Writing Effective Tests

Writing Effective Tests#

Note

Source: Python-specific — no direct equivalent in the C# edition. Examples test the Rational class from The Rational Class and illustrate best practices applicable to any Python module.

Good tests are specific, independent, and cover edge cases. Each test function should check one thing and have a name that explains what scenario it is testing.

Testing the Rational Class#

Assuming Rational is defined in rational.py:

from rational import Rational

def test_normalisation():
    r = Rational(6, -10)
    assert str(r) == "-3/5"

def test_addition():
    f = Rational(-3, 5)
    h = Rational(1, 2)
    assert str(f + h) == "-1/10"

def test_multiplication():
    assert Rational(2, 3) * Rational(3, 4) == Rational(1, 2)

def test_equality():
    assert Rational(2, 4) == Rational(1, 2)
    assert Rational(1, 3) != Rational(1, 4)

def test_whole_number():
    assert str(Rational(6, 3)) == "2"

def test_float_conversion():
    assert float(Rational(1, 4)) == 0.25

Testing for Exceptions#

Use pytest.raises as a context manager to verify that an exception is raised in the expected situation:

import pytest
from rational import Rational

def test_zero_denominator():
    with pytest.raises(ValueError):
        Rational(1, 0)

If the block inside with pytest.raises(ValueError): does not raise ValueError, the test fails.

Edge Cases#

Always test:

  • Zero: factorial(0), Rational(0, 5)

  • Negative inputs: Rational(-1, 2), Rational(1, -2)

  • Boundary values: the smallest and largest valid inputs

  • Already-normalised values: Rational(1, 2) should stay 1/2

  • Idempotent operations: adding zero, multiplying by one

def test_add_zero():
    r = Rational(3, 4)
    assert r + Rational(0, 1) == r

def test_multiply_by_one():
    r = Rational(3, 4)
    assert r * Rational(1) == r

def test_negative_numerator_and_denominator():
    assert str(Rational(-2, -3)) == "2/3"

Organising Tests in a Class#

For a large module, group related tests in a class prefixed with Test:

class TestRationalArithmetic:
    def test_add(self):
        assert Rational(1, 3) + Rational(1, 6) == Rational(1, 2)

    def test_sub(self):
        assert Rational(3, 4) - Rational(1, 4) == Rational(1, 2)

class TestRationalComparison:
    def test_less_than(self):
        assert Rational(1, 3) < Rational(1, 2)

pytest discovers and runs these automatically with no extra configuration.

One Assert per Test (Guideline)#

When a test contains many assertions and fails, it can be hard to tell which assertion caused the failure. Prefer one logical check per test function so failure messages are immediately actionable.