Terry Reedy wrote: > Roger wrote: >>> And, just for completeness, the "is" test is canonical precisely because >>> the interpreter guarantees there is only ever one object of type None, >>> so an identity test is always appropriate. Even the copy module doesn't >>> create copies ... >>> >> >> Does the interpreter guarantee the same for False and True bools? > > Yes. Bool(x) should return one of the two existing instances. > > In 2.x, the *names* 'True' and 'False' can be rebound because bool is > new and people write > try: > False,True > except NameError: > False,True = 0,1 > > to make code back compatible. > I would claim that the ability to rebind True and False is a simple bug, though one not likely to be fixed in an 2.x release. The code above doesn't rebind True and False in interpreters that have them ...
> In 3.0, the names are keywords, just like 'None' and cannot be rebound, > so x is True is guaranteed to answer whether x *is* the true object. > > Back before rebinding 'None' was prohibited, 'is None' was not > completely guaranteed either (absent reading the rest of a file to be > sure no rebinding would be done). > And that was a bug too, in this case one that *was* removed in 2.4, I believe. Don't have 2.3 lying around just now. Python 2.4.3 (#1, May 24 2008, 13:47:28) [GCC 4.1.2 20070626 (Red Hat 4.1.2-14)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> None = 3 SyntaxError: assignment to None regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
