Multiple parameters in PyTest

Introduction

Using multiple parameters lets you test many input combinations easily. It saves time and finds more bugs.

You want to check a function with different pairs of inputs.

You need to test how two or more values work together.

You want to avoid writing many similar test functions.

You want clear test reports showing each input combination.

You want to catch errors that happen only with certain input pairs.

Syntax

PyTest

@pytest.mark.parametrize(("param1", "param2"), [(val1, val2), (val3, val4)])
def test_func(param1, param2):
    # test code here

List parameter names as a tuple or string with comma.

Provide a list of tuples with values for each test run.

Examples

Tests sum of pairs (1,2) and (3,4) to be positive.

PyTest

@pytest.mark.parametrize(("a", "b"), [(1, 2), (3, 4)])
def test_sum(a, b):
    assert a + b > 0

Tests difference for three pairs with string parameter names.

PyTest

@pytest.mark.parametrize("x,y", [(0, 0), (5, 10), (-1, 1)])
def test_diff(x, y):
    assert x - y <= 5

Sample Program

This test checks division with multiple pairs. It expects an error when dividing by zero and correct division otherwise.

PyTest

import pytest

@pytest.mark.parametrize(("num", "divisor"), [(10, 2), (15, 3), (9, 0)])
def test_division(num, divisor):
    if divisor == 0:
        with pytest.raises(ZeroDivisionError):
            result = num / divisor
    else:
        result = num / divisor
        assert result * divisor == num

Important Notes

Use descriptive parameter names for clarity.

Each tuple in the list must match the number of parameters.

pytest shows each parameter set as a separate test in reports.

Summary

Multiple parameters let you test many input combinations easily.

Use @pytest.mark.parametrize with tuples of parameter names and value lists.

This helps find bugs and keeps tests simple and clear.

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