Parametrize with IDs in PyTest - Build an Automation Script

Test addition function with multiple inputs using pytest parametrize with IDs

Preconditions (2)

Step 1: Create a test function that adds two numbers

Step 2: Use pytest.mark.parametrize to run the test with multiple input pairs

Step 3: Assign custom IDs to each input set for better test report readability

Step 4: Run the tests using pytest

Step 5: Observe that each test case is identified by its custom ID in the test report

✅ Expected Result: All test cases pass and the test report shows each test case with its custom ID

Automation Requirements - pytest

Assertions Needed:

Assert that the sum of two numbers equals the expected result

Best Practices:

Use @pytest.mark.parametrize decorator with ids parameter for clarity

Keep test function simple and focused

Use descriptive IDs for each test case

Write clear assertion messages

Automated Solution

PyTest

import pytest

@pytest.mark.parametrize(
    "a, b, expected",
    [
        (1, 2, 3),
        (5, 5, 10),
        (10, 0, 10)
    ],
    ids=["one_plus_two", "five_plus_five", "ten_plus_zero"]
)
def test_addition(a, b, expected):
    result = a + b
    assert result == expected, f"Expected {a} + {b} to be {expected}, but got {result}"

This test uses pytest.mark.parametrize to run the test_addition function three times with different inputs.

The ids parameter assigns friendly names to each test case. This helps when reading test reports because you see names like one_plus_two instead of just numbers.

The assertion checks if the sum is correct and gives a clear message if it fails.

This approach keeps tests simple, readable, and easy to maintain.

Common Mistakes - 3 Pitfalls

Not using the ids parameter in parametrize

Using complex logic inside the test function

Hardcoding expected results inside the test function instead of parametrize

Bonus Challenge

Now add data-driven testing with 3 different inputs including negative numbers and zero, each with descriptive IDs

Show Hint

Practice

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

easy

A. To change the order of test execution

B. To skip certain test cases during execution

C. To label each test case for easier identification in reports

D. To automatically generate test data

Solution

1. Step 1: Understand the role of ids in parametrize

   The ids parameter assigns names to each test case, making it easier to identify which input caused a failure.

2. Step 2: Compare with other options

   Skipping tests, changing order, or generating data are not functions of ids.

3. Final Answer:

   To label each test case for easier identification in reports -> Option C

4. Quick Check:

   Parametrize ids = labels [OK]

Hint: IDs name test cases for clear failure reports [OK]

Common Mistakes:

• Thinking IDs skip tests
• Believing IDs reorder tests
• Assuming IDs generate data

2. Which of the following is the correct syntax to add IDs to a parametrize decorator in pytest?

easy

A. @pytest.mark.parametrize('x,y', [(1,2), (3,4)], ids=['first', 'second'])

B. @pytest.mark.parametrize('x,y', [(1,2), (3,4)], id=['first', 'second'])

C. @pytest.mark.parametrize('x,y', [(1,2), (3,4)], names=['first', 'second'])

D. @pytest.mark.parametrize('x,y', [(1,2), (3,4)], labels=['first', 'second'])

Solution

1. Step 1: Identify the correct parameter name

   The correct parameter to assign IDs is ids, not id, names, or labels.

2. Step 2: Check the syntax structure

   The syntax uses @pytest.mark.parametrize with the argument names, list of tuples, and ids as a list of strings.

3. Final Answer:

   @pytest.mark.parametrize('x,y', [(1,2), (3,4)], ids=['first', 'second']) -> Option A

4. Quick Check:

   Use ids parameter correctly [OK]

Hint: Use 'ids' keyword exactly for naming test cases [OK]

Common Mistakes:

• Using 'id' instead of 'ids'
• Using 'names' or 'labels' which are invalid
• Forgetting to pass a list for IDs

3. Given the code below, what will be the test ID shown for the second test case?

@pytest.mark.parametrize('num', [10, 20], ids=['ten', 'twenty'])
def test_value(num):
    assert num > 5

medium

A. test_value[twenty]

B. test_value[10]

C. test_value[1]

D. test_value[20]

Solution

1. Step 1: Understand how IDs map to test cases

   The ids list assigns 'ten' to the first input (10) and 'twenty' to the second input (20).

2. Step 2: Identify the second test case ID

   The second test case uses the second ID, which is 'twenty'.

3. Final Answer:

   test_value[twenty] -> Option A

4. Quick Check:

   Second ID label = 'twenty' [OK]

Hint: IDs replace values in test names [OK]

Common Mistakes:

• Assuming default numeric index is used
• Confusing value with ID label
• Ignoring the IDs parameter

4. Identify the error in the following pytest parametrize usage:

@pytest.mark.parametrize('a,b', [(1,2), (3,4)], ids='case1,case2')
def test_sum(a, b):
    assert a + b > 0

medium

A. Tuple values must be strings

B. Parameter names must be a list, not a string

C. Test function must return a value

D. IDs should be a list, not a single string

Solution

1. Step 1: Check the type of ids argument

   The ids parameter expects a list of strings, but here it is given a single string with commas.

2. Step 2: Understand the impact of wrong ids type

   Passing a string instead of a list causes pytest to treat it as an iterable of characters, leading to incorrect IDs or errors.

3. Final Answer:

   IDs should be a list, not a single string -> Option D

4. Quick Check:

   IDs must be list of strings [OK]

Hint: Pass IDs as list, not comma string [OK]

Common Mistakes:

• Passing comma-separated string instead of list
• Using wrong parameter type for IDs
• Confusing parameter names with IDs

5. You want to parametrize a test with inputs [(0, 'zero'), (1, 'one'), (2, 'two')] but want to skip the test case where the number is zero using IDs. Which approach correctly uses ids to help identify the skipped test case?

hard

A. @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=lambda val: f"skip_{val[0]}" if val[0]==0 else f"num_{val[0]}")

B. @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=['skip_zero', 'num_one', 'num_two'])

C. @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=['zero', 'one', 'two'])

D. @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=['num_0', 'num_1', 'num_2'])

Solution

1. Step 1: Understand the goal of IDs for skipping

   We want to label the zero case clearly as 'skip_zero' to identify it easily in reports.

2. Step 2: Evaluate each option's IDs usage

   @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=['skip_zero', 'num_one', 'num_two']) uses a list of strings with 'skip_zero' for zero, which is clear and correct. The lambda in @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=lambda val: f"skip_{val[0]}" if val[0]==0 else f"num_{val[0]}") is corrected to accept a single tuple argument as pytest passes. The options with ids=['num_0', 'num_1', 'num_2'] and ids=['zero', 'one', 'two'] do not label zero as skipped.

3. Final Answer:

   @pytest.mark.parametrize('num,word', [(0,'zero'),(1,'one'),(2,'two')], ids=['skip_zero', 'num_one', 'num_two']) -> Option B

4. Quick Check:

   Use explicit ID list to mark skipped case [OK]

Hint: Use explicit ID list to label skipped cases [OK]

Common Mistakes:

• Using lambda incorrectly for IDs
• Not labeling skipped cases clearly
• Using generic IDs without skip info