In python is this the only way to get the number of elements:
arr.__len__()
If so, why the strange syntax?
+33
A:
mylist = [1,2,3,4,5]
len(mylist)
The same works for tuples:
mytuple = (1,2,3,4,5)
len(mytuple)
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. So even objects you may not consider to be lists of elements could still be length-checked. This includes strings, queues, trees, etc.
Soviut
2009-02-05 21:29:48
What's the difference between len and __len__?
Joan Venge
2009-02-05 21:30:57
len() is a language command, __len__() is a method on container types.
Soviut
2009-02-05 21:32:54
len() is a global, builtin function; __len__() is a method that object can implement. len(foo) usually ends up calling foo.__len__().
Tim Lesher
2009-02-05 21:33:34
Good one, thanks.
Joan Venge
2009-02-05 21:35:44
Also in this case, shouldn't the __len__ be prevented to be called directly? Because some people might just use it without knowing.
Joan Venge
2009-02-05 21:39:48
it doesn't matter if it gets called directly, the double underscores indicate that its a special method and basically to use at your own risk.
Soviut
2009-02-05 22:10:25
But for cleanest, or tidiness shouldn't these internal methods get hidden?
Joan Venge
2009-02-05 22:30:04
They start with double underscores - what more hiding do you want?
David Zaslavsky
2009-02-05 22:38:39
You mention that by supplying len() each container does not have to implement a .length() method, but how is this different, if each type still implements a __len__() method which gets called by len() anyways? Is different container types handled differently by len()?
Simon Jensen
2009-02-06 00:07:04
... Not debating your answer, merely a question out of interest. I'm not that familiar with the internal workings of Python.
Simon Jensen
2009-02-06 00:09:32
@David: I was thinking more like a private member so you can't access it directly.
Joan Venge
2009-02-06 17:37:03
@Joan: No, nothing in Python is inaccessible because of the dynamic nature and duck typing. Let the explicit public/private thing from Java/.NET go, it simply doesn't apply in Python. Took me a while to get used to too.
Soviut
2009-02-06 19:33:23
@Simon: len() doesn't handle them differently, its just that different containers have different ways of collecting their lengths (a tree container may only return the number of top level branches it has, etc.) and __len__ is a universal entry point for implementing this.
Soviut
2009-02-06 19:34:36
@Simon: the bit about "don't all need to implement .length()" is confusing. Container types still need to implement a method for returning their length; the point is that it's a standardized protocol, not an ad-hoc method you have to look up for each type. The double-underscores signify this.
Carl Meyer
2009-02-08 16:20:37
I've tried rewording my answer just a little to be more clear.
Soviut
2009-02-08 20:14:22
+2
A:
Just use len(arr):
>>> import array
>>> arr = array.array('i')
>>> arr.append('2')
>>> arr.__len__()
1
>>> len(arr)
1
Tim Lesher
2009-02-05 21:30:05
Usually we just assume that when a newbie says "array" she means "list"; +1 for not assuming that :)
nosklo
2009-02-06 00:03:18
I saw that, and saw that there was already an answer using list. The only reason I added an answer was in case the question really was using an array, and was about to ask, "Well, yes, but what about an array?" I upvoted Soviut's answer anyway. :-)
Tim Lesher
2009-02-09 15:45:02
+8
A:
The preferred way to get the length of any python object is to pass it as an argument to the len function. Internally, python will then try to call the special __len__ method of the object that was passed.
David Locke
2009-02-05 21:32:43
+3
A:
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.
rz
2009-02-05 21:34:00
the markup turns __len__ into bolded len. put backticks around it to make it read as code: `__len__()`.
Autoplectic
2009-02-06 00:00:51
underscores have special meaning in StackOverflow's formatting so they won't show up so you must escape. I fixed it for you, next time read your answer in the preview area before posting :)
nosklo
2009-02-06 00:01:16
+2
A:
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
UncleZeiv
2009-02-06 09:15:37