How to build a Python backend?
(Part 1: internal architecture) 2/2

3. Test it!

Note that for the sake of simplicity, we kept the testing part out of the way so far... shame on us! This is one of the main reasons we split all of our code this way! Keep in mind that all the components can be easily tested both separately or in-context. Let me give you an example. Our API calls the application service which then calls the repository and then returns a todo list converted from the domain object.

Here is a simple test for our repository in myapp/infrastructure/database/test_todo_entry_repository.py:

from tempfile import TemporaryDirectory

import pytest as pytest

from myapp.container import ApplicationContainer
from myapp.domain.todo import TodoEntry
from myapp.infrastructure.database.todo_entry_repository import TodoEntryPickleRepository


@pytest.fixture()
def repository():
    with TemporaryDirectory() as tmp_dir:
        container = ApplicationContainer()

        container.configuration.storage_dir.from_value(tmp_dir)
        yield container.todo_entry_repository()


def test_add_and_get(repository: TodoEntryPickleRepository):
    entry = TodoEntry.create_from_content("test")
    repository.add(entry)
    assert entry == repository.get(entry.id)

If you don't know Pytest Fixture, it's a generator that can be used as an input of a test function. It's very useful if you have context to manage. Here, our repository uses a local directory to store our objects, so we have to create the directory before the test, and delete it with all its content even if the test fails. To do so, we use tempfile.TemporaryDirectory providing context about where the directory is created when we enter it, and wipe it when we leave it. With our fixture, every function that uses the repository input now has a fresh storage directory sent with yield, and only when the test finishes the fixture does the code continue to be executed. Eventually the context is closed and the directory is deleted.

Let's install pytest in the test environment (so it doesn't end up in our final package) and run it. With poetry you can run poetry shell to get a new shell with a virtual environment activated, where all your dependencies are available including tools like pytest:

poetry add --dev pytest
poetry shell
pytest

With this test, we find out that the repository does not open the files in "write in bytes" mode but in "read in utf-8" mode which is the default in Python. So I've added the mode="wb" in my add function and the mode="rb" in my get function.