Question

1. why would anyone use double underscores
2. why not just do len([1,2,3])?

my question is specifically What do the underscores mean?

Answer 1

The one reason I’ve had to use x.__len__() rather than len(x) is due to the limitation that len can only return an int, whereas __len__ can return anything.

You might quite reasonably argue that only an integer should be returned, but if the length of your object is greater than sys.maxsize then you have no choice except to return a long instead:

>>> class A(object):
...     def __len__(self):
...         return 2**80
...
>>> a = A()
>>> len(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to int
>>> a.__len__()
1208925819614629174706176L

This isn’t just a theoretical limitation, I’ve done a fair bit of work with bit containers where (with 32-bit Python) the len function stops working when the object reaches just 256MB.