Dynamic fixture selection in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dynamic fixture selection with parameter

What is the output of the following pytest test when run with pytest -s?

PyTest

import pytest

@pytest.fixture
def data_a():
    return "Data A"

@pytest.fixture
def data_b():
    return "Data B"

@pytest.fixture(params=["data_a", "data_b"])
def dynamic_data(request):
    return request.getfixturevalue(request.param)

def test_dynamic(dynamic_data):
    print(dynamic_data)
    assert dynamic_data in ["Data A", "Data B"]

A Data B

B Data B\nData A

C Data A

D Data A\nData B

💡 Hint

Think about how pytest runs tests with parameterized fixtures.

The fixture 'dynamic_data' is parameterized with two fixture names. pytest runs the test twice, once with 'data_a' and once with 'data_b'. So the print outputs 'Data A' then 'Data B'.

❓ assertion

intermediate

Correct assertion for dynamic fixture value

Given a dynamic fixture that returns either 10 or 20, which assertion correctly verifies the value inside the test?

PyTest

import pytest

@pytest.fixture(params=[10, 20])
def number(request):
    return request.param

def test_number(number):
    # Which assertion is correct here?
    pass

A assert number == 10 and number == 20

B assert number in [10, 20]

C assert number == 10 or number == 20

D assert number != 10 or number != 20

💡 Hint

Check which assertion logically matches the possible values.

Option B correctly asserts that number is one of the two possible values. Option B works but is less clear. Option B is always false. Option B is always true and does not test correctly.

🔧 Debug

advanced

Identify the error in dynamic fixture usage

What error will this pytest code raise when running the test?

PyTest

import pytest

@pytest.fixture(params=["fix1", "fix2"])
def dynamic(request):
    return request.getfixturevalue("fix3")

@pytest.fixture
def fix1():
    return "Fix 1"

@pytest.fixture
def fix2():
    return "Fix 2"

def test_dynamic(dynamic):
    assert dynamic in ["Fix 1", "Fix 2"]

A FixtureLookupError: The fixture 'fix3' is not found

B TypeError: getfixturevalue() missing required positional argument

C AssertionError: dynamic value not in expected list

D No error, test passes

💡 Hint

Check the fixture name passed to getfixturevalue.

The code tries to get a fixture named 'fix3' which does not exist, causing a FixtureLookupError.

❓ framework

advanced

Best practice for dynamic fixture selection in pytest

Which approach is best to dynamically select a fixture based on a test parameter in pytest?

A Use pytest hooks to modify fixture behavior at runtime

B Use if-else inside the test to call fixture functions directly

C Use a parameterized fixture with request.getfixturevalue to call fixtures by name

D Use global variables to store fixture data and select dynamically

💡 Hint

Consider pytest's built-in features for fixture parameterization.

Option C uses pytest's recommended way to select fixtures dynamically by name. Options B and C break fixture isolation and reusability. Option C is complex and not intended for this purpose.

🧠 Conceptual

expert

Understanding fixture scope impact on dynamic fixture selection

If you have a session-scoped dynamic fixture that selects between two fixtures with different scopes (one 'function' and one 'session'), what is the expected behavior when running tests?

A pytest raises an error because fixtures with different scopes cannot be mixed dynamically

B The dynamic fixture adopts the broader scope of the selected fixtures automatically

C The dynamic fixture runs with the narrower scope of the selected fixture for each test run

D The test results are unpredictable and may cause flaky tests

💡 Hint

Think about pytest's rules for fixture scope hierarchy.

pytest does not allow mixing fixtures with incompatible scopes dynamically because it cannot guarantee consistent fixture lifetimes, so it raises a scope mismatch error.