.gitignore

/.vscode
__pycache__
.pytest_cache
log.txt

README.md

stackoverflow question 58270958

Example code for stackoverflow question 58270958 showing how to spy on BackgroundTasks.add_task(...) with pytest.

How to run

git clone https://gist.github.com/dcba361f7b5cd6aba636e5c8a4b1aad6.git
cd dcba361f7b5cd6aba636e5c8a4b1aad6
pipenv install
pipenv run pytest

main.py

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def write_notification(email: str, message=""):
    with open("log.txt", mode="w") as email_file:
        content = f"notification for {email}: {message}"
        email_file.write(content)


@app.post("/send-notification/{email}")
async def send_notification(email: str, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_notification, email, message="some notification")
    return {"message": "Notification sent in the background"}

Pipfile

[[source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true

[dev-packages]
pytest = "*"
black = "*"
requests = "*"
pytest-mock = "*"

[packages]
fastapi = "*"

[requires]
python_version = "3.7"

[pipenv]
allow_prereleases = true

Pipfile.lock

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    }
}

test_main.py

import pytest
from fastapi.testclient import TestClient
from fastapi import BackgroundTasks
from requests import Response

from main import app, write_notification
from unittest.mock import mock_open


@pytest.fixture
def client():
    return TestClient(app)


def test_send_notification(
    client, mocker,
):
    mock_file_open = mocker.patch(
        "main.open", new=mock_open()
    )  # https://docs.python.org/3/library/unittest.mock.html#mock-open
    spy = mocker.spy(BackgroundTasks, "add_task")

    r: Response = client.post("/send-notification/f00@example.com")

    # validate the HTTP response
    assert r.status_code == 200
    assert r.json() == {"message": "Notification sent in the background"}

    # validate the add_task call was made
    spy.assert_called_with(
        mocker.ANY,  # add_task() is a method, and as such its first argument is `self`
        write_notification,
        "f00@example.com",
        message="some notification",
    )

    # validate the background task did its job
    assert mocker.call("log.txt", mode="w") in mock_file_open.call_args_list
    handle = mock_file_open()
    handle.write.assert_called_once_with(
        "notification for f00@example.com: some notification"
    )