Checking exception attributes in PyTest - Build an Automation Script

Verify exception type and message when dividing by zero

Preconditions (2)

Step 1: Write a function that divides two numbers

Step 2: Call the function with divisor as zero inside a test

Step 3: Catch the exception using pytest.raises

Step 4: Check that the exception type is ZeroDivisionError

Step 5: Check that the exception message contains 'division by zero'

✅ Expected Result: Test passes confirming the exception type and message are as expected

Automation Requirements - pytest

Assertions Needed:

Assert exception type is ZeroDivisionError

Assert exception message contains 'division by zero'

Best Practices:

Use pytest.raises context manager to catch exceptions

Use 'as exc_info' to access exception attributes

Assert exception message with 'in' operator for flexibility

Automated Solution

PyTest

import pytest

def divide(a: float, b: float) -> float:
    return a / b

def test_divide_by_zero_raises():
    with pytest.raises(ZeroDivisionError) as exc_info:
        divide(10, 0)
    assert 'division by zero' in str(exc_info.value)

This test defines a simple divide function that divides two numbers.

In the test test_divide_by_zero_raises, we use pytest.raises as a context manager to catch the ZeroDivisionError when dividing by zero.

The as exc_info part captures the exception details, allowing us to check the exception message.

We assert that the string 'division by zero' is part of the exception message to confirm the error is as expected.

Common Mistakes - 3 Pitfalls

Not using pytest.raises and instead using try-except manually

{'mistake': 'Asserting exact exception message equality', 'why_bad': 'Exception messages can vary slightly between Python versions or environments, causing fragile tests.', 'correct_approach': "Use 'in' operator to check if expected text is contained in the exception message."}

{'mistake': 'Not accessing exception message via exc_info.value', 'why_bad': 'Without exc_info, you cannot check exception attributes properly.', 'correct_approach': "Use 'as exc_info' in pytest.raises to access exception object and its message."}

Bonus Challenge

Now add data-driven testing to check exception messages for dividing by zero with different numerators

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