Membership operators (in, not in) in Python

Introduction

Membership operators help you check if something is inside a group or not. They make it easy to find out if a value exists in a list, string, or other collections.

Checking if a name is in a list of invited guests before allowing entry.

Finding out if a word contains a certain letter.

Verifying if a key exists in a dictionary before using it.

Seeing if a number is part of a set of allowed values.

Syntax

Python

value in collection
value not in collection

in returns True if the value is found inside the collection.

not in returns True if the value is NOT found inside the collection.

Examples

Checks if 5 is in the list. Result is True.

Python

5 in [1, 2, 3, 4, 5]

Checks if letter 'a' is NOT in the string 'hello'. Result is False.

Python

'a' not in 'hello'

Checks if 'key' is a key in the dictionary. Result is True.

Python

'key' in {'key': 10, 'other': 20}

Sample Program

This program checks if 'banana' is in the fruits list and prints a message. Then it checks if 'orange' is not in the list and prints another message.

Python

fruits = ['apple', 'banana', 'cherry']

# Check if 'banana' is in the list
if 'banana' in fruits:
    print('Banana is in the list!')

# Check if 'orange' is not in the list
if 'orange' not in fruits:
    print('Orange is not in the list!')

Important Notes

Membership operators work with many types like lists, strings, tuples, sets, and dictionaries (checks keys).

They return True or False, so you can use them directly in conditions.

Summary

in checks if a value is inside a collection.

not in checks if a value is NOT inside a collection.

Use them to write clear and simple checks for membership.

Practice

1. What does the in operator do in Python?

easy

A. Creates a new collection

B. Adds a value to a collection

C. Removes a value from a collection

D. Checks if a value exists inside a collection

Solution

1. Step 1: Understand the purpose of in

   The in operator is used to check if a value is present inside a collection like a list, string, or tuple.

2. Step 2: Compare with other options

   Options A, B, and C describe actions unrelated to membership checking.

3. Final Answer:

   Checks if a value exists inside a collection -> Option D

4. Quick Check:

   in means membership check [OK]

Hint: Remember: in means 'is inside' [OK]

Common Mistakes:

• Confusing in with adding or removing items
• Thinking in changes the collection
• Mixing in with comparison operators

2. Which of the following is the correct syntax to check if the string 'apple' is NOT in the list fruits?

easy

A. if 'apple' not in fruits:

B. if 'apple' not fruits:

C. if not 'apple' fruits:

D. if 'apple' !in fruits:

Solution

1. Step 1: Recall correct syntax for not in

   The correct syntax to check absence is value not in collection.

2. Step 2: Evaluate each option

   if 'apple' not in fruits: matches correct syntax. if 'apple' not fruits: misses in. if not 'apple' fruits: misses in. if 'apple' !in fruits: uses invalid operator !in.

3. Final Answer:

   if 'apple' not in fruits: -> Option A

4. Quick Check:

   Use not in as two words [OK]

Hint: Use not in as two words, no symbols [OK]

Common Mistakes:

• Writing !in instead of not in
• Omitting in after not
• Using not 'value' in which is valid but less clear

3. What is the output of this code?

letters = ['a', 'b', 'c']
print('d' in letters)
print('a' not in letters)

medium

A. True\nTrue

B. False\nTrue

C. False\nFalse

D. True\nFalse

Solution

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

   'd' is not in ['a', 'b', 'c'], so 'd' in letters is False.

2. Step 2: Check if 'a' is not in the list

   'a' is in the list, so 'a' not in letters is False, so print('a' not in letters) prints False.

3. Final Answer:

   False False is incorrect because the second print outputs False, but the first print outputs False, so the correct output is False False which matches False\nFalse.

4. Quick Check:

   'd' in letters = False, 'a' not in letters = False [OK]

Hint: Check each membership separately [OK]

Common Mistakes:

• Assuming 'd' is in the list
• Mixing up in and not in results
• Confusing True/False outputs

4. Find the error in this code snippet:

items = ['pen', 'pencil', 'eraser']
if 'pen' not items:
    print('Pen is missing')

medium

A. Incorrect print statement syntax

B. Wrong list name

C. Missing in after not

D. No error, code is correct

Solution

1. Step 1: Check the membership syntax

   The correct syntax for checking absence is value not in collection. The code misses in after not.

2. Step 2: Verify other parts

   List name and print statement are correct. The only error is missing in.

3. Final Answer:

   Missing in after not -> Option C

4. Quick Check:

   Use not in together [OK]

Hint: Always write not in together [OK]

Common Mistakes:

• Writing not items instead of not in items
• Forgetting in keyword
• Assuming not alone checks membership

5. You have a list words = ['cat', 'dog', 'bird']. You want to print all words that are NOT in the string text = 'I have a dog and a cat'. Which code correctly does this?

hard

A. for w in words:
if w not in text:
print(w)

B. for w in words:
if w in text:
print(w)

C. for w in words:
if not w text:
print(w)

D. for w in words:
if w not text:
print(w)

Solution

1. Step 1: Understand the goal

   We want to print words from the list that are NOT found inside the string text.

2. Step 2: Check each option's logic

   for w in words: if w not in text: print(w) uses if w not in text, which correctly checks absence. for w in words: if w in text: print(w) prints words that ARE in text, opposite of goal. for w in words: if not w text: print(w) has syntax error, missing in after w. for w in words: if w not text: print(w) has syntax error missing in.

3. Final Answer:

   for w in words: if w not in text: print(w) -> Option A

4. Quick Check:

   Use not in for absence check [OK]

Hint: Use if w not in text to find missing words [OK]

Common Mistakes:

• Omitting in keyword in various positions
• Forgetting in keyword
• Printing words that are present instead of absent