How to Use len() in Python
In Python, the built-in len() function returns the length (the number of items) of objects of various types, such as lists and strings.
List length with len()
l = [0, 1, 2]
print(len(l))
# 3
For more details, including handling two-dimensional lists (lists of lists), refer to the following article.
Tuple length with len()
t = (0, 1, 2)
print(len(t))
# 3
Set length with len()
s = {0, 1, 2}
print(len(s))
# 3
For more details on sets, refer to the following article.
Dictionary length with len()
d = {'key_0': 0, 'key_1': 1, 'key_2': 2}
print(len(d))
# 3
String length (number of characters) with len()
s = 'abcde'
print(len(s))
# 5
For more information on finding string lengths, especially how escape sequences and line breaks are handled, refer to the following article.
The special method __len__()
When len() is executed, it calls the special method __len__() of the object. len(x) is equivalent to x.__len__().
l = [0, 1, 2]
print(l.__len__())
# 3
s = 'abcde'
print(s.__len__())
# 5
An error occurs when len() is used with an object that lacks a __len__() method. For example, built-in numeric types such as int and float, as well as bool, do not support len() and will result in an error.
i = 100
print(hasattr(i, '__len__'))
# False
# print(len(i))
# TypeError: object of type 'int' has no len()
hasattr() is a built-in function that determines whether an object has the specified attribute or method.
Custom classes can define the __len__() method to specify how len() calculates their length. This method should return an integer of 0 or greater. For demonstration, it returns 100 here.
class MyClass:
def __len__(self):
return 100
mc = MyClass()
print(len(mc))
# 100