What is __ len __ in python?

It's often the case that the "typical" behavior of a built-in or operator is to call (with different and nicer syntax) suitable magic methods (ones with names like __whatever__) on the objects involved. Often the built-in or operator has "added value" (it's able to take different paths depending on the objects involved) -- in the case of len vs __len__, it's just a bit of sanity checking on the built-in that is missing from the magic method:

>>> class bah(object):
...   def __len__(self): return "an inch"
... 
>>> bah().__len__()
'an inch'
>>> len(bah())
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'str' object cannot be interpreted as an integer

When you see a call to the len built-in, you're sure that, if the program continues after that rather than raising an exception, the call has returned an integer, non-negative, and <= sys.maxsize -- when you see a call to xxx.__len__(), you have no certainty (except that the code's author is either unfamiliar with Python or up to no good;-).

Other built-ins provide even more added value beyond simple sanity checks and readability. By uniformly designing all of Python to work via calls to builtins and use of operators, never through calls to magic methods, programmers are spared from the burden of remembering which case is which. (Sometimes an error slips in: until 2.5, you had to call foo.next() -- in 2.6, while that still works for backwards compatibility, you should call next(foo), and in 3.*, the magic method is correctly named __next__ instead of the "oops-ey" next!-).

So the general rule should be to never call a magic method directly (but always indirectly through a built-in) unless you know exactly why you need to do that (e.g., when you're overriding such a method in a subclass, if the subclass needs to defer to the superclass that must be done through explicit call to the magic method).

• Syntax
• Background len()
• Example Custom __len__()
• Default __len__() Implementation
• What’s the Difference Between len(x) and x.__len__()?
• Where to Go From Here?

Syntax

object.__len__(self)

The Python __len__ method returns a positive integer that represents the length of the object on which it is called. It implements the built-in len() function. For example, if you call len(x) an object x, Python internally calls x.__len__() to determine the length of object x.

We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

💡 Useful Knowledge: If the __bool__() dunder method is not defined, Python internally returns __len__() != 0 to determine whether the object’s associated Boolean value is True or False.

Background len()

Python’s built-in function len() returns the length of the given string, array, list, tuple, dictionary, or any other iterable.

The type of the return value is an integer that represents the number of elements in this iterable.

Python len() – A Simple Guide

Before we learn more about the __len__() dunder method, let’s have a look at a couple of basic len() examples:

>>> friends = ['Alice', 'Bob', 'Carl', 'Ann']
>>> len(friends)
4
>>> friends.extend([1, 2, 3])
>>> len(friends)
7
>>> len('hello world')
11
>>> len('hi')
2
>>> len((1, 2, 3))
3
>>> len({42, 21})
2
>>> age = {'Alice': 18, 'Bob': 21}
>>> len(age)
2
>>> age['Carl'] = 33
>>> len(age)
3

Example Custom __len__()

In the following example, you create a custom class Data and overwrite the __len__() method so that it returns a dummy number.

class Data:
    def __len__(self):
        return 42


a = Data()

print(len(a))
# 42

print(bool(a))
# True    --> Because 42 != 0

If you hadn’t defined the __len__() method, Python would’ve raised an error:

Default __len__() Implementation

If you call len(x) on an object on which the x.__len__() dunder method is not defined, Python will raise a TypeError: object of type '...' has no len(). To fix this error, simply define the __len__() method in the class definition before calling len() on an object.

class Data:
    pass


a = Data()
print(len(a))

Here’s the error message:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in 
    print(len(a))
TypeError: object of type 'Data' has no len()

What’s the Difference Between len(x) and x.__len__()?

The result of len(x) and x.__len__() is the same: both return the number of elements in the object, i.e., more generally, its length.

Have a look at this example:

>>> len([1, 2, 3])
3
>>> [1, 2, 3].__len__()
3

The difference between len(x) and x.__len__() is only syntactical sugar. The former built-in function calls the latter method internally to implement the correct behavior. So, there’s no semantic difference between both ways to obtain the length of an object.

References:

• https://docs.python.org/3/reference/datamodel.html

Where to Go From Here?

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To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

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