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

Show Hint

Practice

1. What is the correct way to capture an exception and check its message using pytest.raises?

easy

A. Use pytest.raises(ValueError) as exc_info: outside a with block.

B. Use pytest.raises(ValueError) without as exc_info and check exc_info.message.

C. Use with pytest.raises() as exc_info: and check exc_info.error.

D. Use with pytest.raises(ValueError) as exc_info: and then check exc_info.value.

Solution

1. Step 1: Use pytest.raises with as exc_info

   This syntax captures the exception info object for inspection.

2. Step 2: Access the exception instance via exc_info.value

   The actual exception object is stored in exc_info.value, allowing attribute checks like the message.

3. Final Answer:

   Use with pytest.raises(ValueError) as exc_info: and then check exc_info.value. -> Option D

4. Quick Check:

   Exception captured = exc_info.value [OK]

Hint: Always use 'as exc_info' to access exception details [OK]

Common Mistakes:

• Not using 'as exc_info' to capture exception
• Trying to access exc_info.message instead of exc_info.value
• Using pytest.raises outside a with block

2. Which of the following is the correct syntax to check the exception message text inside a pytest test?

easy

A. assert exc_info.message == 'error occurred'

B. assert exc_info.value.message == 'error occurred'

C. assert str(exc_info.value) == 'error occurred'

D. assert exc_info.error == 'error occurred'

Solution

1. Step 1: Access exception instance via exc_info.value

   The exception object is stored in exc_info.value.

2. Step 2: Convert exception to string to get message

   Using str(exc_info.value) returns the exception message text.

3. Final Answer:

   assert str(exc_info.value) == 'error occurred' -> Option C

4. Quick Check:

   Exception message = str(exc_info.value) [OK]

Hint: Use str(exc_info.value) to get exception message text [OK]

Common Mistakes:

• Trying to access exc_info.message which does not exist
• Using exc_info.error which is invalid
• Assuming exc_info.value.message always exists

3. Given the code below, what will the assertion check inside the test?

def divide(x, y):
    if y == 0:
        raise ZeroDivisionError('Cannot divide by zero')
    return x / y

def test_divide():
    with pytest.raises(ZeroDivisionError) as exc_info:
        divide(5, 0)
    assert str(exc_info.value) == 'Cannot divide by zero'

medium

A. The test raises an error because exc_info is not defined.

B. The test passes because the exception message matches exactly.

C. The test fails because the exception message is different.

D. The test fails because the exception type is wrong.

Solution

1. Step 1: The function raises ZeroDivisionError with correct message

   Calling divide(5, 0) raises ZeroDivisionError with message 'Cannot divide by zero'.

2. Step 2: The test captures exception and asserts message

   Using pytest.raises captures the exception, and str(exc_info.value) matches the expected message.

3. Final Answer:

   The test passes because the exception message matches exactly. -> Option B

4. Quick Check:

   Exception type and message match = pass [OK]

Hint: Match exception message string exactly with str(exc_info.value) [OK]

Common Mistakes:

• Assuming exception message is accessed by exc_info.message
• Not using 'as exc_info' to capture exception
• Confusing exception type with message

4. Identify the error in the following pytest test code that checks exception attributes:

def test_error():
    with pytest.raises(ValueError) as exc_info:
        raise ValueError('Invalid input')
    assert exc_info.message == 'Invalid input'

medium

A. Using exc_info.message instead of str(exc_info.value) to check message.

B. Not raising the exception inside the with block.

C. Missing parentheses in pytest.raises call.

D. Using as exc_info outside the with block.

Solution

1. Step 1: Check how exception message is accessed

   The exception message is not stored in exc_info.message, but must be accessed via str(exc_info.value).

2. Step 2: Correct assertion syntax

   Replace assert exc_info.message == 'Invalid input' with assert str(exc_info.value) == 'Invalid input'.

3. Final Answer:

   Using exc_info.message instead of str(exc_info.value) to check message. -> Option A

4. Quick Check:

   Exception message accessed via str(exc_info.value) [OK]

Hint: Use str(exc_info.value) to get exception message, not exc_info.message [OK]

Common Mistakes:

• Trying to access exc_info.message which does not exist
• Not raising exception inside with block
• Incorrect pytest.raises syntax

5. You want to test a custom exception MyError that has an attribute code. Which pytest code correctly checks both the exception type and the code attribute value?

hard

A. with pytest.raises(MyError) as exc_info:
raise MyError('fail', code=404)
assert exc_info.value.code == 404

B. with pytest.raises(MyError) as exc_info:
raise MyError('fail', code=404)
assert exc_info.code == 404

C. with pytest.raises(MyError):
raise MyError('fail', code=404)
assert exc_info.value.code == 404

D. with pytest.raises(MyError) as exc_info:
raise MyError('fail', code=404)
assert str(exc_info.value) == 404

Solution

1. Step 1: Capture exception with as exc_info

   This allows access to the exception instance and its attributes.

2. Step 2: Access custom attribute code on exc_info.value

   The exception instance is exc_info.value, so exc_info.value.code accesses the attribute.

3. Final Answer:

   with pytest.raises(MyError) as exc_info: raise MyError('fail', code=404) assert exc_info.value.code == 404 -> Option A

4. Quick Check:

   Custom attribute checked via exc_info.value.code [OK]

Hint: Check custom attributes on exc_info.value, not exc_info directly [OK]

Common Mistakes:

• Trying to access exc_info.code instead of exc_info.value.code
• Not using 'as exc_info' to capture exception
• Comparing str(exc_info.value) to a number