Python __len__: A Beginner’s Guide


Python provides a built-in function called `__len__` which is used to return the length of an object. The `__len__` function can be defined in our own classes and can be used to return the length of any custom object we create. In this blog post we’ll be discussing Python __len__ method in detail.

The `__len__` function returns an integer that represents the number of elements in the object. It is commonly used with collection objects like lists, tuples, sets, and dictionaries.

It’s important to note that not all objects have a defined length. In such cases, calling `len` on these objects will raise a `TypeError`. Therefore, it’s important to always check whether or not an object has a defined length before calling `len`.

Understanding how to use the `__len__` function in Python is essential for any programmer who works with collections or custom objects. With this knowledge, you can easily determine the length of your objects and make informed decisions about how to manipulate them. Let’s learn some more!

Understanding the __len__ method

One of the most commonly used special methods in Python is the `__len__` method. This method allows us to determine the length of an object. The `__len__` method is called automatically when we use the built-in `len()` function on an object.

Let’s take a look at an example. Say we have a list of numbers:

my_list = [1, 2, 3, 4, 5]

If we want to know the length of this list, we can use the `len()` function:


This will output `5`, which is the length of the list.

Now let’s see how we can define our own `__len__` method for a custom class.

class MyClass:
    def __init__(self, my_list):
        self.my_list = my_list
    def __len__(self):
        return len(self.my_list)

In this example, we have defined a custom class `MyClass` with an attribute `my_list`. We have also defined a `__len__` method that returns the length of the `my_list` attribute. Now if we create an instance of this class and call the `len()` function on it, it will return the length of the `my_list` attribute:

my_class = MyClass([1, 2, 3, 4, 5])

This will output `5`, which is the length of the `my_list` attribute in our custom class.

Using __len__ with built-in Python data types

The `__len__()` method is a built-in Python method that returns the length of an object. It can be used with various built-in data types such as strings, lists, tuples, sets, and dictionaries.

When used with strings, `__len__()` returns the number of characters in the string:

my_string = "Hello World"
print(len(my_string)) # Output: 11

When used with lists, tuples, and sets, `__len__()` returns the number of elements in the container:

my_list = [1, 2, 3]
print(len(my_list)) # Output: 3

my_tuple = (4, 5, 6)
print(len(my_tuple)) # Output: 3

my_set = {7, 8, 9}
print(len(my_set)) # Output: 3

When used with dictionaries, `__len__()` returns the number of key-value pairs in the dictionary:

my_dict = {'a': 1, 'b': 2, 'c': 3}
print(len(my_dict)) # Output: 3

In summary, using `__len__()` with built-in Python data types is a simple and useful way to determine the length or size of an object.

Implementing __len__ in user-defined classes

In Python, we can define our own classes and objects. Sometimes, we may want to know the length of an object of our own class. To achieve this, we can implement the `__len__` method in our class.

The `__len__` method should return the number of elements in the object. For example, if we have a class called `MyList` that represents a list of integers, we can implement `__len__` as follows:

class MyList:
    def __init__(self, data): = data

    def __len__(self):
        return len(

In this example, `MyList` takes a list of integers as an argument to its constructor and stores it in an instance variable called `data`. The `__len__` method simply returns the length of `data`.

Now, let’s create an object of `MyList` and check its length:

>>> my_list = MyList([1, 2, 3])
>>> len(my_list)

As expected, the length of `my_list` is 3.

We can also implement `__len__` in other classes that represent collections of items, such as dictionaries or sets. The important thing is to make sure that the method returns an integer that represents the number of items in the collection.

In summary, implementing `__len__` in user-defined classes allows us to get the length of objects created from those classes using the built-in `len()` function.

Best practices for using __len__

When using the `__len__` method in Python, there are some best practices to keep in mind.

Firstly, it’s important to ensure that the `__len__` method returns an integer value that accurately represents the length of the object. This means that if your object has a variable length, such as a list or tuple, the `__len__` method should return the current length of the object.

Secondly, it’s important to make sure that the `__len__` method is implemented consistently across all instances of the object. This means that if you have multiple instances of an object, they should all return the same length when their `__len__` method is called.

Thirdly, it’s generally a good idea to implement the `__len__` method for any custom objects you create in Python. This allows users of your code to easily determine the length of your objects and use them in a consistent way with other built-in Python objects.

Finally, it’s worth noting that some built-in Python objects, such as dictionaries and sets, do not have a `__len__` method implemented by default. In these cases, you can still determine their length using the built-in `len()` function.


In conclusion, the `__len__` method is a powerful tool in Python that allows you to customize the behavior of the built-in `len()` function for your own objects. By implementing this method in your classes, you can define what it means for an object to have a length and allow users of your code to easily determine the size of your object.

Remember that the `__len__` method should always return an integer that represents the length of your object. It should also be implemented in a way that is consistent with how you define equality and comparison operations for your object.

Overall, understanding how to use `__len__` can greatly improve the functionality and usability of your Python code. So whether you’re working on a small personal project or a large-scale application, consider implementing this method in your own classes to take full advantage of its benefits.
Interested in learning more? Check out our Introduction to Python course!

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