Re: [pygame] Problems with getting keyboard input. [identity vs equality]

[Jonathan Atkinson <jonathan_atkinson@yahoo.com>]

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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