Multiple parameters in PyTest - Build an Automation Script

Test addition function with multiple input pairs

Preconditions (1)

Step 1: Call add(1, 2) and verify the result is 3

Step 2: Call add(0, 0) and verify the result is 0

Step 3: Call add(-1, 1) and verify the result is 0

Step 4: Call add(100, 200) and verify the result is 300

✅ Expected Result: The add function returns the correct sum for each input pair

Automation Requirements - pytest

Assertions Needed:

Assert that add(a, b) returns the expected sum for each input pair

Best Practices:

Use @pytest.mark.parametrize to run the test with multiple input sets

Write clear assertion messages

Keep test function simple and focused

Automated Solution

PyTest

import pytest

# Function to test
def add(a, b):
    return a + b

@pytest.mark.parametrize("a,b,expected", [
    (1, 2, 3),
    (0, 0, 0),
    (-1, 1, 0),
    (100, 200, 300)
])
def test_add_multiple_parameters(a, b, expected):
    result = add(a, b)
    assert result == expected, f"Expected add({a}, {b}) to be {expected}, but got {result}"

This test uses @pytest.mark.parametrize to run the test_add_multiple_parameters function multiple times with different inputs.

Each time, it calls the add function with parameters a and b, then checks if the result matches the expected sum.

The assertion includes a message to help understand failures clearly.

Common Mistakes - 3 Pitfalls

Not using @pytest.mark.parametrize and writing separate test functions for each input

Missing assertion messages

Using print statements instead of assertions

Bonus Challenge

Now add data-driven testing with 3 additional input pairs including floats and zeros

Show Hint

Practice

1. What is the main purpose of using @pytest.mark.parametrize with multiple parameters in pytest?

easy

A. To run the same test function with different sets of input values

B. To skip tests conditionally based on input values

C. To group multiple test functions into one

D. To generate test reports automatically

Solution

1. Step 1: Understand the role of @pytest.mark.parametrize

   This decorator allows running a test multiple times with different inputs.

2. Step 2: Recognize the benefit of multiple parameters

   Using multiple parameters lets you test combinations of inputs easily in one test function.

3. Final Answer:

   To run the same test function with different sets of input values -> Option A

4. Quick Check:

   Multiple parameters = multiple input sets [OK]

Hint: Parametrize runs tests repeatedly with different inputs [OK]

Common Mistakes:

• Thinking it skips tests instead of running multiple times
• Confusing it with grouping tests
• Assuming it generates reports automatically

2. Which of the following is the correct syntax to parametrize a test function with two parameters a and b in pytest?

easy

A. @pytest.mark.parametrize((a, b), [(1, 2), (3, 4)])

B. @pytest.mark.parametrize('a, b', [(1, 2), (3, 4)])

C. @pytest.mark.parametrize([a, b], [(1, 2), (3, 4)])

D. @pytest.mark.parametrize('a b', [(1, 2), (3, 4)])

Solution

1. Step 1: Recall the correct parameter names format

   Pytest expects a string with parameter names separated by commas, e.g., 'a, b'.

2. Step 2: Check the value list format

   Values must be a list of tuples matching the parameters, e.g., [(1, 2), (3, 4)].

3. Final Answer:

   @pytest.mark.parametrize('a, b', [(1, 2), (3, 4)]) -> Option B

4. Quick Check:

   Parameter names as string with commas [OK]

Hint: Use a string with comma-separated names for parameters [OK]

Common Mistakes:

• Using tuple or list instead of string for parameter names
• Missing commas between parameter names
• Using space instead of comma to separate names

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

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. First test fails, second test passes

Solution

1. Step 1: Evaluate first test with inputs (1, 2)

   Sum is 1 + 2 = 3, assertion 3 == 3 passes.

2. Step 2: Evaluate second test with inputs (3, 4)

   Sum is 3 + 4 = 7, assertion 7 == 3 fails.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   Check sums against 3 [OK]

Hint: Calculate sums for each tuple and compare [OK]

Common Mistakes:

• Assuming all tests pass without checking values
• Ignoring that assertion compares to fixed number 3
• Confusing which test input causes failure

4. Identify the error in this pytest code using multiple parameters:

import pytest

@pytest.mark.parametrize('a, b', [1, 2, 3])
def test_multiply(a, b):
    assert a * b > 0

medium

A. The test function must return a value

B. The parameter names should be a list, not a string

C. The assertion syntax is incorrect

D. The parameter values should be tuples, not integers

Solution

1. Step 1: Check the parameter values format

   Values must be a list of tuples matching parameters (a, b), but here it's a list of integers.

2. Step 2: Understand why this causes error

   Pytest tries to unpack each integer into two variables, causing a TypeError.

3. Final Answer:

   The parameter values should be tuples, not integers -> Option D

4. Quick Check:

   Values must match parameter count [OK]

Hint: Use tuples for multiple parameters values [OK]

Common Mistakes:

• Passing a flat list instead of list of tuples
• Thinking parameter names must be a list
• Expecting test function to return a value

5. You want to test a function is_even_sum(a, b) that returns True if the sum of a and b is even. Which @pytest.mark.parametrize setup with a single decorator correctly tests all combinations of a from [1, 2] and b from [3, 4] respectively?

hard

A. @pytest.mark.parametrize('a', [1, 2])
@pytest.mark.parametrize('b', [3, 4])

B. @pytest.mark.parametrize('a, b', [(1, 2), (3, 4)])

C. @pytest.mark.parametrize('a, b', [(1, 3), (1, 4), (2, 3), (2, 4)])

D. @pytest.mark.parametrize('a, b', [(1, 3), (2, 4)])

Solution

1. Step 1: Understand the need to test all combinations

   We want to test every pair from a=[1,2] and b=[3,4], so all 4 pairs must be covered.

2. Step 2: Recognize the benefit of multiple parameters

   @pytest.mark.parametrize('a, b', [(1, 3), (1, 4), (2, 3), (2, 4)]) lists all pairs explicitly: (1,3), (1,4), (2,3), (2,4).

3. Final Answer:

   @pytest.mark.parametrize('a, b', [(1, 3), (1, 4), (2, 3), (2, 4)]) -> Option C

4. Quick Check:

   All pairs listed explicitly [OK]

Hint: List all pairs explicitly for full coverage [OK]

Common Mistakes:

• Missing some pairs in the list
• Using separate decorators without knowing cartesian product effect
• Confusing parameter order or values