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Python len() function

Updated on Jan 07, 2020

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

Its syntax is as follows:

len(obj) -> length
PARAMETER DESCRIPTION
obj obj can be a string, list, dictionary, tuple etc.

Here is an example:

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>>> 
>>> len([1, 2, 3, 4, 5]) # length of list
5
>>> 
>>> print(len({"spande", "club", "diamond", "heart"})) # length of set
4
>>>
>>> print(len(("alpha", "beta", "gamma"))) # length of tuple
3
>>> 
>>> print(len({ "mango": 10, "apple": 40, "plum": 16 })) # length of dictionary
3

Try it out:

# length of list
print(len([1, 2, 3, 4, 5])) 

# length of set
print(len({"spande", "club", "diamond", "heart"}))
 
# length of tuple
print(len(("alpha", "beta", "gamma")))

# length of dictionary
print(len({ "mango": 10, "apple": 40, "plum": 16 }))

Ironically, the len() function doesn't work with a generator. Trying to call len() on a generator object will result in TypeError exception.

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>>>
>>> def gen_func(): 
...     for i in range(5):
...         yield i
... 
>>>
>>>
>>> len(gen_func())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'generator' has no len()
>>>

Try it out:

def gen_func(): 
    for i in range(5):
        yield i
 
print(len(gen_func()))

len() with User defined objects #

To use len() on user-defined objects you will have to implement the __len__() method.

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>>> 
>>> class Stack:
... 
...     def __init__(self):
...         self._stack = []
... 
...     def push(self, item):
...         self._stack.append(item)
... 
...     def pop(self):
...         self._stack.pop()
... 
...     def __len__(self):
...         return len(self._stack)
... 
>>> 
>>> s = Stack()
>>> 
>>> len(s)
0
>>> 
>>> s.push(2)
>>> s.push(5)
>>> s.push(9)
>>> s.push(12)
>>> 
>>> len(s)
4
>>>

Try it out:

class Stack: 
    def __init__(self):
        self._stack = []

    def push(self, item):
        self._stack.append(item)

    def pop(self):
        self._stack.pop()

    def __len__(self):
        return len(self._stack)
 

s = Stack()

print(len(s))


s.push(2)
s.push(5)
s.push(9)
s.push(12)

print(len(s))

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