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)