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

Concept Flow - List length and membership test

Start with a list

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Check length with len()

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Use 'in' to test membership

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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.