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Re: [pygame] Problems with getting keyboard input. [identity vs equality]
As a follow-up, I've taken my code, made it half-useful and created a little demo with it; do you think this would be suitable for the PyGame source repository as an example of handling keyboard input?
--jon
On Tue, 25 Jun 2002 19:22:20 +0000
Jonathan Atkinson <jonathan_atkinson@yahoo.com> wrote:
> Danny,
>
> Thanks for clearing that up :-)
>
> --jon
>
> On Tue, 25 Jun 2002 12:05:10 -0700 (PDT)
> Danny Yoo <dyoo@hkn.eecs.berkeley.edu> wrote:
>
> >
> >
> > On Tue, 25 Jun 2002, Jonathan Atkinson wrote:
> >
> > > Thanks, that's helped. I guess it's a syntactical thing then: what use
> > > does 'is' serve? I assumed it was directly equivalent to '==', but it
> > > looks like I was wrong. Secondly, why does using 'is' work with K_ESCAPE
> > > and K_SPACE but not with K_UP?
> >
> > All things in Python are objects, even integers. When we want to check if
> > two objects are equal, we should use '==', but if we want to see if
> > they're the same object --- identical --- then we should use 'is' instead.
> >
> >
> > The difference is somewhat subtle, but here's an example with lists that
> > might clear it up:
> >
> > ###
> > >>> mylist1 = [3, 1, 4, 1, 5, 9]
> > >>> mylist2 = [3, 1, 4, 1, 5, 9]
> > >>> mylist1 == mylist2
> > 1
> > >>> mylist1 is mylist2
> > 0
> > ###
> >
> > 'mylist1' and 'mylist2' are distinct lists, so the 'is' check says that
> > they're not identical, even though they're "equal". But now let's try
> > something else:
> >
> > ###
> > >>> mylist3 = mylist2
> > >>> mylist2 == mylist3
> > 1
> > >>> mylist2 is mylist3
> > 1
> > ###
> >
> > Here, 'is' tells us that mylist2 and mylist3 are naming the same list.
> > This is significant because if we make changes to the object named by
> > mylist2, we'll see that "change" in mylist3, but only because they both
> > name the same object:
> >
> > ###
> > >>> mylist2.append(2)
> > >>> mylist1
> > [3, 1, 4, 1, 5, 9]
> > >>> mylist2
> > [3, 1, 4, 1, 5, 9, 2]
> > >>> mylist3
> > [3, 1, 4, 1, 5, 9, 2]
> > ###
> >
> >
> >
> >
> > Going to your question, K_UP is a numerical code for 273:
> >
> > ###
> > >>> pygame.locals.K_UP
> > 273
> > ###
> >
> > and if we're curious, we can also lookup the values of K_SPACE and
> > K_ESCAPE:
> >
> > ###
> > >>> pygame.locals.K_SPACE
> > 32
> > >>> pygame.locals.K_ESCAPE
> > 27
> > ###
> >
> > There's one main difference between K_UP and the other two constants, and
> > it's a silly-but-significant observation: K_UP is larger than the other
> > two numbers!
> >
> >
> > It turns out that all numbers that are less than around 100 are cached by
> > Python, so that there's only one copy of each small number throughout the
> > Python system. This allows 'is' to work perfectly well on small integers,
> > but not on larger values:
> >
> > ###
> > >>> 90 is 89 + 1
> > 1
> > >>> 273 == 272+1
> > 1
> > >>> 273 is 272+1
> > 0
> > ###
> >
> >
> >
> >
> > > It seems a little illogical, unless I'm missing something.
> >
> > It's not illogical; it's just that I think 'is' is not the right
> > comparison operation in almost all cases.
> >
> >
> > Hope this helps!
> >
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keypresses.py
dude.gif