Parametrize with IDs in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pytest parametrize with custom IDs

What will be the output when running this pytest test with the given parametrize and custom IDs?

PyTest

import pytest

@pytest.mark.parametrize("input,expected", [(2, 4), (3, 9), (4, 16)], ids=["two", "three", "four"])
def test_square(input, expected):
    assert input * input == expected

if __name__ == "__main__":
    import sys
    import pytest
    sys.exit(pytest.main([__file__, "-q", "--tb=line"]))

A 3 passed (parametrize) in 0.01s

B 3 passed in 0.01s

C 3 passed (two, three, four) in 0.01s

D 3 failed (two, three, four) in 0.01s

💡 Hint

Look at how the ids parameter affects the test report output.

Using ids in pytest.mark.parametrize replaces the default numeric IDs with custom strings. The test report shows these custom IDs next to the test count.

❓ assertion

intermediate

Correct assertion for parametrize with IDs

Given this pytest parametrize with IDs, which assertion correctly checks the test ID in a hook?

PyTest

import pytest

@pytest.mark.parametrize("num,expected", [(1, 1), (2, 4)], ids=["one", "two"])
def test_square(num, expected):
    assert num * num == expected

# Hook to check test ID
@pytest.hookimpl(tryfirst=True)
def pytest_runtest_setup(item):
    # Which assertion is correct here?
    pass

A assert item.callspec.id == "one" or item.callspec.id == "two"

B assert item.name == "test_square[one]" or item.name == "test_square[two]"

C assert item.callspec.params == {"num": 1, "expected": 1}

D assert item.callspec.id in ["one", "two"]

💡 Hint

Check how to access the test ID string from the item object.

The item.callspec.id attribute holds the string ID for the current parameter set. Using in checks if it matches any expected ID.

🔧 Debug

advanced

Debug why custom IDs are not shown in pytest report

Why does this pytest parametrize code not show the custom IDs in the test report?

PyTest

import pytest

@pytest.mark.parametrize("x,y", [(1,2), (3,4)], ids=lambda val: f"val_{val}")
def test_sum(x, y):
    assert x + y > 0

A Because the ids function receives each parameter tuple, not individual values

B Because the ids function must return a list, not a lambda

C Because the ids function receives the whole tuple, not individual values

D Because the ids parameter only accepts strings, not functions

💡 Hint

Check the expected input type for the ids callable in parametrize.

The ids callable receives each parameter tuple as a whole, not individual values. The lambda expects a single value, so it fails to produce correct IDs.

🧠 Conceptual

advanced

Purpose of IDs in pytest parametrize

What is the main purpose of using the ids parameter in pytest.mark.parametrize?

A To change the order in which tests run

B To provide custom names for each test case for better readability in reports

C To skip certain test cases based on their ID

D To group tests into categories for parallel execution

💡 Hint

Think about how test reports display parameterized tests.

The ids parameter allows you to assign meaningful names to each parameter set, making test reports easier to understand.

❓ framework

expert

Implementing dynamic IDs for complex parameters in pytest

You have a list of dictionaries as parameters for a pytest test. How do you implement ids to generate readable test IDs showing the 'name' key from each dictionary?

PyTest

import pytest

params = [
    {"name": "alpha", "value": 1},
    {"name": "beta", "value": 2},
    {"name": "gamma", "value": 3}
]

@pytest.mark.parametrize("param", params, ids=??? )
def test_value(param):
    assert param["value"] > 0

A ids=[d["name"] for d in params]

B ids=[str(d["value"]) for d in params]

C ids=lambda d: str(d)

D ids=lambda d: d["name"]

💡 Hint

Remember that ids can be a list of strings or a callable.

When parameters are complex objects, providing a list of strings for ids is the simplest way to show meaningful test IDs. A lambda would receive each dict but must return a string; however, pytest expects a list of strings or a callable that receives each parameter. Here, a list comprehension is clearer and correct.