Getting the length of an array in Python
In Python, is the following the only way to get the number of elements?
arr.__len__()
If so, why the strange syntax?
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 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 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.
Python uses duck typing: it doesn't care about what an object is, as long as it has the appropriate interface for the situation at hand. When you call the built-in function len() on an object, you are actually calling its internal __len__ method. A custom object can implement this interface and len() will return the answer, even if the object is not conceptually a sequence.
For a complete list of interfaces, have a look here: http://docs.python.org/reference/datamodel.html#basic-customization
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