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A pytest plugin for testing TCP clients

Project description

pytest-tcpclient is a pytest plugin that helps you write better TCP clients.

It provides two fixtures, tcpserver and tcpserver_factory.

Behind the scenes, the tcpserver fixture creates an in-process TCP server that listens on a port to which the client can connect and send messages and from which it can receive replies.

The tcpserver fixture is used to express expectations about what messages the client sends and also to send replies to it. If any expectation is unfulfilled, the test will fail with a diagnostic message.

Contents

Hello!

Here’s an example of a passing test that uses the tcpserver fixture:

import asyncio
import pytest


@pytest.mark.asyncio()
async def test_hello(tcpserver):

    # ==========================================================================
    # Establish expectations and replies on the server side.
    #
    # In this case, the client is expected to connect and then send a specific
    # message. If that occurs, the server will send a reply. It is expected
    # that the client will then disconnect.

    tcpserver.expect_connect()
    tcpserver.expect_bytes(b"Hello, server!")
    tcpserver.send_bytes(b"Hello, client!")
    tcpserver.expect_disconnect()

    # ==========================================================================
    # Client interacts with server.
    #
    # It connects, sends the expected message and then receives the reply.
    # Finally, it disconnects from the server.

    reader, writer = await asyncio.open_connection(None, tcpserver.service_port)

    writer.write(b"Hello, server!")
    assert await reader.readexactly(14) == b"Hello, client!"

    writer.close()
    await writer.wait_closed()

    # ==========================================================================
    # Synchronise with server. Test failures, if any, will be reported here. In
    # this case, there are no failures.

    await tcpserver.join()

Here’s the result:

============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0, pluggy-1.0.0
rootdir: /home/anders/src/pytest-tcpclient, configfile: pyproject.toml
plugins: mock-3.10.0, asyncio-0.20.2, cov-4.0.0, tcpclient-0.7.26.dev1+gad4c8b2.d20221115
asyncio: mode=strict
collected 1 item

examples/test_hello.py .                                                 [100%]

============================== 1 passed in 0.03s ===============================

Test Failure

This example demonstrates test failure.

It is similar to the previous example except that the client does not send the expected message. As a result, the server times out while waiting for that message and the test fails.

import asyncio
import pytest


@pytest.mark.asyncio()
async def test_expect_bytes_times_out(tcpserver):

    # --------------------------------------------------------------------------
    # Server expectations. The server just expects the client to connect, send
    # a message and then disconnect.

    tcpserver.expect_connect()
    tcpserver.expect_bytes(b"Hello, world!")
    tcpserver.expect_disconnect()

    # --------------------------------------------------------------------------
    # The client connects but it does not send the message and it does not close
    # the connection.

    reader, writer = await asyncio.open_connection(None, tcpserver.service_port)

    # --------------------------------------------------------------------------
    # The server will time out waiting for the expected message. The test will
    # fail with a diagnostic message.

    await tcpserver.join()

Here’s the result:

============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0, pluggy-1.0.0
rootdir: /home/anders/src/pytest-tcpclient, configfile: pyproject.toml
plugins: mock-3.10.0, asyncio-0.20.2, cov-4.0.0, tcpclient-0.7.26.dev1+gad4c8b2.d20221115
asyncio: mode=strict
collected 1 item

examples/test_expect_bytes_times_out.py F                                [100%]

=================================== FAILURES ===================================
_________________________ test_expect_bytes_times_out __________________________

tcpserver = <pytest_tcpclient.plugin.MockTcpServer object at 0x7fda575068b0>

    @pytest.mark.asyncio()
    async def test_expect_bytes_times_out(tcpserver):

        # --------------------------------------------------------------------------
        # Server expectations. The server just expects the client to connect, send
        # a message and then disconnect.

        tcpserver.expect_connect()
        tcpserver.expect_bytes(b"Hello, world!")
        tcpserver.expect_disconnect()

        # --------------------------------------------------------------------------
        # The client connects but it does not send the message and it does not close
        # the connection.

        reader, writer = await asyncio.open_connection(None, tcpserver.service_port)

        # --------------------------------------------------------------------------
        # The server will time out waiting for the expected message. The test will
        # fail with a diagnostic message.

>       await tcpserver.join()
E       Failed: Timed out waiting for b'Hello, world!'

examples/test_expect_bytes_times_out.py:26: Failed
=========================== short test summary info ============================
FAILED examples/test_expect_bytes_times_out.py::test_expect_bytes_times_out
============================== 1 failed in 1.04s ===============================

Sending and receiving length-delimited “frames”

A common pattern for TCP communication is to send and receive discrete messages. One way to represent the boundaries of these messages is to prepend a 4-byte binary integer in network (big-endian) ordering that is the length of the payload. In pytest-tcpclient this convention is called “framing”.

Here’s an example of testing that a client sends a frame (expect_frame):

import asyncio
import pytest
import struct


@pytest.mark.asyncio()
async def test_expect_frame_success(tcpserver):

    tcpserver.expect_connect()
    tcpserver.expect_frame(b"Goodbye, world")

    reader, writer = await asyncio.open_connection(None, tcpserver.service_port)

    payload = b"Goodbye, world"

    # Here's the 4-byte length header
    writer.write(struct.pack(">I", len(payload)))

    # Here's the payload
    writer.write(payload)

    # Done
    writer.close()

    await writer.wait_closed()

    await tcpserver.join()

Here’s the result:

============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0, pluggy-1.0.0
rootdir: /home/anders/src/pytest-tcpclient, configfile: pyproject.toml
plugins: mock-3.10.0, asyncio-0.20.2, cov-4.0.0, tcpclient-0.7.26.dev1+gad4c8b2.d20221115
asyncio: mode=strict
collected 1 item

examples/test_expect_frame_success.py .                                  [100%]

============================== 1 passed in 0.02s ===============================

Here’s an example where the server sends a frame (send_frame) and the client receives it:

import asyncio
import pytest
import struct


@pytest.mark.asyncio()
async def test_send_frame_success(tcpserver):
    tcpserver.expect_connect()
    reader, writer = await asyncio.open_connection(None, tcpserver.service_port)

    # The server immediately sends a frame
    tcpserver.send_frame(b"Hello")

    # The client receives the frame. First the header and then the payload.
    header_bytes = await reader.readexactly(4)
    message_length, = struct.unpack(">I", header_bytes)
    assert await reader.readexactly(message_length) == b"Hello"

    writer.close()
    await writer.wait_closed()

Here’s the result:

============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.2.0, pluggy-1.0.0
rootdir: /home/anders/src/pytest-tcpclient, configfile: pyproject.toml
plugins: mock-3.10.0, asyncio-0.20.2, cov-4.0.0, tcpclient-0.7.26.dev1+gad4c8b2.d20221115
asyncio: mode=strict
collected 1 item

examples/test_send_frame_success.py .                                    [100%]

============================== 1 passed in 0.02s ===============================

Development

If you want to use a virtual environment, do that first and activate it. You can use any virtual environment system you like. However, if you want to use virtualenv (and you already have virtualenv installed) you could do this:

$ virtualenv -p3.8 venv

Next, make the project:

$ make

That will do the following:

  • Install all the dependencies

  • Run the tests

  • Generate a coverage report

  • Fail if the coverage is below 100%

Build configuration

Build configuration is mostly stored in pyproject.toml. Idealy, it would all be in there. However, there are two exceptions.

First, setuptools has been chosen as the build system. Unfortunately, to install pytest-tcpclient in editable mode, a minimial setup.py must be and has been provided.

Packages that pytest-tcpclient requires to run are listed in pyproject.toml.

Packages required for development (testing, coverage and linting) are listed in dev_dependencies.txt.

tox has been configured (in tox.ini) to install those packages before running the tests.

setuptools has been configured to supply the option dev for those extra packages. For example, the Makefile has the following command to initialise the virtual environment:

$ python -m pip install -e .[dev]

Default make target is style_and_test

The default target in the Makfile is style_and_test which first calls the linter, then runs the tests and, finally, checks that code coverage is 100%

tox

tox is only used for CI builds. See .github/workflows/build.yml.

make detects changes to configuration files

If any of the build system configuration files, make will reinstall all dependencies.

Continuous Integration and Deployment

There is a workflow (.github/workflows/build.yml) that will build and test pull requests with tox.

There is another workflow (.github/workflows/publish.yml) that is triggered by the appearance of new version tags on the main branch. It will build and test the code and additionally publish the package to pypi.

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