Some Notes on Writing Reliable Tests for Flask Applications with Pytest

Today I Learnt

What to Test?

Unit tests should focus on testing a small unit of code in isolation:

• Database models
• Utility functions

Functional tests should focus on how the view functions operate:

• Nominal conditions (GET, POST, etc.) for a view function
• Invalid HTTP methods are handled properly for a view function
• Invalid data is passed to a view function

Documenting Tests

Martin Fowler provides a really idiomatic way of describing tests in his blog post on GivenWhenThen:

"""
GIVEN a User model
WHEN a new User is created
THEN check the email, hashed_password, and role fields are defined correctly
"""

• GIVEN - what are the initial conditions for the test?
• WHEN - what is occurring that needs to be tested?
• THEN - what is the expected response?

Testing for Exceptions

Asserting an exception is raised:

import pytest

def test_raises_exception():
    with pytest.raises(ZeroDivisionError):
        1 / 0

Asserting an exception is not raised:

def my_division_function(a, b):
    return a / b

def test_code_raises_no_exception():
    """
    Assert your python code raises no exception.    
    """
    try:
        my_division_function(10, 5)
    except ZeroDivisionError as exc:
        assert False, f"'10 / 5' raised an exception {exc}"

Pytest Fixtures

Fixtures initialise tests to a known state to run tests in a predictable and repeatable manner, these should be created in your conftest.py file.

In Pytest, fixtures can be run with different scopes:

• function - run once per test function (default scope)
• class - run once per test class
• module - run once per module (e.g., a test file)
• session - run once per session

For example, to create a User as a fixture:

from project.models import User

@pytest.fixture(scope='module')
def new_user():
    user = User('[email protected]', 'Password1!')
    return user

@pytest.fixture(scope='module')

This decorator specifies that a function is a fixture with a module-level scope. So this fixture will be called once per test module.

def new_user()

Is the function that creates an instance of a User using valid arguments.

To use this fixture, in other tests you pass it in as the first argument to a subsequent test, e.g.,

def test_new_user_with_fixture(new_user):
    """
    GIVEN a User model
    WHEN a new User is created
    THEN check the email, hashed_password, and role fields are defined correctly
    """
    assert new_user.email == '[email protected]'
    assert new_user.hashed_password != 'Password1!'
    assert new_user.role == 'user'