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209

answers:

4

Is there any case where len(someObj) does not call someObj's __len__ function?

I recently replaced the former with the latter in a (sucessful) effort to speed up some code. I want to make sure there's not some edge case somewhere where len(someObj) is not the same as someObj.__len__().

A: 

According to Mark Pilgrim, it looks like no. len(someObj) is the same as someObj.__len__();

Cheers!

inkedmn
A: 

I think the answer is that it will always work -- according to the Python docs:

__len__(self):

Called to implement the built-in function len(). Should return the length of the object, an integer >= 0. Also, an object that doesn't define a __nonzero__() method and whose __len__() method returns zero is considered to be false in a Boolean context.

Andrew Jaffe
+7  A: 

What kind of speedup did you see? I cannot imagine it was noticeable was it?

From http://mail.python.org/pipermail/python-list/2002-May/147079.html

in certain situations there is no difference, but using len() is preferred for a couple reasons.

first, it's not recommended to go calling the __methods__ yourself, they are meant to be used by other parts of python.

len() will work on any type of sequence object (lists, tuples, and all). __len__ will only work on class instances with a __len__ method.

len() will return a more appropriate exception on objects without length.

Crescent Fresh
It was about half a second on a program that ran for one minute. It's probably because I called len 2,443,519 times. As I was writing the question I realized that I should probably reduce the number of times I'm calling len.
David Locke
@David: Yeah you missed mentioning the 2,443,519 part. Holy hell ;)
Crescent Fresh
I personally wouldn't consider the extra 1/120th speedup to make it worth the code ugliness, but that's your call.
Eli Courtwright
@Eli, Normally I would agree with you. In this case I'm trying to benchmark the same problem in multiple languages.
David Locke
FYI: I was able to remove 2,363,276 of those calls to len and that sped things up by another second and a half.
David Locke
+3  A: 

If __len__ returns a length over sys.maxsize, len() will raise an exception. This isn't true of calling __len__ directly. (In fact you could return any object from __len__ which won't be caught unless it goes through len().)

Benjamin Peterson
Note that, since `len` is supposed to be return the number of elements in a collection, returning something bigger than `sys.maxsize` is almost certainly nonsense.
Mike Graham