@pytest.mark.parametrize decorator - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple parametrize test

What will be the output when running this pytest test with @pytest.mark.parametrize?

PyTest

import pytest

@pytest.mark.parametrize('x,y', [(1,2), (3,4)])
def test_sum(x, y):
    assert x + y == 3

A One test passes, one test fails

B Both tests pass

C Both tests fail

D SyntaxError due to parametrize usage

💡 Hint

Check the sum of each pair and compare with 3

The first pair (1,2) sums to 3, so the assertion passes. The second pair (3,4) sums to 7, so the assertion fails.

❓ assertion

intermediate

Correct assertion for parametrize test

Which assertion correctly tests that the product of x and y equals 6 in this parametrize test?

PyTest

import pytest

@pytest.mark.parametrize('x,y', [(1,6), (2,3), (3,2)])
def test_product(x, y):
    # Choose the correct assertion below

A assert x * y == 6

B assert x + y == 6

C assert x - y == 6

D assert x / y == 6

💡 Hint

Product means multiplication

The test checks if multiplying x and y equals 6. Only option A uses multiplication correctly.

🔧 Debug

advanced

Identify the error in parametrize usage

What error will this pytest code raise when run?

PyTest

import pytest

@pytest.mark.parametrize('x,y', [(1, 2), (3, 4)])
def test_values(x, y):
    assert x < y

A TypeError: cannot unpack non-iterable int object

B No error, test runs successfully

C AssertionError: test fails

D SyntaxError: invalid syntax in parametrize decorator

💡 Hint

Check the format of the list passed to parametrize

The list should contain tuples of two values, but here it contains integers, so unpacking fails.

🧠 Conceptual

advanced

Purpose of @pytest.mark.parametrize

What is the main purpose of using @pytest.mark.parametrize in pytest tests?

A To mark tests as expected to fail

B To skip tests conditionally based on input

C To run the same test function multiple times with different input values

D To group multiple tests into one test suite

💡 Hint

Think about repeating tests with different data

Parametrize allows running one test function many times with different inputs to check multiple cases easily.

❓ framework

expert

Combining parametrize with fixtures

Given this pytest code, what will be the output when running the test?

PyTest

import pytest

@pytest.fixture
def base_value():
    return 10

@pytest.mark.parametrize('increment', [1, 2])
def test_increment(base_value, increment):
    assert base_value + increment in [11, 12]

A Fixture not found error

B First test passes, second test fails

C Both tests fail

D Both tests pass

💡 Hint

Check the sum of base_value and increment for each param

The fixture returns 10. Adding increments 1 and 2 gives 11 and 12, both in the allowed list, so both tests pass.

Practice

1. What is the main purpose of the @pytest.mark.parametrize decorator in pytest?

easy

A. To skip a test function during test execution

B. To run the same test function multiple times with different input values

C. To mark a test as expected to fail

D. To group multiple test functions into a test suite

Solution

1. Step 1: Understand the decorator's role

   @pytest.mark.parametrize allows running one test function multiple times with different inputs.

2. Step 2: Compare options with this role

   Only To run the same test function multiple times with different input values describes running the same test with different inputs, matching the decorator's purpose.

3. Final Answer:

   To run the same test function multiple times with different input values -> Option B

4. Quick Check:

   Parametrize = multiple inputs [OK]

Hint: Parametrize means run test many times with different data [OK]

Common Mistakes:

• Confusing parametrize with skipping tests
• Thinking it groups tests instead of repeating one
• Mixing it up with marking expected failures

2. Which of the following is the correct syntax to use @pytest.mark.parametrize for testing a function with inputs 1 and 2?

easy

A. @pytest.mark.parametrized('input', [1, 2])

B. @pytest.parametrize('input', (1, 2))

C. @pytest.mark.parametrize(input, [1, 2])

D. @pytest.mark.parametrize('input', [1, 2])

Solution

1. Step 1: Check correct decorator name and syntax

   The correct decorator is @pytest.mark.parametrize with parameters as strings and list of values.

2. Step 2: Validate each option

   @pytest.mark.parametrize('input', [1, 2]) uses correct spelling, quotes around parameter name, and list syntax. Others have syntax errors or wrong names.

3. Final Answer:

   @pytest.mark.parametrize('input', [1, 2]) -> Option D

4. Quick Check:

   Correct spelling and syntax = @pytest.mark.parametrize('input', [1, 2]) [OK]

Hint: Decorator name is mark.parametrize with string param name [OK]

Common Mistakes:

• Misspelling 'parametrize' as 'parametrized'
• Forgetting quotes around parameter name
• Using wrong brackets or missing @ symbol

3. What will be the output when running this pytest code?

import pytest

@pytest.mark.parametrize('x,y', [(1,2), (3,4)])
def test_sum(x, y):
    assert x + y == 3

medium

A. First test passes, second test fails

B. Both tests fail

C. Both tests pass

D. Syntax error during test collection

Solution

1. Step 1: Analyze test cases and assertion

   Test runs twice: (1,2) and (3,4). Assertion checks if sum equals 3.

2. Step 2: Evaluate each case

   1+2=3 passes; 3+4=7 fails assertion.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   Sum check true only for (1,2) [OK]

Hint: Check each input pair sum against assertion [OK]

Common Mistakes:

• Assuming all tests pass without checking values
• Confusing tuple unpacking in parametrize
• Ignoring assertion logic

4. Identify the error in this pytest code using @pytest.mark.parametrize:

@pytest.mark.parametrize('num', [1, 2, 3])
def test_even():
    assert num % 2 == 0

medium

A. Wrong decorator name

B. List of values should be a tuple

C. Missing parameter in test function definition

D. Assertion logic is incorrect

Solution

1. Step 1: Check function parameters

   Test function must accept the parameter named in parametrize ('num'), but test_even() has no parameters.

2. Step 2: Confirm other parts

   Decorator name and list syntax are correct; assertion logic is valid for even check.

3. Final Answer:

   Missing parameter in test function definition -> Option C

4. Quick Check:

   Function must accept parametrize variables [OK]

Hint: Test function needs parameters matching parametrize names [OK]

Common Mistakes:

• Forgetting to add parameters to test function
• Thinking list must be tuple
• Confusing assertion logic with syntax error

5. You want to test a function is_valid_age(age) that returns True if age is between 18 and 65 inclusive. Which @pytest.mark.parametrize usage correctly tests ages 17, 18, 30, 65, and 70 with expected results?

hard

A. @pytest.mark.parametrize(['age', 'expected'], [(17, False), (18, True), (30, True), (65, True), (70, False)])

B. @pytest.mark.parametrize('age', [17, 18, 30, 65, 70], 'expected', [False, True, True, True, False])

C. @pytest.mark.parametrize('age,expected', {17: False, 18: True, 30: True, 65: True, 70: False})

D. @pytest.mark.parametrize(age,expected, [(17, False), (18, True), (30, True), (65, True), (70, False)])

Solution

1. Step 1: Understand parametrize syntax for multiple parameters

   When passing multiple parameters, use a string with comma-separated names or a list of strings as first argument, and a list of tuples as second.

2. Step 2: Evaluate each option

   A: @pytest.mark.parametrize(age,expected, [(17, False), (18, True), (30, True), (65, True), (70, False)]) misses quotes around parameter names, causing error. B has wrong syntax with multiple arguments. C uses a dict, which is invalid. D: @pytest.mark.parametrize(['age', 'expected'], [(17, False), (18, True), (30, True), (65, True), (70, False)]) uses list of strings correctly.

3. Step 3: Choose best practice

   @pytest.mark.parametrize(['age', 'expected'], [(17, False), (18, True), (30, True), (65, True), (70, False)]) is correct and clear syntax for multiple parameters.

4. Final Answer:

   @pytest.mark.parametrize(['age', 'expected'], [(17, False), (18, True), (30, True), (65, True), (70, False)]) -> Option A

5. Quick Check:

   List of param names + list of tuples = @pytest.mark.parametrize(['age', 'expected'], [(17, False), (18, True), (30, True), (65, True), (70, False)]) [OK]

Hint: Use list of param names and list of tuples for multiple inputs [OK]

Common Mistakes:

• Passing multiple parameter lists separately
• Using dict instead of list of tuples
• Confusing string vs list for parameter names