List length and membership test in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you find the number of items in a list in Python?

Use the len() function. For example, len(my_list) returns the number of items in my_list.

beginner

What does the expression item in my_list check?

It checks if item is present anywhere inside my_list. It returns True if found, otherwise False.

beginner

What will len([]) return?

It returns 0 because the list is empty and has no items.

beginner

How can you check if the number 5 is NOT in a list called numbers?

Use the expression 5 not in numbers. It returns True if 5 is not found in the list.

beginner

What is the output of this code?

my_list = [1, 2, 3, 4]
print(len(my_list))
print(3 in my_list)
print(5 in my_list)

The output is:

4
True
False

Explanation:
- len(my_list) is 4 because there are 4 items.
- 3 in my_list is True because 3 is in the list.
- 5 in my_list is False because 5 is not in the list.

What does len([10, 20, 30]) return?

A 3

B 30

C 0

D Error

The list has 3 items, so len() returns 3.

Which expression checks if 'apple' is in the list fruits?

A 'apple' not in fruits

B fruits in 'apple'

C 'apple' in fruits

D len('apple') in fruits

The correct syntax to check membership is item in list.

What will len([]) output?

A 1

B 0

C None

D Error

An empty list has zero items, so len([]) returns 0.

What does 7 not in [1, 2, 3, 4] return?

A True

B False

C 7

D Error

Since 7 is not in the list, the expression returns True.

If my_list = ['a', 'b', 'c'], what is 'd' in my_list?

A Error

B True

C 'd'

D False

'd' is not in the list, so the expression returns False.

Explain how to find the length of a list and check if an item is inside it in Python.

Think about how you count items and how you ask if something is present.

Describe what happens when you use not in with a list.

It's like asking if something is missing.

Practice

1.

What does the len() function do when used with a list in Python?

easy

A. It removes the last item from the list.

B. It checks if an item is in the list.

C. It adds a new item to the list.

D. It returns the number of items in the list.

Solution

1. Step 1: Understand the purpose of len()

   The len() function counts how many elements are inside a list.

2. Step 2: Compare with other options

   Checking membership uses in, adding uses append(), removing uses pop().

3. Final Answer:

   It returns the number of items in the list. -> Option D

4. Quick Check:

   len() counts items [OK]

Hint: Remember: len() counts items, in checks presence [OK]

Common Mistakes:

• Confusing len() with in operator
• Thinking len() adds or removes items
• Mixing up list methods with functions

2.

Which of the following is the correct syntax to check if the number 5 is in the list nums?

nums = [1, 3, 5, 7]

easy

A. if 5 inside nums:

B. if nums in 5:

C. if 5 in nums:

D. if nums contains 5:

Solution

1. Step 1: Recall correct membership syntax

   Python uses in to check if an item is inside a list: item in list.

2. Step 2: Check each option

   Only if 5 in nums: matches correct syntax; others are invalid Python.

3. Final Answer:

   if 5 in nums: -> Option C

4. Quick Check:

   Use in for membership test [OK]

Hint: Use 'item in list' to check membership [OK]

Common Mistakes:

• Writing 'inside' or 'contains' instead of 'in'
• Reversing order like 'list in item'
• Using invalid keywords for membership

3.

What will be the output of this code?

fruits = ['apple', 'banana', 'cherry']
print(len(fruits))
print('banana' in fruits)
print('orange' in fruits)

medium

A. 3\nTrue\nFalse

B. 3\nFalse\nTrue

C. Error\nTrue\nFalse

D. 3\nTrue\nTrue

Solution

1. Step 1: Calculate length of the list

   The list has 3 items: 'apple', 'banana', 'cherry', so len(fruits) is 3.

2. Step 2: Check membership for 'banana' and 'orange'

   'banana' is in the list, so 'banana' in fruits is True. 'orange' is not, so 'orange' in fruits is False.

3. Final Answer:

   3 True False -> Option A

4. Quick Check:

   len=3, banana in list=True, orange in list=False [OK]

Hint: Count items with len(), check presence with in [OK]

Common Mistakes:

• Confusing True/False for membership
• Miscounting list length
• Expecting syntax error for correct code

4.

Find the error in this code snippet:

items = [10, 20, 30]
if 20 in items
    print("Found 20")

medium

A. Missing colon ':' after the if condition.

B. Wrong use of 'in' operator.

C. List should be defined with parentheses, not brackets.

D. print statement syntax is incorrect.

Solution

1. Step 1: Check syntax of if statement

   Python requires a colon ':' at the end of the if condition line.

2. Step 2: Identify the missing colon

   The code has if 20 in items without a colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' after the if condition. -> Option A

4. Quick Check:

   if statements need ':' [OK]

Hint: Always put ':' after if condition [OK]

Common Mistakes:

• Forgetting colon after if condition
• Misusing 'in' operator syntax
• Using parentheses instead of brackets for lists

5.

You have a list numbers = [2, 4, 6, 8]. You want to check if the list has exactly 4 items and if the number 5 is not in the list. Which code correctly does this?

hard

A. if len(numbers) = 4 and 5 not in numbers:
print("Condition met")

B. if len(numbers) == 4 and 5 not in numbers:
print("Condition met")

C. if len(numbers) == 4 or 5 in numbers:
print("Condition met")

D. if len(numbers) == 4 and 5 in numbers:
print("Condition met")

Solution

1. Step 1: Check list length condition

   Use len(numbers) == 4 to test if list has exactly 4 items.

2. Step 2: Check membership condition

   Use 5 not in numbers to confirm 5 is not in the list.

3. Step 3: Combine conditions correctly

   Use and to require both conditions true together.

4. Final Answer:

   if len(numbers) == 4 and 5 not in numbers: print("Condition met") -> Option B

5. Quick Check:

   Length check and membership negation combined with and [OK]

Hint: Use 'len() == number' and 'item not in list' with and [OK]

Common Mistakes:

• Using single '=' instead of '==' for comparison
• Using or instead of and for both conditions
• Checking for 5 in list instead of not in