Conditional parametrize in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of conditionally parametrized pytest test

Given the following pytest test code with conditional parametrization, what will be the output when running the test?

import pytest

@pytest.mark.parametrize("input,expected", [
    (1, 2),
    (2, 3),
    pytest.param(3, 4, marks=pytest.mark.skipif(True, reason="skip 3")),
])
def test_increment(input, expected):
    assert input + 1 == expected

PyTest

import pytest

@pytest.mark.parametrize("input,expected", [
    (1, 2),
    (2, 3),
    pytest.param(3, 4, marks=pytest.mark.skipif(True, reason="skip 3")),
])
def test_increment(input, expected):
    assert input + 1 == expected

A All three tests pass

B Two tests pass, one test fails

C Two tests pass, one test is skipped

D All three tests fail

💡 Hint

Look at the skipif mark condition and how it affects test execution.

The third test case is marked to be skipped unconditionally (skipif(True)). So pytest will run only the first two tests which pass.

❓ assertion

intermediate

Assertion outcome with conditional parametrize

Consider this pytest test with conditional parametrization based on a variable:

import pytest

run_extra = False

params = [(1, 2), (2, 3)]
if run_extra:
    params.append((3, 5))

@pytest.mark.parametrize("input,expected", params)
def test_add_one(input, expected):
    assert input + 1 == expected

What will happen when running this test?

PyTest

import pytest

run_extra = False

params = [(1, 2), (2, 3)]
if run_extra:
    params.append((3, 5))

@pytest.mark.parametrize("input,expected", params)
def test_add_one(input, expected):
    assert input + 1 == expected

A Two tests run and both pass

B Three tests run, one fails

C Two tests run, one fails

D Three tests run and all pass

💡 Hint

Check the value of run_extra and how it affects params.

Since run_extra is False, only two test cases are parametrized and both pass.

🔧 Debug

advanced

Identify the error in conditional parametrize usage

What error will this pytest code raise?

import pytest

condition = False

@pytest.mark.parametrize("x", [1, 2] if condition else 3)
def test_example(x):
    assert x > 0

PyTest

import pytest

condition = False

@pytest.mark.parametrize("x", [1, 2] if condition else 3)
def test_example(x):
    assert x > 0

A TypeError: 'int' object is not iterable

B SyntaxError: invalid syntax

C No error, tests run with x=1 and x=2

D ValueError: parametrize values must be a list or tuple

💡 Hint

Check the else part of the conditional expression.

The else part returns an int (3), which is not iterable. parametrize expects an iterable of values.

❓ framework

advanced

Best way to conditionally parametrize tests in pytest

You want to run a test with extra parameters only if an environment variable TEST_EXTRA is set to 'yes'. Which pytest approach correctly implements this conditional parametrization?

Options:

PyTest

import os
import pytest

extra_params = [(3, 4)] if os.getenv('TEST_EXTRA') == 'yes' else []
params = [(1, 2), (2, 3)] + extra_params

@pytest.mark.parametrize("input,expected", params)
def test_func(input, expected):
    assert input + 1 == expected

A Use pytest hooks to add parameters dynamically at runtime

B Use pytest.mark.skipif inside parametrize for each extra param

C Use pytest parametrize with indirect=True and check env inside test

D Use a conditional list to build params before @pytest.mark.parametrize as shown

💡 Hint

Think about simple and clear ways to add parameters conditionally.

Building the parameter list conditionally before decorating is the simplest and most readable approach.

🧠 Conceptual

expert

Understanding conditional parametrize impact on test count

If you have this pytest code:

import pytest

@pytest.mark.parametrize("a", [1, 2])
@pytest.mark.parametrize("b", [3, 4] if False else [5])
def test_combined(a, b):
    assert a < 10 and b < 10

How many tests will pytest run?

PyTest

import pytest

@pytest.mark.parametrize("a", [1, 2])
@pytest.mark.parametrize("b", [3, 4] if False else [5])
def test_combined(a, b):
    assert a < 10 and b < 10

A 4 tests

B 2 tests

C 1 test

D 3 tests

💡 Hint

Remember how multiple parametrize decorators combine parameters.

The outer parametrize with b has only one value [5]. The inner parametrize with a has two values [1,2]. Total tests = 2 * 1 = 2.