Python's "in" set operator

Yes, but it also means hash(b) == hash(x), so equality of the items isn't enough to make them the same.

That's right. You could try it in the interpreter like this:

>>> a_set = set(['a', 'b', 'c'])

>>> 'a' in a_set
True

>>>'d' in a_set
False

Yes it can mean so, or it can be a simple iterator. For example: Example as iterator:

a=set(['1','2','3'])
for x in a:
 print ('This set contains the value ' + x)

Similarly as a check:

a=set('ILovePython')
if 'I' in a:
 print ('There is an "I" in here')

edited: edited to include sets rather than lists and strings

Strings, though they are not set types, have a valuable in property during validation in scripts:

yn = input("Are you sure you want to do this? ")
if yn in "yes":
    #accepts 'y' OR 'e' OR 's' OR 'ye' OR 'es' OR 'yes'
    return True
return False

I hope this helps you better understand the use of in with this example.