On Sun, 25 Dec 2011 15:45:10 -0800, Larry Hudson wrote: > On 12/24/2011 11:09 PM, GZ wrote: >> Hi, >> >> I run into a weird problem. I have a piece of code that looks like the >> following: >> >> f(...., a=None, c=None): >> assert (a==None)==(c==None) >> > <...> > > At first glance this looked like it should be a simple boolean "and", > but then I realized that when a and c are both unequal to None, the > result would also be True. This implies the logical approach would be > exclusive-or (^). Try this expression: > > not ((a==None) ^ (c==None))
^ is *bitwise* xor, not boolean xor. Python doesn't offer boolean xor directly, although != comes close. > OTOH, if what you really want is simply to check that both are None (my > first impression), then it's simply: > > (a==None) and (c==None) Replace == with 'is'. > Most of the replies you're getting here seem unnecessarily complicated. == is a more complicated operator than the 'is' operator. That's why the is operator is to be preferred when testing for None -- it is guaranteed to do the right thing, while == is not. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
