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


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


If so, why the strange syntax?

8/25/2018 2:21:18 AM

Accepted Answer

my_list = [1,2,3,4,5]
# 5

The same works for tuples:

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

And strings, which are really just arrays of characters:

my_string = 'hello world'
# 11

It was intentionally done this way so that lists, tuples and other container types 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)
4/18/2018 6:22:14 PM

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 4
len(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.

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow