Checking exception attributes in PyTest - Test Execution Trace

Test Overview

This test checks that a function raises a ValueError with the correct message when given invalid input.

Test Code - pytest

PyTest

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError) as exc_info:
        divide(10, 0)
    assert exc_info.value.args[0] == "Cannot divide by zero"

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test starts	Test runner is ready to execute test_divide_by_zero	—	PASS
2	Calls divide(10, 0)	Function divide is executing with a=10, b=0	—	PASS
3	Raises ValueError with message 'Cannot divide by zero'	Exception is raised inside divide function	—	PASS
4	pytest.raises context captures the ValueError as exc_info	Exception info stored in exc_info variable	—	PASS
5	Assert exc_info.value.args[0] equals 'Cannot divide by zero'	Checking exception message content	Exception message is exactly 'Cannot divide by zero'	PASS
6	Test ends successfully	Test passed with no errors	—	PASS

Failure Scenario

Failing Condition: The divide function does not raise ValueError or raises with wrong message

Execution Trace Quiz - 3 Questions

What does the test check about the exception?

A The exception type and its message content

B Only that an exception is raised, no message check

C That no exception is raised

D The exception stack trace

Key Result

Always check both the type and the message of exceptions to ensure your code fails as expected with clear error information.

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