List length and membership test in Python - Step-by-Step Execution

Concept Flow - List length and membership test

Start with a list

↓

Check length with len()

↓

Use 'in' to test membership

↓

Return length and membership result

We start with a list, then find its length using len(), and check if an item is in the list using 'in'. Finally, we get the results.

Execution Sample

Python

my_list = [2, 4, 6, 8]
length = len(my_list)
has_six = 6 in my_list
print(length, has_six)

This code finds the length of the list and checks if 6 is inside it, then prints both results.

Execution Table

Step	Action	Expression Evaluated	Result	Output
1	Create list	my_list = [2, 4, 6, 8]	[2, 4, 6, 8]
2	Calculate length	len(my_list)	4
3	Check membership	6 in my_list	True
4	Print results	print(length, has_six)		4 True

💡 All steps completed, program ends after printing.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
my_list	undefined	[2, 4, 6, 8]	[2, 4, 6, 8]	[2, 4, 6, 8]	[2, 4, 6, 8]
length	undefined	undefined	4	4	4
has_six	undefined	undefined	undefined	True	True

Key Moments - 2 Insights

Why does len(my_list) return 4 instead of the last number in the list?

len(my_list) counts how many items are in the list, not the value of any item. See step 2 in execution_table where len(my_list) is 4 because there are 4 items.

Why does '6 in my_list' return True even though 6 is not the first item?

'in' checks every item in the list until it finds 6. It doesn't matter where 6 is. Step 3 shows '6 in my_list' is True because 6 is present.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of length?

A 8

B 4

C 6

D Undefined

💡 Hint

Check the 'Result' column at step 2 in execution_table.

At which step does the program check if 6 is in the list?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for membership test.

If we check '5 in my_list' instead of '6 in my_list', what would be the result at step 3?

A Error

B True

C False

D Undefined

💡 Hint

Since 5 is not in the list, membership test returns False.

Concept Snapshot

List length and membership test in Python:
- Use len(list) to get number of items.
- Use 'item in list' to check if item exists.
- len() returns an integer count.
- 'in' returns True or False.
- Both are simple and fast ways to inspect lists.

Full Transcript

This example shows how to find the length of a list and check if a value is inside it. First, we create a list with four numbers. Then, we use len() to count how many items are in the list, which is 4. Next, we check if the number 6 is in the list using '6 in my_list', which returns True because 6 is present. Finally, we print both results: 4 and True. This helps us understand how to quickly get list size and test membership in Python.

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