len() function in Python - Step-by-Step Execution

Concept Flow - len() function

Start with object

↓

Call len() function

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Check object type

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Calculate number of items

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Return length as integer

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Use length in program

The len() function takes an object and returns the number of items it contains as an integer.

Execution Sample

Python

my_list = [10, 20, 30]
length = len(my_list)
print(length)

This code finds how many items are in the list and prints that number.

Execution Table

Step	Action	Object	len() Result	Output
1	Define my_list	[10, 20, 30]
2	Call len(my_list)	[10, 20, 30]	3
3	Assign length = 3
4	Print length			3

💡 Program ends after printing the length 3

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
my_list	undefined	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
length	undefined	undefined	undefined	3	3

Key Moments - 3 Insights

Why does len(my_list) return 3 and not the list itself?

len() counts how many items are inside the list, it does not return the list. See execution_table step 2 where len(my_list) gives 3.

Can len() be used on numbers like 123?

No, len() works on collections like lists, strings, or tuples. Numbers are not collections, so len(123) would cause an error.

What type of value does len() return?

len() always returns an integer representing the count of items, as shown in execution_table step 2 where the result is 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of length after step 3?

A undefined

B [10, 20, 30]

C 3

D 0

💡 Hint

Check the 'len() Result' and 'Action' columns in step 3 of the execution_table.

At which step is the len() function called?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Action' column in the execution_table to find when len(my_list) is called.

If my_list had 5 items instead of 3, what would len(my_list) return at step 2?

A 3

B 5

C 0

D Error

💡 Hint

len() returns the number of items in the object, so it matches the list length.

Concept Snapshot

len(object)
- Returns the number of items in object
- Works on lists, strings, tuples, etc.
- Returns an integer
- Raises error if object has no length
- Use to find size of collections

Full Transcript

The len() function in Python counts how many items are inside a collection like a list or string. When you call len(my_list), Python checks the list and returns the number of items it contains as an integer. For example, if my_list has three items, len(my_list) returns 3. This value can be stored in a variable and printed. len() does not return the list itself, only the count of items. It works only on objects that have a length, like lists, strings, and tuples. Using len() on a number causes an error because numbers don't have length. The returned value is always an integer representing the size of the collection.