The len() function can be used with several different types in Python - both built-in types and library types. For example:

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

How do I get the length of a list?

To find the number of elements in a list, use the builtin function len:

items = []
items.append("apple")
items.append("orange")
items.append("banana")

And now:

len(items)

returns 3.

Explanation

Everything in Python is an object, including lists. All objects have a header of some sort in the C implementation.

Lists and other similar builtin objects with a "size" in Python, in particular, have an attribute called ob_size, where the number of elements in the object is cached. So checking the number of objects in a list is very fast.

But if you're checking if list size is zero or not, don't use len - instead, put the list in a boolean context - it is treated as False if empty, and True if non-empty.

From the docs

len(s)

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).

len is implemented with __len__, from the data model docs:

object.__len__(self)

Called to implement the built-in function len(). Should return the length of the object, an integer >= 0. Also, an object that doesn’t define a __nonzero__() [in Python 2 or __bool__() in Python 3] method and whose __len__() method returns zero is considered to be false in a Boolean context.

And we can also see that __len__ is a method of lists:

items.__len__()

returns 3.

Builtin types you can get the len (length) of

And in fact we see we can get this information for all of the described types:

>>> all(hasattr(cls, '__len__') for cls in (str, bytes, tuple, list, 
                                            range, dict, set, frozenset))
True

Do not use len to test for an empty or nonempty list

To test for a specific length, of course, simply test for equality:

if len(items) == required_length:
    ...

But there's a special case for testing for a zero length list or the inverse. In that case, do not test for equality.

Also, do not do:

if len(items): 
    ...

Instead, simply do:

if items:     # Then we have some items, not empty!
    ...

or

if not items: # Then we have an empty list!
    ...

I explain why here but in short, if items or if not items is more readable and performant than other alternatives.

While this may not be useful due to the fact that it'd make a lot more sense as being "out of the box" functionality, a fairly simple hack would be to build a class with a length property:

class slist(list):
    @property
    def length(self):
        return len(self)

You can use it like so:

>>> l = slist(range(10))
>>> l.length
10
>>> print l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Essentially, it's exactly identical to a list object, with the added benefit of having an OOP-friendly length property.

As always, your mileage may vary.

Besides len you can also use operator.length_hint (requires Python 3.4+). For a normal list both are equivalent, but length_hint makes it possible to get the length of a list-iterator, which could be useful in certain circumstances:

>>> from operator import length_hint
>>> l = ["apple", "orange", "banana"]
>>> len(l)
3
>>> length_hint(l)
3

>>> list_iterator = iter(l)
>>> len(list_iterator)
TypeError: object of type 'list_iterator' has no len()
>>> length_hint(list_iterator)
3

But length_hint is by definition only a "hint", so most of the time len is better.

I've seen several answers suggesting accessing __len__. This is all right when dealing with built-in classes like list, but it could lead to problems with custom classes, because len (and length_hint) implement some safety checks. For example, both do not allow negative lengths or lengths that exceed a certain value (the sys.maxsize value). So it's always safer to use the len function instead of the __len__ method!

And for completeness (primarily educational), it is possible without using the len() function. I would not condone this as a good option DO NOT PROGRAM LIKE THIS IN PYTHON, but it serves a purpose for learning algorithms.

def count(list):   # list is an iterable object but no type checking here!
    item_count = 0
    for item in list:
        item_count += 1
    return item_count

count([1,2,3,4,5])

(The list object must be iterable, implied by the for..in stanza.)

The lesson here for new programmers is: You can’t get the number of items in a list without counting them at some point. The question becomes: when is a good time to count them? For example, high-performance code like the connect system call for sockets (written in C) connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);, does not calculate the length of elements (giving that responsibility to the calling code). Notice that the length of the address is passed along to save the step of counting the length first? Another option: computationally, it might make sense to keep track of the number of items as you add them within the object that you pass. Mind that this takes up more space in memory. See Naftuli Kay‘s answer.

Example of keeping track of the length to improve performance while taking up more space in memory. Note that I never use the len() function because the length is tracked:

class MyList(object):
    def __init__(self):
        self._data = []
        self.length = 0 # length tracker that takes up memory but makes length op O(1) time
        

        # the implicit iterator in a list class
    def __iter__(self):
        for elem in self._data:
            yield elem
            
    def add(self, elem):
        self._data.append(elem)
        self.length += 1
            
    def remove(self, elem):
        self._data.remove(elem)
        self.length -= 1
            
mylist = MyList()
mylist.add(1)
mylist.add(2)
mylist.add(3)
print(mylist.length) # 3
mylist.remove(3)
print(mylist.length) # 2

Answering your question as the examples also given previously:

items = []
items.append("apple")
items.append("orange")
items.append("banana")

print items.__len__()

You can use the len() function to find the length of an iterable in python.

my_list = [1, 2, 3, 4, 5]
print(len(my_list))  # OUTPUT: 5

The len() function also works with strings:

my_string = "hello"
print(len(my_string))  # OUTPUT: 5

So to conclude, len() works with any sequence or collection (or any sized object that defines __len__).

There are three ways that you can find the length of the elements in the list. I will compare the 3 methods with performance analysis here.

Method 1: Using len()

items = []
items.append("apple")
items.append("orange")
items.append("banana")

print(len(items))

output:

3

Method 2: Using Naive Counter Method

items = []
items.append("apple")
items.append("orange")
items.append("banana")

counter = 0
for i in items:
    counter = counter + 1

print(counter)

output:

3

Method 3: Using length_hint()

items = []
items.append("apple")
items.append("orange")
items.append("banana")

from operator import length_hint
list_len_hint = length_hint(items)
print(list_len_hint)

output:

3

Performance Analysis – Naive vs len() vs length_hint()

Note: In order to compare, I am changing the input list into a large set that can give a good amount of time difference to compare the methods.

items = list(range(100000000))

# Performance Analysis
from operator import length_hint
import time

# Finding length of list
# using loop
# Initializing counter

start_time_naive = time.time()
counter = 0
for i in items:
    # incrementing counter
    counter = counter + 1
end_time_naive = str(time.time() - start_time_naive)

# Finding length of list
# using len()
start_time_len = time.time()
list_len = len(items)
end_time_len = str(time.time() - start_time_len)

# Finding length of list
# using length_hint()
start_time_hint = time.time()
list_len_hint = length_hint(items)
end_time_hint = str(time.time() - start_time_hint)

# Printing Times of each
print("Time taken using naive method is : " + end_time_naive)
print("Time taken using len() is : " + end_time_len)
print("Time taken using length_hint() is : " + end_time_hint)

Output:

Time taken using naive method is : 7.536813735961914
Time taken using len() is : 0.0
Time taken using length_hint() is : 0.0

Conclusion

It can be clearly seen that time taken for naive is very large compared to the other two methods, hence len() & length_hint() is the best choice to use.

There is an inbuilt function called len() in python which will help in these conditions.

>>> a = [1,2,3,4,5,6]
>>> len(a)  # Here the len() function counts the number of items in the list.
6

This will work slightly different in the case of string: it counts the characters.

>>> a = "Hello"
>>> len(a)
5

To get the number of elements in any sequential objects, your goto method in Python is len() eg.

a = range(1000) # range
b = 'abcdefghijklmnopqrstuvwxyz' # string
c = [10, 20, 30] # List
d = (30, 40, 50, 60, 70) # tuple
e = {11, 21, 31, 41} # set

len() method can work on all the above data types because they are iterable i.e You can iterate over them.

all_var = [a, b, c, d, e] # All variables are stored to a list
for var in all_var:
    print(len(var))

A rough estimate of the len() method

def len(iterable, /):
    total = 0
    for i in iterable:
        total += 1
    return total

Simple: use len(list) or list.__len__()

In terms of how len() actually works, this is its C implementation:

static PyObject *
builtin_len(PyObject *module, PyObject *obj)
/*[clinic end generated code: output=fa7a270d314dfb6c input=bc55598da9e9c9b5]*/
{
    Py_ssize_t res;

    res = PyObject_Size(obj);
    if (res < 0) {
        assert(PyErr_Occurred());
        return NULL;
    }
    return PyLong_FromSsize_t(res);
}

Py_ssize_t is the maximum length that the object can have. PyObject_Size() is a function that returns the size of an object. If it cannot determine the size of an object, it returns -1. In that case, this code block will be executed:

    if (res < 0) {
        assert(PyErr_Occurred());
        return NULL;
    }

And an exception is raised as a result. Otherwise, this code block will be executed:

    return PyLong_FromSsize_t(res);

res which is a C integer, is converted into a Python int (which is still called a "Long" in the C code because Python 2 had two types for storing integers) and returned.