Python len() Function

The len() function in Python is used to get the number of items in an object. It works with many data types like strings, lists, tuples, dictionaries, and more.

Syntax

len(object)

Parameter:

• object – A sequence (like string, list, tuple) or a collection (like dictionary, set, etc.).

Returns:

• An integer representing the number of items in the object.

Example 1: Length of a String

text = "Hello, World!"
print(len(text))

13

Example 2: Length of a List

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

5

Example 3: Length of a Tuple

values = (10, 20, 30)
print(len(values))

3

Example 4: Length of a Dictionary

person = {"name": "Alice", "age": 25, "city": "New York"}
print(len(person))

3

Note: len() returns the number of keys in the dictionary.

Supported Data Types

• String
• List
• Tuple
• Dictionary
• Set
• Range
• Custom classes with __len__() method

Use Case: Why is len() Useful?

• Checking if a list is empty: if len(my_list) == 0:
• Limiting input size in forms or APIs
• Looping through data and keeping count

Common Mistake

• Wrong: len(5) → Raises TypeError because int has no length

Interview Tip

len() is often used in interview coding problems to control loops, check constraints, or validate data. Know which data types it works on!

Summary

• len() returns the number of items in an object
• Works with sequences and collections
• Raises TypeError if used on unsupported types like int or float

Practice Problem

Ask the user to enter a sentence. Print how many characters it contains:

sentence = input("Enter a sentence: ")
print("Number of characters:", len(sentence))