Membership operators (in, not in) in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the in operator check in Python?

The in operator checks if a value exists inside a sequence like a list, string, or tuple. It returns True if the value is found, otherwise False.

beginner

How does the not in operator work?

The not in operator checks if a value does NOT exist in a sequence. It returns True if the value is missing, otherwise False.

beginner

Example: What is the output of 'a' in 'cat'?

The output is True because the letter 'a' is found inside the string 'cat'.

beginner

Example: What does 5 not in [1, 2, 3, 4] return?

It returns True because the number 5 is not present in the list.

intermediate

Can membership operators be used with dictionaries?

Yes, membership operators check keys in dictionaries. For example, 'key' in dict checks if 'key' is a key in the dictionary.

What does 3 in [1, 2, 3, 4] return?

A True

B False

C Error

D None

3 is present in the list, so 3 in [1, 2, 3, 4] returns True.

What is the result of 'x' not in 'example'?

A True

B False

C Error

D None

The letter 'x' is in 'example', so 'x' not in 'example' returns False.

Which operator checks if an item is NOT in a sequence?

A not in

B in

C ==

D !=

The not in operator checks if an item is missing from a sequence.

What does 'key' in {'key': 1, 'val': 2} check?

A If 'key' is a value

B If 'key' is a string

C If 'key' is a list

D If 'key' is a key

Membership operators check keys in dictionaries, so it checks if 'key' is a key.

What is the output of 10 not in (5, 10, 15)?

A Error

B True

C False

D None

10 is in the tuple, so 10 not in (5, 10, 15) returns False.

Explain how the in and not in operators work in Python with examples.

Think about checking if something is inside or outside a group.

Describe how membership operators behave when used with dictionaries.

Remember dictionaries have keys and values.

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