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From: Robert Kern on 5 Mar 2010 20:08 On 2010-03-05 17:48 PM, Jack Diederich wrote: > On Fri, Mar 5, 2010 at 6:09 PM, Steven D'Aprano > <steve(a)remove-this-cybersource.com.au> wrote: >> On Fri, 05 Mar 2010 15:58:01 -0500, Jack Diederich wrote: >> >>>>> So, the pythonic way to check for True/False should be: >>>>> >>>>>>>> 1 is True >>>>> False >>>> >>>> Why do you need to check for True/False? >>>> >>>> >>> You should never check for "is" False/True but always check for >>> equality. The reason is that many types support the equality (__eq__) >>> and boolen (__bool__ in 3x) protocols. If you check equality these will >>> be invoked, if you check identity ("is") they won't. >> >> Never say never. >> >> If you specifically want to test for True or False themselves, accepting >> no substitutes, then using "is" is the obvious way, and using "==" is >> clearly and obviously wrong because it does accept substitutes: >> >>>>> 1.0 == True >> True >>>>> decimal.Decimal(0, 1) == False >> True > > > Yes, obviously if you _really_ mean to test if something has the > object identity of True or False then an "is" test is the way to go. > I'm just not sure why you would ever do that. Also, I'm not sure how > your assertion matches up with the examples; The examples test for > equality with a float that returns true for __eq__ and a Decimal that > returns false for __eq__. No, both comparisons return True. Decimal(0,1) is equal in value to 0 (and thus False). Comparing it to False using __eq__ returns True. > Both "1.0" and "Decimal(0, 1)" will return > False if the test is "is True" or "is False." Yes. That is exactly what he is asserting. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Bruno Desthuilliers on 8 Mar 2010 03:53
Rolando Espinoza La Fuente a écrit : > On Fri, Mar 5, 2010 at 2:32 PM, mk <mrkafk(a)gmail.com> wrote: >> Arnaud Delobelle wrote: >> >>>>>> 1 == True >>> True >>>>>> 0 == False >>> True >>> >>> So what's your question? >> Well nothing I'm just kind of bewildered: I'd expect smth like that in Perl, >> but not in Python.. Although I can understand the rationale after skimming >> PEP 285, I still don't like it very much. >> > > So, the pythonic way to check for True/False should be: > >>>> 1 is True > False > >>>> 0 is False > False > > instead of ==, right? Nope. The pythonic way is to check for truth value - not for True or False -, and to only use the identity test when wanting to test for identity. |