Python __len__() magic method

Python __len__ is one of the various magic methods in Python programming language, it is basically used to implement the len() function in Python because whenever we call the len() function then internally __len__ magic method is called. It finally returns an integer value that is greater than or equal to zero as it represents the length of the object for which it is called. 

Python __len__() Function Syntax

Syntax: object.__len__()

• object: It is the object whose length is to be determined.                 

Returns: Returns a non negative integer.

Python __len__() Function Example

When the object we are iterating already has  __len__ method defined internally, then len function gives the correct result for the object i.e.

Python3

class GFG:
    def __init__(self, a):
        self.a = a
    def __len__(self):
        return len(self.a)

obj = GFG("GeeksForGeeks")
print(len(obj))

Output:

13

Example 2:

When the object doesn’t have a predefined __len__ method, then while executing the len function it gives a TypeError, but it can be corrected by defining a __len__ method by our own,

Python3

class GFG:

    def __init__(self, item):
        self.item = item
    def __len__(self):
        return 1

obj = GFG("Geeksforgeeks")
print(obj.__len__())

Output:

1

Default __len__() Implementation

When we call len(obj) from the class without defining __len__() methods then it will raise a TypeError: object of type ‘…’ has no len().

Python3

class Length:
    pass

obj = Length()
print(len(obj))

Output:

TypeError                                 Traceback (most recent call last)
<ipython-input-9-c08d3a505e12> in <module>
     3 
     4 obj = Length()
----> 5 print(len(obj))
TypeError: object of type 'Length' has no len()