This document describes what design decisions were made during the development of this library. It also describes how the library works in detail.

This document assumes that you can use the library and have at least limited knowledge about the source code. If you feel that it is not true for you, you may want to read the Tutorial and Howto.

API design

The API should be simple for use to simple cases, but also provide great flexibility for the advanced cases. When increasing flexibility of the API it should not change the simple API unless it is absolutely required.

API compatibility is paramount. API breaking is only allowed when it is on par with the gain of the new functionality.

Adding new parameters to functions which have default value is not considered a breaking API change.

Simple API

API should be kept as simple as possible. It means that describing an expected request and its response should be trivial for the user. For this reason, the API is flat: it contains a handful of functions which have many parameters accepting built-in python types (such as bytes, string, int, etc) in contrast to more classes and functions with less arguments.

This API allows to define an expected request and the response which will be sent back to the client in a single line. This is one of the key features so using the library is not complicated.


def test_query_params(httpserver):
    httpserver.expect_request("/foo", query_string={"user": "user1"}).respond_with_data("OK")

It is simple in the most simple cases, but once the expectation is more specific, the line can grow significantly, so here the user is expected to put the literals into variables:

def test_query_params(httpserver):
    expected_query = {"user": "user1"}
    httpserver.expect_request("/foo", query_string=expected_query).respond_with_data("OK")

If the user wants something more complex, classes are available for this which can be instantiated and then specified for the parameters normally accepting only built-in types.

The easy case should be made easy, with the possibility of making advanced things in a bit more complex way.

Flexible API

The API should be also made flexible as possible but it should not break the simple API and not make the simple API complicated. A good example for this is the respond_with_handler method, which accepts a callable object (eg. a function) which receives the request object and returns the response object.

The user can implement the required logic there.

Adding this flexibility however did not cause any change in the simple API, the simple cases can be still used as before.

Higher-level API

In the early days of this library, it wanted to support the low-level http protocol elements: request status, headers, etc to provide full coverage for the protocol itself. This was made in order to make the most advanced customizations possible.

Then the project received a few PRs adding HeaderValueMatcher and support for authorization which relied on the low-level API to add a higher-level API without breaking it. In the opposite case, adding a low-level API to a high-level would not be possible.


The API provided by pytest-httpserver is transparent. That means that the objects (most importantly the Request and Response objects) defined by werkzeug are visible by the user of pytest-httpserver, there is no wrapping made. This is done by the sake of simplicity.

As werkzeug provides a stable API, there’s no need to change this in the future, however this also limits the library to stick with werkzeug in the long term. Replacing werkzeug to something else would break the API due to this transparency.


This section describes how to work with pytest-httpserver’s requirements. These are the packages used by the library.

Number of requirements

It is required to keep the requirements at minimum. When adding a new library to the package requirements, research in the following topics should be done:

  • code quality

  • activity of the development and maintenance

  • number of open issues, and their content

  • how many people using that library

  • python interpreter versions supported

  • amount of API breaking changes

  • license

Sometimes, it is better to have the own implementation instead of having a tiny library added to the requirements, which may cause compatibility issues.

Requirements version restrictions

In general, the package requirements should have no version restrictions. For example, the werkzeug library has no restrictions, which means that if a new version comes out of it, it is assumed that pytest-httpserver will be able to run with it.

Many people uses this library in an environment having full of other packages and limiting version here will limit their versions in their requirements also. For example if there’s a software using werkzeug 1.0.0 and our requirements have <0.9 specified it will make pytest-httpserver incompatible with their software.

Requirements testing

Currently it is required to test with only the latest version of the required packages. However, if there’s an API breaking change which affects pytest-httpserver, a decision should be made:

  • apply version restrictions, possibly making pytest-httpserver incompatible with some other software

  • add workaround to pytest-httpserver to support both APIs

HTTP server

The chosen HTTP server which drives this library is implemented by the werkzeug library. The reason behind this decision is that werkzeug is used by Flask, a very popular web framework and it provides a proven, stable API in the long term.

Supported python versions

Supporting the latest python versions (such as 3.7 and 3.8 at the time of writing this), is a must. Supporting the older versions is preferred, following the state of the officially supported python versions by PSF.

The library should be tested periodically on the supported versions.

Dropping support for old python versions is possible if supporting would cause an issue or require extensive workaround.

Python support for a given version is also dropped if it is near to the end of support or when a dependency deprecates it - this is needed to move forward with the community in order to support the latest versions of the dependencies.

Testing and coverage

It is not required to have 100% test coverage but all possible use-cases should be covered. Github actions is used to test the library on all the supported python versions, and tox.ini is provided if local testing is desired.

When a bug is reported, there should be a test for it, which would re-produce the error and it should pass with the fix.

Server starting and stopping

The server is started when the first test is run which uses the httpserver fixture. It will be running till the end of the session, and new tests will use the same instance. A cleanup is done between the tests which restores the clean state (no handlers registered, empty log, etc) to avoid cross-talk.

The reason behind this is the time required to stop the server. For some reason, werkzeug (the http server used) needs about 1 second to stop itself. Adding this time to each test is not acceptable in most of the cases.

Note that it is still compatible with pytest-xdist (a popular pytest extension to run the tests in parallel) as in such case, distinct test sessions will be run and those will have their own http server instance.

Fixture scope

Due to the nature of the http server (it is run only once), it seems to be a good recommendation to keep the httpserver fixture session scoped, not function scoped. The problem is that the cleanup which needs to be done between the tests (as the server is run only once, see above), and that cleanup needs to be attached to a function scoped fixture.

HTTP port selection

In early versions of the library, the user had to specify which port the server should be bound. This later changed to have an so-called ephemeral port, which is a random free port number chosen by the kernel. It is good because it guarantees that it will be available and it allows parallel test runnings for example.

In some cases it is not desired (eg if the code being tested has wired-in port number), in such cases it is still possible to specify the port number.

Also, the host can be specified which allows to bind on “” so the server is accessible from the network in case you want to test a javascript code running on a different server in a browser.