Single parameter in PyTest - Test Execution Trace

Test Overview

This test checks if a function correctly doubles a single input number. It verifies the output matches the expected doubled value.

Test Code - pytest

PyTest

import pytest

def double(x):
    return x * 2

@pytest.mark.parametrize('input_value, expected', [
    (2, 4),
    (5, 10),
    (0, 0),
    (-3, -6)
])
def test_double(input_value, expected):
    result = double(input_value)
    assert result == expected, f"Expected {expected} but got {result}"

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	—	PASS
2	pytest collects test_double with 4 parameter sets	Test cases ready to run with inputs (2,4), (5,10), (0,0), (-3,-6)	—	PASS
3	Runs test_double with input_value=2, expected=4	Function double called with 2	Check if double(2) == 4	PASS
4	Runs test_double with input_value=5, expected=10	Function double called with 5	Check if double(5) == 10	PASS
5	Runs test_double with input_value=0, expected=0	Function double called with 0	Check if double(0) == 0	PASS
6	Runs test_double with input_value=-3, expected=-6	Function double called with -3	Check if double(-3) == -6	PASS
7	All parameterized tests completed	All assertions passed	—	PASS

Failure Scenario

Failing Condition: The double function returns incorrect value for any input

Execution Trace Quiz - 3 Questions

What does the test_double function verify?

A That the double function returns input squared

B That the double function returns input plus 2

C That the double function returns input multiplied by 2

D That the double function returns input divided by 2

Key Result

Using parameterized tests helps check multiple input cases efficiently with one test function.

Practice

1. What is the purpose of using a single parameter in a pytest test function?

easy

A. To define multiple test functions at once

B. To make the test function run only once

C. To skip the test function during execution

D. To pass different values to the test function for repeated testing

Solution

1. Step 1: Understand pytest parameterization

   Pytest allows passing parameters to test functions to run the same test with different inputs.

2. Step 2: Role of a single parameter

   A single parameter lets you supply multiple values to the test, so it runs multiple times with each value.

3. Final Answer:

   To pass different values to the test function for repeated testing -> Option D

4. Quick Check:

   Single parameter = multiple test runs [OK]

Hint: Single parameter means testing with different inputs [OK]

Common Mistakes:

• Thinking single parameter defines multiple tests
• Confusing parameter with skipping tests
• Assuming single parameter runs test once only

2. Which of the following is the correct syntax to use a single parameter with pytest's @pytest.mark.parametrize decorator?

easy

A. @pytest.mark.parametrize('num', 1, 2, 3)

B. @pytest.mark.parametrize(num, [1, 2, 3])

C. @pytest.mark.parametrize('num', [1, 2, 3])

D. @pytest.mark.parametrize(['num'], 1, 2, 3)

Solution

1. Step 1: Check parameter name format

   The parameter name must be a string inside quotes, e.g., 'num'.

2. Step 2: Check values format

   The values must be inside a list or iterable, e.g., [1, 2, 3].

3. Final Answer:

   @pytest.mark.parametrize('num', [1, 2, 3]) -> Option C

4. Quick Check:

   Parametrize syntax = string + list [OK]

Hint: Parameter name in quotes, values in list brackets [OK]

Common Mistakes:

• Not quoting the parameter name
• Passing values without list brackets
• Using wrong argument structure

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

import pytest

@pytest.mark.parametrize('x', [2, 4])
def test_double(x):
    assert x * 2 == 4

medium

A. First test passes, second test fails

B. Both tests fail

C. First test fails, second test passes

D. Both tests pass

Solution

1. Step 1: Evaluate first test with x=2

   2 * 2 = 4, assertion is true, test passes.

2. Step 2: Evaluate second test with x=4

   4 * 2 = 8, assertion 8 == 4 is false, test fails.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   2*2=4 pass, 4*2=8 fail [OK]

Hint: Check each input separately for assertion [OK]

Common Mistakes:

• Assuming all tests pass without checking values
• Mixing up which input causes failure
• Ignoring assertion logic

4. Identify the error in this pytest code using a single parameter:

import pytest

@pytest.mark.parametrize('val', 1, 2, 3)
def test_positive(val):
    assert val > 0

medium

A. Parameter name should not be in quotes

B. Missing list brackets around parameter values

C. Test function must have two parameters

D. Assertion should use == instead of >

Solution

1. Step 1: Check parametrize values format

   Values must be inside a list or iterable, e.g., [1, 2, 3].

2. Step 2: Identify missing brackets

   Code passes values as separate arguments instead of a single list, causing syntax error.

3. Final Answer:

   Missing list brackets around parameter values -> Option B

4. Quick Check:

   Values need list brackets [OK]

Hint: Values must be in a list or tuple [OK]

Common Mistakes:

• Passing values without brackets
• Removing quotes from parameter name
• Wrong assertion operator

5. Given this test function, how can you modify it to test multiple string inputs using a single parameter?

def test_is_uppercase(s):
    assert s.isupper()

hard

A. Add @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) above the function

B. Change function to accept two parameters s and t

C. Use a for loop inside the test to iterate over strings

D. Call test_is_uppercase multiple times manually

Solution

1. Step 1: Understand parameterizing tests

   Using @pytest.mark.parametrize allows running the test multiple times with different inputs.

2. Step 2: Apply parametrize decorator with string list

   Adding @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) runs test_is_uppercase twice with each string.

3. Final Answer:

   Add @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) above the function -> Option A

4. Quick Check:

   Parametrize decorator runs test with multiple inputs [OK]

Hint: Use @pytest.mark.parametrize with list of inputs [OK]

Common Mistakes:

• Trying to pass multiple parameters instead of one
• Using loops inside test instead of parametrize
• Calling test manually instead of using pytest features