Writing Complex Tests

For many tests, writing one or more static HTML files is sufficient. However there are a large class of tests for which this approach is insufficient, including:

  • Tests that require cross-domain access

  • Tests that depend on setting specific headers or status codes

  • Tests that need to inspect the browser sent request

  • Tests that require state to be stored on the server

  • Tests that require precise timing of the response.

To make writing such tests possible, we are using a number of server-side components designed to make it easy to manipulate the precise details of the response:

  • wptserve, a custom python HTTP server.

  • pywebsocket, an existing websockets server

This document will concentrate on the features of wptserve available to test authors.

Introduction to wptserve

wptserve is a python-based web server. By default it serves static files in the testsuite. For more sophisticated requirements, several mechanisms are available to take control of the response. These are outlined below.


Suitable for:

  • Cross domain requests
  • Adding headers or status codes to static files
  • Controlling the sending of static file bodies

Pipes are designed to allow simple manipulation of the way that static files are sent without requiring any custom code. They are also useful for cross-origin tests because they can be used to activate a substitution mechanism which can fill in details of ports and server names in the setup on which the tests are being run.

Pipes are indicated by adding a query string to a request for a static resource, with the parameter name pipe. The value of the query should be a | serperated list of pipe functions. For example to return a .html file with the status code 410 and a Content-Type of text/plain, one might use:


There are a selection of pipe functions provided with wptserve and more may be added if there are good use cases.


Used to subsitute variables from the server environment, or from the request into the response. A typical use case is for testing cross-domain since the exact domain name and ports of the servers are generally unknown.

Substitutions are marked in a file using a block delimited by {{ and }}. Inside the block the following variables are avalible:

  • {{host}} - the host name of the server exclusing any subdomain part.
  • {{domains[]}} - the domain name of a particular subdomain e.g. {{domains[www]}} for the www subdomain.
  • {{ports[][]}} - The port number of servers, by protocol e.g. {{ports[http][1]}} for the second (i.e. non-default) http server.
  • {{headers[]}} - The HTTP headers in the request e.g. {{headers[X-Test]}} for a hypothetical X-Test header.
  • {{GET[]}} - The query parameters for the request e.g. {{GET[id]}} for an id parameter sent with the request.

So, for example, to write a javascript file called xhr.js that does a cross domain XHR test to a different subdomain and port, one would write in the file:

var server_url = "http://{{domains[www]}}:{{ports[http][1]}}/path/to/resource";
//Create the actual XHR and so on

The file would then be included as:

<script src="xhr.js?pipe=sub"></script>


Used to set the HTTP status of the response, for example:



Used to add or replace http headers in the response. Takes two or three arguments; the header name, the header value and whether to append the header rather than replace an existing header (default: False). So, for example, a request for:


causes example.html to be returned with a text/plain content type whereas:


Will cause example.html to be returned with both text/html and text/plain content-type headers.


Used to send only part of a response body. Takes the start and, optionally, end bytes as arguments, although either can be null to indicate the start or end of the file, respectively. So for example:


Would result in a response with a body containing 10 bytes of example.txt including byte 10 but excluding byte 20.


Would cause all bytes from byte 10 of example.txt to be sent, but:


Would send the first 20 bytes of example.txt.


Used to send the body of a response in chunks with delays. Takes a single argument that is a microsyntax consisting of colon-separated commands. There are three types of commands:

  • Bare numbers represent a number of bytes to send

  • Numbers prefixed d indicate a delay in seconds

  • Numbers prefixed r must only appear at the end of the command, and indicate that the preceding N items must be repeated until there is no more content to send.

In the absence of a repetition command, the entire remainder of the content is sent at once when the command list is exhausted. So for example:


causes a 1s delay before sending the entirety of example.txt.


causes 100 bytes of example.txt to be sent, followed by a 1s delay, and then the remainder of the file to be sent. On the other hand:


Will cause the file to be sent in 100 byte chunks separated by a 1s delay until the whole content has been sent.

asis files

Suitable for:

  • Static, HTTP-non-compliant responses

asis files are simply files with the extension .asis. They are sent byte for byte to the server without adding a HTTP status line, headers, or anything else. This makes them suitable for testing situations where the precise bytes on the wire are static, and control over the timing is unnecessary, but the response does not conform to HTTP requirements.

py files

Suitable for:

  • All tests requiring dynamic responses
  • Tests that need to store server side state.

The most flexible mechanism for writing tests is to use .py files. These are interpreted as code and are suitable for the same kinds of tasks that one might achieve using cgi, PHP or a similar technology. Unlike cgi or PHP, the file is not executed directly and does not produce output by writing to stdout. Instead files must contain (at least) a function named main, with the signature:

def main(request, response):

Here request is a Request object that contains details of the request, and response is a Response object that can be used to set properties of the response. Full details of these objects is provided in the wptserve documentation.

In many cases tests will not need to work with the response object directly. Instead they can set the status, headers and body simply by returning values from the main function. If any value is returned, it is interpreted as the response body. If two values are returned they are interpreted as headers and body, and three values are interpreted as status, headers, body. So, for example:

def main(request, response):
    return "TEST"

creates a response with no non-default headers and the body TEST. Headers can be added as follows:

def main(request, response):
    return ([("Content-Type", "text/plain"), ("X-Test", "test")],

And a status code as:

def main(request, response):
    return (410,
            [("Content-Type", "text/plain"), ("X-Test", "test")],

A custom status string may be returned by using a tuple code, string in place of the code alone.

At the other end of the scale, some tests require precision over the exact bytes sent over the wire and their timing. This can be achieved using the writer property of the response, which exposes a ResponseWriter object that allows wither writing specific parts of the request or direct access to the underlying socket.

For full documentation on the facilities available in .py files, see the wptserve documentation.