How to Use len() Function in Python: Syntax and Examples

In Python, use the len() function to get the number of items in a collection like a string, list, or tuple. Just pass the collection inside the parentheses, for example, len(my_list) returns the count of elements in my_list.

Syntax

The len() function takes one argument, which is the collection you want to measure. It returns an integer representing the number of items in that collection.

• len(s): where s is a string, list, tuple, dictionary, or other collection.
• The function counts elements, characters, or keys depending on the type.

python

len(s)

Example

This example shows how to use len() with different types: a string, a list, and a dictionary.

python

my_string = "hello"
my_list = [10, 20, 30, 40]
my_dict = {"a": 1, "b": 2, "c": 3}

print(len(my_string))  # Counts characters
print(len(my_list))    # Counts list items
print(len(my_dict))    # Counts dictionary keys

Output

5
4
3

Common Pitfalls

Some common mistakes when using len() include:

• Passing a number instead of a collection causes an error.
• Expecting len() to count nested items inside collections (it only counts top-level items).
• Using len() on None or unsupported types raises an error.

python

wrong_value = 123
# print(len(wrong_value))  # This will cause a TypeError

correct_value = [1, 2, 3]
print(len(correct_value))  # Correct usage, outputs 3

Output

3

Quick Reference

Usage	Description	Example
len(s)	Returns number of items in collection s	len([1,2,3]) → 3
len('text')	Returns number of characters in string	len('hello') → 5
len(dict)	Returns number of keys in dictionary	len({'a':1,'b':2}) → 2

Key Takeaways

Use len() to find the number of items in strings, lists, tuples, dictionaries, and other collections.

Pass only collections or sequences to len(); passing numbers or None causes errors.

len() counts top-level elements, not nested items inside collections.

The function returns an integer representing the count.

Remember len() works on many built-in types, making it very versatile.