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Mock subclass of BaseSelector



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This is a collection of Python classes designed to help to test TCP servers based on selectors. The mockselector package provides everything needed to easily write unittest TestCases simulating incoming connections and the associated input data


From PyPI

Starting from 0.1.1, mockselector is available on PyPI. If you just want to use it in your projects, it is the recommended way:

pip install mockselector

Of course, this can be done in a relevant venv if you do not want to have it in your main installation.

Note: the test folder is only available in the source distribution.

From Github

This is the recommended way if you want to contribute or simply tweak mockselector to your own requirements. You can get a local copy by downloading a zipfile but if you want to make changes, you should rather clone the repository to have access to all git goodies:

git clone

You can then install it in your main Python installation or in a venv with:

pip install -e .

or on Windows with the launcher:

py -m pip install -e .

pip should be used to install it for the first time in order to have setuptools-scm to generate the file from git metadata. Once this is done, can be used with no special issue.

Alternatively you can manually install setuptools-scm:

pip install setuptools-scm
python install
Special handling of

mockselector relies on setuptools-scm to automatically extract a version number from git metadata and store it in a file for later use. The requires the availability of both git (which should not be a problem when the project is downloaded from Github), and setuptools-scm. If it fails because one is not available or because git metadata is not there (if you only downloaded a zip archive from Github), the version is set to 0.0.0

For that reason, if you do not use git to download the sources, you should download a source distribution from PyPI, because the latter contains a valid

pip uses the pyproject.toml file with respect to PEP-518 and PEP-517 to know that setuptools-scm is required before the build.

Basic use

Once installed, you can easily import it in your tests.

from mockselector.selector import MockSocket, ListenSocket, MockSelector

MockSelector is a selectors.BaseSelector subclass. At creation time it takes an iterable of objects. Those objects can be:

  • a socket.socket (or a Mock) that will be returned by a select call along with an EVENT_READ event
  • a pair (socket, event) that will be returned - this allows to pass EVENT_WRITE events
  • an iterable of above elements. They will be returned in a list by a single select call as simultaneous events

MockSocket is a specialization of a Mock(socket.socket). Its initializer takes an iterable of byte strings or functions returning byte strings. The functions can be used as a run time side effect to set a flag in a server and allow a clean exit from the main loop. The byte strings are returned one at a time by the recv method. When the iterable is exhausted, recv returns an empty byte string (b'') to mimic a client close or shutdown on the socket.

ListenSocket is used to mimic a listening socket. Its initializer takes an iterable of socket.socket objects (including plain Mock or MockSocket objects) or callables returning an object like that. The socket objects are returned one at a time by the accept method.

Typical use

Facing a main server loop close to:

        s = socket.socket()
        s.bind(('', self.port))
        sel = DefaultSelector()
        sel.register(s, EVENT_READ)
        while not self.stop:
            for key, event in
                if key.fileobj == s:
                    c, _ = s.accept()
                    sel.register(c, EVENT_READ)
                    c = key.fileobj
                    data = c.recv(1024)
                    if len(data) == 0:
                        # process received data

You can do:

    def test_run_stop(self):
        def do_stop(serv):
            serv.stop = True
            return b''
        serv = ...                  # an instance or the serveur to test
        c1 = MockSocket([...])      # a client with its data
        c2 = MockSocket([..., lambda: do_stop(serv)]) # another client asking for end of server loop
        s = ListenSocket((c1, c2))
        sel = MockSelector([s, c1, s, c2, c2, (c1, c2), c1, c2, c2]) # ordered list of events
        with patch('socket.socket') as socket, \
                patch('miniserv.DefaultSelector') as selector:
            socket.return_value = s
            selector.return_value = sel

You can find a full code example in the and files in the tests folder

Advanced use and contribution

If you want to tailor the package, it already contains a number of tests. You can run all of them from the top folder:

python install -e    # edit mode of install to use the local folder
python -m unittest discover

I will be glad to receive issues that would help to improve this project...

Disclaimer: beta quality

Even if the package has a nice test coverage (> 95%), it currently only contains what I need to test another project of mine. It might not meet your own requirements, or main contain Still Unidentified Bugs...

It is still a 0.x version, so the API is not guaranteed to be stable.


That work is licenced under a MIT Licence. See LICENSE.txt


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