter to support the two resolutions of 1024 and 480: resolutions = 1024, 480.

Finally we state to go on with the static app static: next = static. With Paste each filter app needs some other app (or filter app) to filter.

Of course, this is only a very plain sample for a WSGI/Paste setup. You could also create much more complex pipelines with several other filters and adapting images from Plone, Diazo or other content providers.

Also the content produced by pail could be mangled by further filters in a WSGI pipeline; that’s up to you. See the respective paste.deploy documentation for details about Paste configuration files.


Create a virtual environment, activate it, and install pail:

$ virtualenv py27   # also Python 2.6, 3.2, 3.3 should work
$ source py27/bin/activate
(py27)$ pip install pail

Create a Paste config in pailstatic.ini:

# pailstatic.ini
# run an HTTP server on port 8000
use = egg:Paste#http
host =
port = 8000

# filter all requests through pail
use = egg:pail
resolutions = 1024, 480
next = static

# serve static content...
use = egg:Paste#static
# ...from this local directory
document_root = %(here)s/static/

Now create the static content:

(py27)$ mkdir static/
(py27)$ cd static/

Create an HTML file named index.html like this:

    <title>My test page</title>
                      screen.width,screen.height)+'; path=/';
    <div>Some Text</div>
    <img  style="width: 100%;" href="myimage.jpg" />

and copy some image file, preferably a wide one (1024+ pixels width), into the static/ dir. Rename the image file to myimage.jpg.

Now install the missing paster packages and start the server:

(py27)$ pip install PasteScript
(py27)$ paster serve pailstatic.ini

Now, browsing http://localhost:8000/ you should see the generated page with the image included. Nothing special. Nothing? If you are using a desktop and the original image (put into the static dir) was wider than 1024 pixels, while the desktop has a maximum resolution of 1024px, it should automatically have been downscaled to 1024 px width. The same page watched from a mobile device with <= 480 px screen width should automatically get an image with width 480 px.

You can force that switch on a single machine by replacing the Math.max() expression in the JavaScript part to some fixed value like 480, 960, or whatever you want.

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