Conditional parametrize in PyTest - Build an Automation Script

Test addition function with conditional parameters

Preconditions (2)

Step 1: Create a test function that adds two numbers

Step 2: Use pytest parametrize to test multiple input pairs

Step 3: Add a condition to skip some parameters based on a flag

Step 4: Run the tests

Step 5: Verify that only the parameters meeting the condition are tested

✅ Expected Result: Tests run only for the parameters that meet the condition, others are skipped or not run

Automation Requirements - pytest

Assertions Needed:

Verify the addition result is correct for each parameter set

Verify tests are skipped or not run for parameters failing the condition

Best Practices:

Use @pytest.mark.parametrize with a conditional filter

Use clear test names for each parameter set

Avoid hardcoding test data inside the test function

Use pytest skip markers or conditional logic inside parametrize

Automated Solution

PyTest

import pytest

# Flag to control which parameters to test
run_extended_tests = False

# Define test data with a condition
params = [
    (1, 2, 3, True),    # (a, b, expected, run_test)
    (3, 5, 8, True),
    (10, 20, 30, False),
    (0, 0, 0, True)
]

# Filter parameters based on the flag
filtered_params = [ (a, b, expected) for a, b, expected, run_test in params if run_test or run_extended_tests ]

@pytest.mark.parametrize("a,b,expected", filtered_params)
def test_addition(a, b, expected):
    assert a + b == expected

This test script uses pytest.mark.parametrize to run the test_addition function with multiple sets of inputs.

The params list includes a boolean flag run_test to indicate if the test case should run by default.

The filtered_params list comprehension filters the parameters based on the run_extended_tests flag. If run_extended_tests is False, only parameters with run_test=True are included.

This way, you can control which test cases run conditionally without changing the test code.

The assertion checks that the sum of a and b equals the expected result.

Common Mistakes - 3 Pitfalls

Hardcoding all parameters without condition

Using if statements inside the test function to skip tests

Not providing descriptive test ids for parameters

Bonus Challenge

Add data-driven testing with 3 different input sets and a condition to run only even sums

Show Hint

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