Telethon uses Pytest, for testing, Tox for environment setup, and pytest-asyncio and pytest-cov for asyncio and coverage integration.

While reading the full documentation for these is probably a good idea, there is a lot to read, so a brief summary of these tools is provided below for convienience.

Brief Introduction to Pytest

Pytest is a tool for discovering and running python tests, as well as allowing modular reuse of test setup code using fixtures.

Most Pytest tests will look something like this:

from module import my_thing, my_other_thing

def test_my_thing(fixture):
    assert my_thing(fixture) == 42

async def test_my_thing(event_loop):
    assert await my_other_thing(loop=event_loop) == 42

Note here:

  1. The test imports one specific function. The role of unit tests is to test that the implementation of some unit, like a function or class, works. It’s role is not so much to test that components interact well with each other. I/O, such as connecting to remote servers, should be avoided. This helps with quickly identifying the source of an error, finding silent breakage, and makes it easier to cover all possible code paths.

    System or integration tests can also be useful, but are currently out of scope of Telethon’s automated testing.

  2. A function test_my_thing is declared. Pytest searches for files starting with test_, classes starting with Test and executes any functions or methods starting with test_ it finds.

  3. The function is declared with a parameter fixture. Fixtures are used to request things required to run the test, such as temporary directories, free TCP ports, Connections, etc. Fixtures are declared by simply adding the fixture name as parameter. A full list of available fixtures can be found with the pytest --fixtures command.

  4. The test uses a simple assert to test some condition is valid. Pytest uses some magic to ensure that the errors from this are readable and easy to debug.

  5. The pytest.mark.asyncio fixture is provided by pytest-asyncio. It starts a loop and executes a test function as coroutine. This should be used for testing asyncio code. It also declares the event_loop fixture, which will request an asyncio event loop.

Brief Introduction to Tox

Tox is a tool for automated setup of virtual environments for testing. While the tests can be run directly by just running pytest, this only tests one specific python version in your existing environment, which will not catch e.g. undeclared dependencies, or version incompatabilities.

Tox environments are declared in the tox.ini file. The default environments, declared at the top, can be simply run with tox. The option tox -e py36,flake can be used to request specific environments to be run.

Brief Introduction to Pytest-cov

Coverage is a useful metric for testing. It measures the lines of code and branches that are exercised by the tests. The higher the coverage, the more likely it is that any coding errors will be caught by the tests.

A brief coverage report can be generated with the --cov option to tox, which will be passed on to pytest. Additionally, the very useful HTML report can be generated with --cov --cov-report=html, which contains a browsable copy of the source code, annotated with coverage information for each line.