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

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)

The way you take a length of anything for which that makes sense (a list, dictionary, tuple, string, ...) is to call len on it.

l = [1,2,3,4]s = 'abcde'len(l) #returns 4len(s) #returns 5

The reason for the "strange" syntax is that internally python translates len(object) into object.__len__(). This applies to any object. So, if you are defining some class and it makes sense for it to have a length, just define a __len__() method on it and then one can call len on those instances.

Just use len(arr):

>>> import array>>> arr = array.array('i')>>> arr.append('2')>>> arr.__len__()1>>> len(arr)1