On 2017 Mar 2 , at 2:53 a, Stephan Houben <stephanh42@gmail.com> wrote:
A crucial difference between a set and a type is that you cannot explicitly iterate over the elements of a type, so while we could implement
x in int
to do something useful, we cannot make
for x in int: print(x)
__contains__ was introduced to provide a more efficient test than to simply iterate over the elements one by one. I don’t see why something has to be iterable in order to implement __contains__, though. class PositiveInts(int): def __contains__(self, x): return x > 0 N = PostiveInts()
Because if we could, we could implement Russell's paradox in Python:
R = set(x for x in object if x not in x)
print(R in R)
object is not the equivalent of the paradoxical set of all sets. It’s closer to the set of all valid Python values. That includes all valid Python set values, but a Python set is not mathematical set; it’s a *finite* collection of *hashable* values.