boolean xor

[Aahz Maruch]

In article <93i7ne$5dk$1 at nnrp1.deja.com>,  <ndev42 at yahoo.com> wrote:
>Nicolas removed that attribution for Aahz:
>>
>> I need a boolean (not bit-wise) xor function.  I've got the following
>> function, but my logic skills are rusty enough that I'm wondering if
>> this is the "right" way to do it (I've validated that it produces the
>> correct outputs, at least):
>>
>> def xor(a,b):
>>     return not ( (a and b) or (not (a or b)) )
>
>A boolean xor is a detector of differences. Knowing that, you can
>simply write:
>
>def xor(a,b):
>	return not(a==b)

That works only if the test is against equality, not "truthfullness" by
Python's standards.  For example, I was using my xor() function this
way:

class Token:
    def __init__(self, URL=None, Shutdown=None):
        if not xor(URL, Shutdown):
            raise "Must assign one of URL or Shutdown"

In this scenario, one would expect URL to be a string and Shutdown to be
an integer; comparing for equality would be useless.
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