Asserting exceptions (pytest.raises) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assert that a ValueError is raised.

PyTest

import pytest

def test_value_error():
    with pytest.[1](ValueError):
        int('abc')

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

Araises

Braises_error

Cassert

Dexpect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'assert' instead of 'raises' causes syntax errors.

Using a wrong function name like 'raises_error' will fail.

The pytest.raises context manager is used to check if the specified exception is raised.

2fill in blank

medium

Complete the code to check the exception message contains 'invalid literal'.

PyTest

import pytest

def test_error_message():
    with pytest.raises(ValueError) as [1]:
        int('abc')
    assert 'invalid literal' in [1].value.args[0]

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

Aexception

Berror

Cexcinfo

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not matching the assertion line causes NameError.

Forgetting to use 'as' to capture exception info.

The variable after as captures the exception info object, commonly named excinfo.

3fill in blank

hard

Fix the error in the code to properly assert a ZeroDivisionError is raised.

PyTest

import pytest

def test_division():
    with pytest.raises([1]):
        result = 1 / 0

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

AValueError

BZeroDivisionError

CTypeError

DIndexError

Attempts:

3 left

💡 Hint

Common Mistakes

Using ValueError or other exceptions causes the test to fail.

Misspelling the exception name.

The exception raised by dividing by zero is ZeroDivisionError.

4fill in blank

hard

Fill both blanks to assert a KeyError is raised and check its message.

PyTest

import pytest

def test_key_error():
    with pytest.[1](KeyError) as [2]:
        d = {}
        d['missing']

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

Araises

Berror

Cexcinfo

Dexpect

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong context manager name causes syntax errors.

Using a variable name not matching the assertion line.

Use raises as the context manager and excinfo to capture the exception info.

5fill in blank

hard

Fill all three blanks to assert a TypeError is raised and check the exception message contains 'unsupported operand'.

PyTest

import pytest

def test_type_error():
    with pytest.[1](TypeError) as [2]:
        result = 'a' + 1
    assert '[3]' in [2].value.args[0]

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

Araises

Bexcinfo

Cunsupported operand

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong exception type or variable name.

Checking for wrong message text.

Use raises to catch the exception, excinfo to capture it, and check the message contains unsupported operand.

Practice

1. What is the main purpose of using pytest.raises in a test?

easy

A. To ignore exceptions and continue the test

B. To skip a test when an exception occurs

C. To print the exception message during test execution

D. To check if a specific exception is raised by the code inside the with block

Solution

1. Step 1: Understand the role of pytest.raises

   pytest.raises is used to verify that a certain exception is raised by the code inside its with block.

2. Step 2: Compare options with the purpose

   Only To check if a specific exception is raised by the code inside the with block correctly states that it checks for a specific exception. Other options describe unrelated behaviors.

3. Final Answer:

   To check if a specific exception is raised by the code inside the with block -> Option D

4. Quick Check:

   pytest.raises checks exceptions = A [OK]

Hint: Remember: pytest.raises confirms expected errors happen [OK]

Common Mistakes:

• Thinking pytest.raises skips tests
• Assuming it prints exceptions automatically
• Believing it ignores exceptions

2. Which of the following is the correct syntax to assert that a ValueError is raised when calling func() using pytest.raises?

easy

A. pytest.raises(ValueError, func())

B. with pytest.raises(ValueError): func()

C. with pytest.raises('ValueError'): func()

D. pytest.raises(ValueError): func()

Solution

1. Step 1: Recall correct pytest.raises syntax

   The correct way is to use a with statement with pytest.raises(ExceptionType): followed by the code that should raise the exception.

2. Step 2: Evaluate each option

   with pytest.raises(ValueError): func() uses the correct with block and exception type. pytest.raises(ValueError, func()) calls the function inside the argument, which runs before checking. with pytest.raises('ValueError'): func() uses a string instead of the exception class. pytest.raises(ValueError): func() misses the with keyword.

3. Final Answer:

   with pytest.raises(ValueError): func() -> Option B

4. Quick Check:

   Use with + pytest.raises(Exception): code [OK]

Hint: Always use 'with pytest.raises(Exception):' before the code [OK]

Common Mistakes:

• Calling the function inside pytest.raises() arguments
• Using exception name as string instead of class
• Omitting the 'with' keyword

3. What will be the output of the following test code?

import pytest

def test_zero_division():
    with pytest.raises(ZeroDivisionError) as exc_info:
        1 / 0
    assert 'division by zero' in str(exc_info.value)

medium

A. Test passes because ZeroDivisionError is raised and message matches

B. Test fails because exception message is not checked correctly

C. Test fails because ZeroDivisionError is not raised

D. SyntaxError due to incorrect use of pytest.raises

Solution

1. Step 1: Analyze the code behavior

   The code inside the with pytest.raises(ZeroDivisionError) as exc_info: block raises a ZeroDivisionError by dividing 1 by 0.

2. Step 2: Check the assertion

   The assertion verifies that the exception message contains 'division by zero', which is true for this error.

3. Final Answer:

   Test passes because ZeroDivisionError is raised and message matches -> Option A

4. Quick Check:

   Exception raised and message checked = D [OK]

Hint: Use exc_info.value to check exception message text [OK]

Common Mistakes:

• Not using 'as exc_info' to capture exception details
• Checking wrong exception message text
• Expecting no exception raised

4. Identify the error in this test code snippet:

def test_key_error():
    with pytest.raises(KeyError):
        d = {'a': 1}
        value = d['b']
    assert value == 1

medium

A. The variable 'value' is used outside the with block where it may not be assigned

B. The exception type should be ValueError, not KeyError

C. The dictionary key 'b' exists, so no exception is raised

D. pytest.raises cannot be used with dictionary lookups

Solution

1. Step 1: Understand variable scope and exception flow

   The line value = d['b'] raises a KeyError, so value is never assigned inside the with block.

2. Step 2: Identify the error in assertion

   After the with block, the code tries to assert value == 1, but value is undefined because the assignment never happened due to the exception.

3. Final Answer:

   The variable 'value' is used outside the with block where it may not be assigned -> Option A

4. Quick Check:

   Exception stops assignment, so variable undefined after [OK]

Hint: Don't use variables assigned inside exception blocks outside [OK]

Common Mistakes:

• Assuming variable is assigned despite exception
• Confusing exception types
• Thinking pytest.raises can't handle dict key errors

5. You want to test a function parse_int(s) that should raise ValueError with message containing 'invalid literal' when given a non-integer string. Which pytest code correctly asserts this behavior and checks the error message?

hard

A. with pytest.raises(ValueError):
parse_int('abc')
assert 'invalid literal' in e.value

B. pytest.raises(ValueError, parse_int('abc'))
assert 'invalid literal' in e.value

C. with pytest.raises(ValueError) as e:
parse_int('abc')
assert 'invalid literal' in str(e.value)

D. with pytest.raises('ValueError') as e:
parse_int('abc')
assert 'invalid literal' in str(e.value)

Solution

1. Step 1: Use correct syntax to capture exception info

   Use with pytest.raises(ValueError) as e: to catch the exception and store info in e.

2. Step 2: Check error message properly

   After the block, check if the string 'invalid literal' is in str(e.value). This converts the exception to string for message checking.

3. Step 3: Eliminate incorrect options

   with pytest.raises(ValueError): parse_int('abc') assert 'invalid literal' in e.value tries to use e outside the block without capturing it. pytest.raises(ValueError, parse_int('abc')) assert 'invalid literal' in e.value calls the function inside pytest.raises incorrectly and uses e undefined. with pytest.raises('ValueError') as e: parse_int('abc') assert 'invalid literal' in str(e.value) uses exception name as string instead of class.

4. Final Answer:

   with pytest.raises(ValueError) as e: parse_int('abc') assert 'invalid literal' in str(e.value) -> Option C

5. Quick Check:

   Capture exception with 'as' and check message string [OK]

Hint: Use 'as' to capture exception and check message with str(e.value) [OK]

Common Mistakes:

• Using exception name as string instead of class
• Calling function inside pytest.raises arguments
• Accessing exception info outside 'with' without 'as'