Testing Web sites with twill

twill was initially designed for testing Web sites, although since then people have also figured out that it's good for browsing unsuspecting Web sites.

Using twill-sh

The simplest way to test Web sites is to write one or more twill scripts and then simply run

twill-sh [ -u initial_url ] script(s)

either from the command-line (for development purposes), via a cron job (to check to see if sites are up and responding), or from your functional or unit tests (see below).

twill-sh will try to run each script given to it on the command line once, and will report the number of scripts that failed. The exit value of the script will be 0 if there are no failures, so you can use it in a shell script easily enough.

twill-sh will gather scripts from directories, so you can create a whole directory hierarchy containing your scripts and they will all be gathered and run, in standard lexical order.

The -u flag can be used to give twill-sh an initial URL; this is equivalent to placing a "go (initial_url)" command at the top of the script, but is particularly handy for test frameworks where the URL might change depending on the developer.

Stress testing

You can use the twill-fork script to do some stress testing. The syntax is

twill-fork -n <number to execute> -p <number of processes> script [ scripts... ]

For example,

twill-fork -n 500 -p 10 test-script

will fork 10 times and run test-script 50 times in each process. twill-fork will record the time it takes to run all of the scripts specified on the command and print a summary at the end.

The time recorded is not the CPU time used. (This would lead to an inaccurate estimate because the client code uses blocking calls to retrieve Web pages.) Rather, the time recorded is the clock time measured between the start and end of script execution.

Try twill-fork -h to get a list of other command line arguments.

Note that twill-fork still needs a lot of work...

Unit testing

twill can be used in unit testing, and it contains some Python support infrastructure for this purpose.

As an example, here's the code from twill's own unit test, testing the unit-test support code:

import os
import testlib
import twill.unit
import twilltestserver
from quixote.server.simple_server import run as quixote_run

def test():
    # port to run the server on

    # create a function to run the server
    def run_server_fn():
        quixote_run(twilltestserver.create_publisher, port=PORT)

    # abspath to the script
    script = os.path.join(testlib.testdir, 'test-unit-support.twill')

    # create test_info object
    test_info = twill.unit.TestInfo(script, run_server_fn, PORT)

    # run tests!

Here, I'm unit testing the Quixote application twilltestserver, which is run by quixote_run (a.k.a. quixote.server.simple_server.run) on port PORT, using the twill script test-unit-support.twill. That script contains this code:

# starting URL is provided to it by the unit test support framework.

go ./multisubmitform
code 200

A few things to note:

  • the initial URL is set based on the URL reported by TestInfo, which calculates it based on the PORT argument. (This can be overriden by subclasses.)
  • TestInfo contains code to (a) run the server function in a new process, and (b) run the twill script against that server. It then kills the server after script completion.
  • You can also pass a 'sleep' argument to the TestInfo constructor that specifies how many seconds to wait for the server to start before executing the script.

Testing WSGI applications "in-process"

twill has some built-in support for testing WSGI applications.

twill contains two functions, add_wsgi_intercept and remove_wsgi_intercept, that allow Python applications to redirect HTTP calls into a WSGI application "in-process", without going via an external Internet call. This is particularly useful for unit tests, where setting up an externally available Web server can be inconvenient.

For example, the following code redirects all localhost:80 calls to the given WSGI app:

def create_app():
    return wsgi_app

twill.add_wsgi_intercept('localhost', 80, create_app)

See the tests/test-wsgi-intercept.py unit test for more information.