Checking exception attributes in PyTest - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check that the exception message contains the word 'error'.

PyTest

import pytest

def test_error_message():
    with pytest.raises(ValueError) as exc_info:
        raise ValueError('This is an error message')
    assert 'error' in exc_info.value.[1][0]

Drag options to blanks, or click blank then click option'

Aargs

Btext

Cmessage

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'message' attribute which does not exist on the exception object.

Trying to access exc_info.message directly.

The args attribute of an exception contains the message as a tuple. Checking if 'error' is in exc_info.value.args verifies the message.

2fill in blank

medium

Complete the code to assert that the exception's first argument equals 'Invalid input'.

PyTest

import pytest

def test_invalid_input():
    with pytest.raises(ValueError) as exc_info:
        raise ValueError('Invalid input')
    assert exc_info.value.args[[1]] == 'Invalid input'

Drag options to blanks, or click blank then click option'

A-1

B1

C0

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 which is out of range for a single-argument exception.

Using negative index incorrectly.

The first argument of the exception message is at index 0 in the args tuple.

3fill in blank

hard

Fix the error in the code to correctly check the exception message contains 'timeout'.

PyTest

import pytest

def test_timeout_error():
    with pytest.raises(TimeoutError) as exc_info:
        raise TimeoutError('Connection timeout')
    assert 'timeout' in exc_info.value.[1][0]

Drag options to blanks, or click blank then click option'

Aargs

Btext

Cmsg

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent attributes like 'message' or 'msg'.

Trying to access exc_info.message directly.

The exception message is stored in args, not in message or other attributes.

4fill in blank

hard

Fill both blanks to check the exception type and that its message equals 'File not found'.

PyTest

import pytest

def test_file_error():
    with pytest.raises([1]) as exc_info:
        raise FileNotFoundError('File not found')
    assert exc_info.value.args[[2]] == 'File not found'

Drag options to blanks, or click blank then click option'

AFileNotFoundError

BValueError

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong exception type in pytest.raises.

Using index 1 instead of 0 for the message.

The exception type to catch is FileNotFoundError. The message is the first argument at index 0.

5fill in blank

hard

Fill all three blanks to assert the exception type, check the message contains 'denied', and verify the error code attribute equals 403.

PyTest

import pytest

class PermissionErrorWithCode(PermissionError):
    def __init__(self, message, code):
        super().__init__(message)
        self.code = code

def test_permission_error():
    with pytest.raises([1]) as exc_info:
        raise PermissionErrorWithCode('Access denied', 403)
    assert 'denied' in exc_info.value.args[[2]]
    assert exc_info.value.[3] == 403

Drag options to blanks, or click blank then click option'

APermissionErrorWithCode

B0

Ccode

DPermissionError

Attempts:

3 left

💡 Hint

Common Mistakes

Using the base PermissionError instead of the custom subclass.

Checking the message at wrong index.

Trying to access a non-existent attribute instead of 'code'.

The test catches the custom exception PermissionErrorWithCode. The message is the first argument at index 0. The custom attribute code is checked for the error code.

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