Is arr.__len__() the preferred way to get the length of an array in Python?

Joan Venge picture Joan Venge · Feb 5, 2009 · Viewed 1.7M times · Source

In Python, is the following the only way to get the number of elements?

arr.__len__()

If so, why the strange syntax?

Answer

Soviut picture Soviut · Feb 5, 2009
my_list = [1,2,3,4,5]
len(my_list)
# 5

The same works for tuples:

my_tuple = (1,2,3,4,5)
len(my_tuple)
# 5

And strings, which are really just arrays of characters:

my_string = 'hello world'
len(my_string)
# 11

It was intentionally done this way so that lists, tuples and other container types or iterables didn't all need to explicitly implement a public .length() method, instead you can just check the len() of anything that implements the 'magic' __len__() method.

Sure, this may seem redundant, but length checking implementations can vary considerably, even within the same language. It's not uncommon to see one collection type use a .length() method while another type uses a .length property, while yet another uses .count(). Having a language-level keyword unifies the entry point for all these types. So even objects you may not consider to be lists of elements could still be length-checked. This includes strings, queues, trees, etc.

The functional nature of len() also lends itself well to functional styles of programming.

lengths = map(len, list_of_containers)