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.

Practice

1. Which of the following statements correctly checks if the string 'apple' is in the list fruits = ['apple', 'banana', 'cherry'] using pytest assertions?

easy

A. assert fruits in 'apple'

B. assert 'apple' not in fruits

C. assert 'apple' in fruits

D. assert 'apple' == fruits

Solution

1. Step 1: Understand the membership check

   The in keyword checks if an item exists inside a collection like a list.

2. Step 2: Apply the correct assertion

   To confirm 'apple' is in the list, use assert 'apple' in fruits. This passes if 'apple' is found.

3. Final Answer:

   assert 'apple' in fruits -> Option C

4. Quick Check:

   Use in to check presence [OK]

Hint: Use 'in' to check presence, 'not in' for absence [OK]

Common Mistakes:

• Using 'not in' instead of 'in' when checking presence
• Reversing the order like 'fruits in apple'
• Using equality instead of membership check

2. Which of the following is the correct syntax to assert that the number 5 is NOT in the list numbers = [1, 2, 3, 4] using pytest?

easy

A. assert 5 in numbers

B. assert numbers not in 5

C. assert 5 not numbers

D. assert 5 not in numbers

Solution

1. Step 1: Identify the correct membership operator

   To check that 5 is not in the list, use not in keyword.

2. Step 2: Write the correct assertion syntax

   The correct syntax is assert 5 not in numbers. This passes if 5 is absent.

3. Final Answer:

   assert 5 not in numbers -> Option D

4. Quick Check:

   Use 'not in' to check absence [OK]

Hint: Remember 'not in' checks absence, syntax: assert x not in y [OK]

Common Mistakes:

• Misplacing 'not' like 'assert 5 not numbers' (SyntaxError)
• Swapping operands like 'numbers not in 5' which is invalid
• Using 'in' instead of 'not in' for absence check

3. What will be the result of this pytest assertion?

items = ['pen', 'pencil', 'eraser']
assert 'marker' not in items

medium

A. Test passes because 'marker' is not in the list

B. Test fails because 'marker' is in the list

C. SyntaxError due to wrong assertion

D. Test passes because 'marker' is in the list

Solution

1. Step 1: Check if 'marker' is in the list

   The list contains 'pen', 'pencil', and 'eraser'. 'marker' is not present.

2. Step 2: Evaluate the assertion

   The assertion checks that 'marker' is not in items, which is true, so the test passes.

3. Final Answer:

   Test passes because 'marker' is not in the list -> Option A

4. Quick Check:

   Not in list means assertion passes [OK]

Hint: If item absent and 'not in' used, test passes [OK]

Common Mistakes:

• Assuming test fails if item is missing
• Confusing 'in' and 'not in' logic
• Thinking syntax error occurs with 'not in'

4. Identify the error in this pytest assertion and select the correct fix:

colors = ['red', 'green', 'blue']
assert 'yellow' in colors not

medium

A. Use 'assert not 'yellow' in colors' instead

B. SyntaxError: 'not' should come before 'in' as 'not in'

C. Replace 'in' with 'not in' and remove 'not' at the end

D. No error, assertion is correct

Solution

1. Step 1: Analyze the assertion syntax

   The expression ends with 'colors not' which is invalid syntax in Python.

2. Step 2: Correct the membership operator usage

   The correct operator for checking absence is 'not in', written together, not separated.

3. Final Answer:

   SyntaxError: 'not' should come before 'in' as 'not in' -> Option B

4. Quick Check:

   Use 'not in' together, no trailing 'not' [OK]

Hint: Write 'not in' together, never split or reorder [OK]

Common Mistakes:

• Writing 'in colors not' instead of 'not in colors'
• Using 'assert colors not in 'yellow'' which reverses order
• Omitting 'not' or placing it incorrectly

5. You want to write a pytest test to confirm that none of the forbidden words ['spam', 'scam', 'fraud'] appear in the user input string text. Which assertion correctly checks this?

hard

A. assert all(word not in text for word in ['spam', 'scam', 'fraud'])

B. assert any(word in text for word in ['spam', 'scam', 'fraud'])

C. assert 'spam' not in text and 'scam' not in text or 'fraud' not in text

D. assert 'spam' in text or 'scam' in text or 'fraud' in text

Solution

1. Step 1: Understand the requirement

   We want to ensure none of the forbidden words appear in the text, so all must be absent.

2. Step 2: Use a generator with 'all' and 'not in'

   The expression all(word not in text for word in forbidden_words) returns True only if every word is not in text.

3. Step 3: Evaluate other options

   assert any(word in text for word in ['spam', 'scam', 'fraud']) checks if any forbidden word is present (opposite). assert 'spam' not in text and 'scam' not in text or 'fraud' not in text has incorrect logic mixing 'and' and 'or'. assert 'spam' in text or 'scam' in text or 'fraud' in text checks presence, not absence.

4. Final Answer:

   assert all(word not in text for word in ['spam', 'scam', 'fraud']) -> Option A

5. Quick Check:

   Use all() with 'not in' for absence of all items [OK]

Hint: Use all() with 'not in' to confirm all items absent [OK]

Common Mistakes:

• Using any() instead of all() for absence check
• Mixing 'and' and 'or' incorrectly in conditions
• Checking only one forbidden word instead of all