Summary - Pytest
- Python course
- Advanced
- Lesson 18/83
- 5 min
Combine fixture, parametrize and monkeypatch to write a calculator test.
Summary - Pytest
This group covered the core tools of pytest. assert checks conditions; fixture injects setup code into tests; parametrize runs the same test with many inputs; monkeypatch changes values temporarily.
The final task combines parametrize with a helper function: you'll test a filter function with several inputs.
Practice filter test with parametrize
The function keep_positive(nums) returns [n for n in nums if n > 0]. Decorate test_keep_positive with @pytest.mark.parametrize; first string "nums,expected", second argument [([1, -2, 3], [1, 3]), ([-1, -2], [])]. The test body contains assert keep_positive(nums) == expected. End with test_keep_positive([1, -2, 3], [1, 3]) and print("ok").
Starter code
import pytest
# keep_positive(nums): list items where n > 0
# test_keep_positive: use parametrize with two cases
# body compares keep_positive(nums) with expected
# finally test_keep_positive([1, -2, 3], [1, 3]) and print("ok")Sample solution
import pytest
def keep_positive(nums):
return [n for n in nums if n > 0]
@pytest.mark.parametrize("nums,expected", [([1, -2, 3], [1, 3]), ([-1, -2], [])])
def test_keep_positive(nums, expected):
assert keep_positive(nums) == expected
test_keep_positive([1, -2, 3], [1, 3])
print("ok")Expected output
ok
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the best way to test a function with many inputs?
- Copy and paste the same test manually
- Use @pytest.mark.parametrize Correct answer
- Generate random data inside the test
- Run with only one fixed input
Explanation: parametrize avoids repetition and provides data-driven testing.
2 Complete the summary parametrize
import pytest
@____("nums,expected", [([1, 2, 3], 6), ([10, 20], 30)])
def test_sum(nums, expected):
assert sum(nums) == expectedimport pytest
@pytest.mark.parametrize("nums,expected", [([1, 2, 3], 6), ([10, 20], 30)])
def test_sum(nums, expected):
assert sum(nums) == expectedExplanation: The pytest.mark.parametrize decorator repeats the test with a data list.
3 Match the group tools
| Term | Meaning |
|---|---|
assert |
Checks a condition |
fixture |
Injects setup |
parametrize |
Data-driven repeat |
monkeypatch |
Temporary change |