Conditional parametrize in PyTest - Test Execution Trace

Test Overview

This test uses pytest's parametrize decorator with a condition to run different inputs. It verifies the function returns expected results only for allowed inputs.

Test Code - pytest

PyTest

import pytest

def is_even(n):
    return n % 2 == 0

# Parametrize only even numbers
@pytest.mark.parametrize('num', [n for n in [2, 3, 4, 5] if n % 2 == 0])
def test_is_even(num):
    assert is_even(num) is True

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	—	PASS
2	pytest collects test_is_even with parametrize values filtered to even numbers [2, 4]	Test cases ready for num=2 and num=4	—	PASS
3	Test runs test_is_even with num=2	Function is_even called with 2	assert is_even(2) is True	PASS
4	Test runs test_is_even with num=4	Function is_even called with 4	assert is_even(4) is True	PASS
5	Test ends with all parameterized cases passed	Test report shows 2 passed tests	—	PASS

Failure Scenario

Failing Condition: If the test runs with an odd number like 3 or 5, assertion fails

Execution Trace Quiz - 3 Questions

What values does the test actually run with?

A [2, 4]

B [2, 3, 4, 5]

C [3, 5]

D []

Key Result

Use conditional filtering in parametrize to run tests only on relevant inputs, avoiding unnecessary failures.

Practice

1. What is the main purpose of using pytest.param with skipif inside @pytest.mark.parametrize?

easy

A. To mark tests as expected failures

B. To run all test cases regardless of conditions

C. To conditionally skip specific test cases based on environment or conditions

D. To parametrize tests without any conditions

Solution

1. Step 1: Understand pytest.param with skipif

   This syntax allows marking individual parameters to be skipped if a condition is true.

2. Step 2: Purpose of conditional skipping

   This helps run only relevant tests in certain environments, skipping others automatically.

3. Final Answer:

   To conditionally skip specific test cases based on environment or conditions -> Option C

4. Quick Check:

   Conditional skipping = skipif with pytest.param [OK]

Hint: Use pytest.param with skipif to skip tests conditionally [OK]

Common Mistakes:

• Thinking skipif skips all tests, not just specific params
• Confusing skipif with xfail
• Not using pytest.param inside parametrize

2. Which of the following is the correct syntax to skip a parameter when Python version is less than 3.8 using pytest.mark.parametrize?

easy

A. @pytest.mark.parametrize('x', [pytest.param(1, marks=pytest.mark.skipif(sys.version_info < (3,8), reason='Old Python'))])

B. @pytest.mark.parametrize('x', [pytest.param(1, skipif=sys.version_info < (3,8))])

C. @pytest.mark.parametrize('x', [pytest.param(1, marks=skipif(sys.version_info < (3,8)))])

D. @pytest.mark.parametrize('x', [pytest.param(1, marks=pytest.skipif(sys.version_info < (3,8)))])

Solution

1. Step 1: Identify correct usage of skipif mark

   The skipif mark must be used inside pytest.mark.skipif with a reason string.

2. Step 2: Check syntax correctness

   @pytest.mark.parametrize('x', [pytest.param(1, marks=pytest.mark.skipif(sys.version_info < (3,8), reason='Old Python'))]) correctly uses pytest.mark.skipif with condition and reason inside marks parameter.

3. Final Answer:

   @pytest.mark.parametrize('x', [pytest.param(1, marks=pytest.mark.skipif(sys.version_info < (3,8), reason='Old Python'))]) -> Option A

4. Quick Check:

   Use pytest.mark.skipif with marks and reason [OK]

Hint: Use marks=pytest.mark.skipif(condition, reason='') inside pytest.param [OK]

Common Mistakes:

• Using skipif without pytest.mark
• Omitting the reason argument
• Passing skipif as a direct argument instead of marks

3. Given the code below, what will be the output when running pytest on Python 3.7?

import sys
import pytest

@pytest.mark.parametrize('num', [
    pytest.param(1, marks=pytest.mark.skipif(sys.version_info < (3,8), reason='Skip on old Python')),
    2,
    3
])
def test_numbers(num):
    assert num > 0

medium

A. All three tests run and pass

B. Test with num=1 is skipped; tests with num=2 and num=3 run and pass

C. All tests are skipped

D. Test with num=1 fails due to skipif syntax error

Solution

1. Step 1: Analyze skipif condition with Python 3.7

   sys.version_info < (3,8) is True for Python 3.7, so param with num=1 is skipped.

2. Step 2: Check other parameters

   Parameters 2 and 3 have no skip marks, so their tests run and pass the assertion.

3. Final Answer:

   Test with num=1 is skipped; tests with num=2 and num=3 run and pass -> Option B

4. Quick Check:

   skipif True skips param; others run [OK]

Hint: Check skipif condition against environment to predict skipped tests [OK]

Common Mistakes:

• Assuming all tests run regardless of skipif
• Confusing skipif with xfail
• Thinking skipif causes syntax error

4. Identify the error in the following code snippet that uses conditional parametrize:

import pytest

@pytest.mark.parametrize('val', [
    pytest.param(10, marks=pytest.mark.skipif('sys.version_info < (3,8)', reason='Old Python')),
    20
])
def test_val(val):
    assert val > 0

medium

A. The skipif condition is a string instead of a boolean expression

B. Missing import of sys module

C. pytest.param cannot be used inside parametrize

D. The reason argument is missing in skipif

Solution

1. Step 1: Check skipif condition type

   The condition is passed as a string, but skipif expects a boolean expression, not a string.

2. Step 2: Confirm other parts

   sys module is not imported, but the main error is the string condition. pytest.param usage and reason argument are correct.

3. Final Answer:

   The skipif condition is a string instead of a boolean expression -> Option A

4. Quick Check:

   skipif condition must be boolean, not string [OK]

Hint: skipif condition must be a boolean expression, not a string [OK]

Common Mistakes:

• Passing condition as string instead of boolean
• Ignoring missing sys import (not main error here)
• Misusing pytest.param outside parametrize

5. You want to run a test with parameters mode set to 'fast', 'slow', and 'medium'. However, you want to skip the 'slow' mode test only if an environment variable SKIP_SLOW is set to '1'. Which is the correct way to write this conditional parametrize in pytest?

hard

A. import os
@pytest.mark.parametrize('mode', [
'fast',
pytest.param('slow', marks=pytest.mark.skipif('os.getenv("SKIP_SLOW") == "1"')),
'medium'
])

B. import os
@pytest.mark.parametrize('mode', [
'fast',
pytest.param('slow', skipif=os.getenv('SKIP_SLOW') == '1'),
'medium'
])

C. import os
@pytest.mark.parametrize('mode', [
'fast',
pytest.param('slow', marks=pytest.skipif(os.getenv('SKIP_SLOW') == '1')),
'medium'
])

D. import os
@pytest.mark.parametrize('mode', [
'fast',
pytest.param('slow', marks=pytest.mark.skipif(os.getenv('SKIP_SLOW') == '1', reason='Skip slow mode')),
'medium'
])

Solution

1. Step 1: Use os.getenv to check environment variable

   os.getenv('SKIP_SLOW') returns the value of the environment variable as a string.

2. Step 2: Apply skipif mark correctly

   Use pytest.mark.skipif with a boolean condition and a reason inside marks parameter of pytest.param.

3. Step 3: Verify syntax correctness

   import os @pytest.mark.parametrize('mode', [ 'fast', pytest.param('slow', marks=pytest.mark.skipif(os.getenv('SKIP_SLOW') == '1', reason='Skip slow mode')), 'medium' ]) correctly uses marks=pytest.mark.skipif(condition, reason='...') and imports os.

4. Final Answer:

   import os @pytest.mark.parametrize('mode', [ 'fast', pytest.param('slow', marks=pytest.mark.skipif(os.getenv('SKIP_SLOW') == '1', reason='Skip slow mode')), 'medium' ]) -> Option D

5. Quick Check:

   Use marks=pytest.mark.skipif with env check and reason [OK]

Hint: Use os.getenv and marks=pytest.mark.skipif with reason to skip params [OK]

Common Mistakes:

• Passing skipif as direct argument instead of marks
• Using string condition instead of boolean
• Omitting reason in skipif mark