Python len() usage and examples

Python built-in functions

The len() function returns the number of items or length of an object.

The syntax of len() is:

len(s)

len() parameter

s-Sequence (string, bytes, tuple, list, or range) or set (dictionary, set, or frozen set)

len() return value

The len() function returns the number of items in an object.

Not passing a parameter or passing an invalid parameter will raise a TypeError exception.

Example1How does len() work with tuples, lists, and ranges?

testList = []
print(testList, 'number of items', len(testList))
testList = [1,　2,　3]
print(testList, 'number of items', len(testList))
testTuple = (1,　2,　3)
print(testTuple, 'number of items', len(testTuple))
testRange = range(1,　10)
print('Item', testRange, 'number of items', len(testRange))

The output when running the program is:

[] number of items is 0
[1,　2,　3] number of items　3
(1,　2,　3) number of items　3
Item range(1,　10) number of items　9

Visit these pages to learn more about the following content:

• Python list

• Python tuple

• Python range() method

Example2How does len() handle strings and bytes?

testString = ''
print('string', testString, 'length is', len(testString))
testString = 'Python'
print('string', testString, 'length is', len(testString))
# byte object
testByte = b'Python'
print('string', testByte, 'length is', len(testByte))
testList = [1,　2,　3]
# Convert to byte object
testByte = bytes(testList)
print('string', testByte, 'length is', len(testByte))

The output when running the program is:

string length is 0
string 'Python' length is　6
string 'b'Python' length is　6
string 'b'\x01\x02\x03Length is　3

Visit these pages to learn more about the following content:

• Python string

• Python bytes() method

Example3：How to use len() with dictionaries and sets?

testSet = {1,　2,　3}
print(testSet, 'Length is', len(testSet))
# Empty Set
testSet = set()
print(testSet, 'Length is', len(testSet))
testDict = {1: 'one',　2: 'two'}
print(testDict, 'Length is', len(testDict))
testDict = {}
print(testDict, 'Length is', len(testDict))
testSet = {1,　2}
# frozenSet
frozenTestSet = frozenset(testSet)
print(frozenTestSet, 'Length is', len(frozenTestSet))

The output when running the program is:

{1,　2,　3} Length is　3
set() Length is 0
{1: 'one',　2: 'two' Length is　2
{} Length is 0
frozenset({1,　2}) Length is　2

Visit these pages to learn more about the following content:

• Python dictionary

• Python set

• Python Frozenset()

Internally, len() calls the object's __len__ method. You can understand len() as:

def len(s):
　　　　return s.__len__()

Therefore, you can assign a custom length to the object (if necessary)

Example4：How to use len() for custom objects?

class Session:
　　　　def __init__(self, number = 0):
　　　　　　self.number = number
　　　　
　　　　def __len__(self):
　　　　　　return self.number
# Default length is 0
s1　= Session()
print(len(s1))
# Given length
s2　= Session(6)
print(len(s2))

The output when running the program is:

0
6

Python built-in functions