len() returns the number of items in a container — the character count of a string, the number of elements in a list/tuple/set, or the number of key-value pairs in a dict.

Syntax

len(s)

Deep Dive Course

len() works on any object that implements the __len__() dunder method — strings, lists, tuples, dicts, sets, and most other built-in containers. Internally, CPython just calls obj.__len__() and returns whatever integer it produces, which is also why you can add len() support to your own classes by defining that method.

1Understanding len()

💡

len() is O(1) for built-in containers — they cache their size — so it's always cheap to call, even inside a loop, unlike counting items manually with a generator.

editor.html

names = ["Ana", "Boris", "Cara"]
print(len(names))

localhost:3000

2Practical Example

Here is a real-world application of len() showing how it is used in production Python code.

editor.html

class Playlist:
    def __init__(self, songs):
        self.songs = songs
    def __len__(self):
        return len(self.songs)

p = Playlist(["Song A", "Song B"])
print(len(p))

localhost:3000

3Best Practices

Follow these guidelines when working with len():

1. Use if not container: rather than len(container) == 0 to check for emptiness — it's more idiomatic and works for any falsy check

2. Define __len__ on custom classes so they work naturally with len() and truthiness checks

3. Remember len() counts characters, not bytes, for a str — use len(s.encode()) if you need the byte count

⚠️

Tip: len() is O(1) for built-in containers — they cache their size — so it's always cheap to call, even inside a loop, unlike counting items manually with a generator.

editor.html

names = ["Ana", "Boris", "Cara"]
print(len(names))

localhost:3000

Examples

Example 01Basic Usage

names = ["Ana", "Boris", "Cara"]
print(len(names))

Example 02Advanced Example

class Playlist:
    def __init__(self, songs):
        self.songs = songs
    def __len__(self):
        return len(self.songs)

p = Playlist(["Song A", "Song B"])
print(len(p))

Best Practices

Use if not container: rather than len(container) == 0 to check for emptiness — it's more idiomatic and works for any falsy check
Define __len__ on custom classes so they work naturally with len() and truthiness checks
Remember len() counts characters, not bytes, for a str — use len(s.encode()) if you need the byte count

Interview Question

Why is len() a built-in function instead of a method on every object, like obj.length() in some other languages?▼

Hint: Consider consistency and how CPython implements it under the hood.

Python's design philosophy favors len(obj) over obj.len() partly for consistency — every container is measured the same way, from the outside — and because it lets CPython optimize the common case: for built-in types, len() reads a size field directly from the underlying C struct instead of dispatching a method call, making it faster than a typical method lookup. Custom classes can still opt in by implementing __len__().

Exercises

MediumPractice using len() in a real scenario.

View Solution

names = ["Ana", "Boris", "Cara"]
print(len(names))

Frequently Asked Questions

Why is len() a built-in function instead of a method on every object, like obj.length() in some other languages?▼

Related Functions

__len__()listrange()