Thread by @raymondh

#Python factlet: The len() function insists that the corresponding __len__() method return a value x such that:

0 ≤ x.__index__() ≤ sys.maxsize

* 3.0 and '3' don't have an __index__ method.
* -1 is too small.
* sys.maxsize+1 is too big.

1/

You could call __len__() successfully, but the len() function fails:

class A:
def __len__(self):
return -1

>>> a = A()

>>> a.__len__()
-1

>>> len(a)
...
ValueError: __len__() should return >= 0

2/

Classes written in C typically implement __len__() with the mp_length or sq_length slot. That constrains them to sys.maxsize limits:

>>> r = range(10**100)
>>> r.__len__()
Traceback (most recent call last):
...
OverflowError: Python int too large to convert to C ssize_t

3/

The upshot is that random's choice() function is size constrained because it calls len(seq):

>>> choice(range(10**100))
...
OverflowError: Python int too large to convert to C ssize_t

Meanwhile randrange() works fine because len() isn't called:

>>> x = randrange(10**100)

4/