Python len()

Summary: in this tutorial, you will learn how to use the Python len() function to get the number of items of an object.

Introduction to the Python len() function #

The len() function returns the number of items (length) of an object. Here’s the syntax of the len() function:

len(s)Code language: Python (python)

In this syntax, the s is an object that you want to get the length. The object can be:

• A sequence such as a string, bytes, tuple, list or range.
• A collection such as a dictionary, a set or a frozen set.
• Or any user-defined object that implements the __len__() method.

When you call the len() function on an object, the len() function will delegate to __len__ method of the object. In other words, it’ll call the __len__() method of the object.

So the following function call:

len(obj)Code language: Python (python)

is equivalent to:

obj.__len__()Code language: Python (python)

Python len() function examples #

Let’s take some examples of using Python len() function.

1) Using the len() function with a sequence #

The following example shows how to use the len() function to get the length of a string, the number of elements in a tuple, and the number of items in a range:

s = 'Python'
print(len(s))  # ? 6

seasons = ('Spring', 'Summer', 'Autumn', 'Winter')
print(len(seasons))  # ? 4


r = range(1, 10)
print(len(r))  # ? 9Code language: Python (python)

2) Using the len() function with a collection #

The following example uses the len() function to get the number of items in a collection:

days = {
    1: 'Sun',
    2: 'Mon',
    3: 'Tue',
    4: 'Wed',
    5: 'Thu',
    6: 'Fri',
    7: 'Sat'
}
print(len(days))  # ? 7

rgb = set(('red', 'green', 'blue'))
print(len(rgb))  # ? 3Code language: Python (python)

3) Using the len() function with user-defined object #

The following example shows how to use the len() function with a user-defined object:

class Employee:
    def __init__(self, name):
        self.name = name
        self.dependents = []

    def add_dependent(self, dependent):
        self.dependents.append(dependent)
        return self

    def __len__(self):
        return len(self.dependents)


employee = Employee('John Doe')
employee.add_dependent('Alice Doe')
employee.add_dependent('Bob Doe')

print(len(employee))  # ? 2Code language: Python (python)

How it works.

First, define the Employee class that has two attributes name and dependents:

class Employee:
    def __init__(self, name):
        self.name = name
        self.dependents = []Code language: Python (python)

Next, define the add_dependent() that adds a dependent to the dependents list:

def add_dependent(self, dependent):
   self.dependents.append(dependent)
   return selfCode language: Python (python)

Then, implement the __len__() method that returns the number of dependents of the employee.

def __len__(self):
   return len(self.dependents)Code language: Python (python)

After that, create an Employee object and add two dependents:

employee = Employee('John Doe')
employee.add_dependent('Alice Doe')
employee.add_dependent('Bob Doe')Code language: Python (python)

Finally, pass the employee object to the len() function. The len() will call the __len__() method of the Employee class.

Summary #

• Use the len() function to get the number of items of an object.
• The len() function delegates the call to __len__() method.