Checking membership (in, not in) in PyTest

Introduction

We check if something is inside a group or not. This helps us confirm if data is where it should be.

To verify if a username exists in a list of registered users.

To check if an error message appears in the output logs.

To confirm that a product ID is not in the list of discontinued items.

To ensure a keyword is present in a text response.

To test if a value is missing from a set of expected results.

Syntax

PyTest

assert item in collection
assert item not in collection

item is what you want to find.

collection can be a list, string, set, or other container.

Examples

Check if 'apple' is in the list of fruits.

PyTest

assert 'apple' in ['apple', 'banana', 'cherry']

Check that the word 'error' is not in the message string.

PyTest

assert 'error' not in 'All tests passed successfully'

Check if number 5 is in the set of numbers.

PyTest

assert 5 in {1, 3, 5, 7}

Sample Program

This test checks if certain items are in or not in collections. All assertions should pass.

PyTest

import pytest

def test_membership():
    fruits = ['apple', 'banana', 'cherry']
    message = 'All tests passed successfully'
    numbers = {1, 3, 5, 7}

    assert 'banana' in fruits
    assert 'error' not in message
    assert 5 in numbers
    assert 'grape' not in fruits

Important Notes

Using in and not in makes tests easy to read and understand.

These checks work with many types like lists, strings, sets, and tuples.

Failed assertions will show which item was missing or wrongly present.

Summary

Use in to check if an item is inside a collection.

Use not in to check if an item is not inside a collection.

These checks help confirm data presence or absence simply and clearly.

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