Checking membership (in, not in) in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of membership test in pytest assertion

What will be the result of running this pytest test function?

PyTest

def test_membership():
    fruits = ['apple', 'banana', 'cherry']
    assert 'banana' in fruits
    assert 'orange' not in fruits

A SyntaxError due to incorrect assert syntax

B Test passes without errors

C AssertionError because 'orange' is in fruits

D AssertionError because 'banana' is not in fruits

💡 Hint

Check if the items are actually in the list and how assert works.

The list contains 'banana' so the first assert passes. 'orange' is not in the list, so the second assert also passes. No errors occur.

❓ assertion

intermediate

Identify the failing assertion

Given the following test code, which assertion will fail when the test runs?

PyTest

def test_colors():
    colors = {'red', 'green', 'blue'}
    assert 'yellow' not in colors
    assert 'green' in colors
    assert 'blue' not in colors
    assert 'red' in colors

A assert 'yellow' not in colors

B assert 'green' in colors

C assert 'blue' not in colors

D assert 'red' in colors

💡 Hint

Check which colors are actually in the set.

'blue' is in the set, so asserting it is not in colors will fail.

🔧 Debug

advanced

Find the error in membership test

Why does this pytest test raise a TypeError?

PyTest

def test_membership_error():
    data = None
    assert 'key' in data

A Because 'in' operator cannot be used with NoneType

B Because 'key' is not a string

C Because assert syntax is incorrect

D Because data is an empty dictionary

💡 Hint

Think about what types support membership tests.

NoneType does not support membership tests with 'in', so Python raises a TypeError.

❓ framework

advanced

Best practice for membership assertion in pytest

Which pytest assertion is the best practice to check that the string 'error' is NOT in the log message variable 'log'?

A assert 'error' not in log

B assert not 'error' in log

C assert 'error' not in log == True

D assert 'error' not in log()

💡 Hint

Consider readability and correctness of Python syntax.

Option A is clear, readable, and correct. Option A is incorrect due to operator precedence (not 'error' evaluates to False, so it checks False in log). Option A is functionally correct but redundant (== True). Option A treats log as a function which may cause error.

🧠 Conceptual

expert

Understanding membership test behavior with custom objects

Consider a class with a custom __contains__ method. Which statement about membership tests using 'in' is true?

PyTest

class Container:
    def __contains__(self, item):
        return item == 'allowed'

c = Container()

A 'allowed' in c returns True, 'denied' in c raises AttributeError

B 'allowed' in c returns False, 'denied' in c returns True

C 'allowed' in c raises TypeError, 'denied' in c returns False

D 'allowed' in c returns True, 'denied' in c returns False

💡 Hint

Look at how __contains__ is implemented and what it returns.

The __contains__ method returns True only if the item is 'allowed'. So membership test returns True for 'allowed' and False otherwise.