[Tutor] Confused about functions

[Titu Kim]

I am making these statement without refering to the
source. If i am wrong, please correct me. 
    To explain the behaviour in changer(), i believe
python by default passes changer a copy of x, but an
object reference to the list L. Thus, changer makes
change on the copy of x but changes the original list
L. 
--- Andrei Kulakov <sill@optonline.net> wrote:
> On Thu, Nov 01, 2001 at 02:08:51AM -0600, Kit
> O'Connell wrote:
> > I am working my way through Learning Python (from
> O'Reilly). I am reading
> > about functions and how they interact with
> variables (on built-in, global,
> > and local levels). The following example code is
> given:
> > 
> > def changer (x,y):
> > 	x = 2
> > 	y[0] = 'spam'
> > 
> > X = 1
> > L = [1, 2]
> > changer (X, L)
> > 
> > at the end of the code, the value of X remains 1,
> because x is only set to
> > 2 on a local level within the function. However, L
> has chaned to ['spam',
> > 2].
> > 
> > I am confused. Why does L change, but X doesn't? I
> hope someone can
> > explain this for me a bit.
> 
> Consider this:
> 
> def changer(x, y):
>     x = 1
>     y = [1,2]
> 
> X = 2
> Y = [3,4]
> changer(X, Y)
> print X, Y # prints 2, [3,4]
> 
> Parts of lists can be changed, on the fly. In your
> code, that's precisely 
> what you do. Lists can often be quite large
> (millions of items) and you
> often want to change just one item (or a few) of
> that list, instead of
> making a copy. Imagine how much more expensive it'd
> be to make a new copy
> of a million-item list and change stuff in the new
> copy, and keep the old
> one unchanged!
> 
> Now you're probably thinking, well, why doesn't the
> integer here behave
> consistently with a list, then? The reason is
> encapsulation of data -
> in a solid program, you want to pass some stuff into
> a function and get
> results. For instance:
> 
> def mult(x, y):
>   return x * y
> 
> Now, if you're thinking at the place in the code
> where you call this
> function, you'll see this:
> 
> result = mult(val1, val2)
> 
> You shouldn't need to look at the function to be
> sure that val1 and val2
> don't change. If mult changed val1, for instance,
> that'd be bad because
> when you look at the line above, it looks like you
> just pass two values
> in and get a result. This makes for a much more
> clearer program: if you
> look through code and try to figure out the flow of
> logic, you don't have
> to constantly jump to that functions code to check
> whether it modified
> a variable or not.
> 
> These behaviours are only the reasonable defaults,
> you can override both.
> 
> This would change X outside the function:
> 
> def changer(x, y):
>   global X
>   X = 1
> 
> This would keep Y list unchanged:
> 
> def changer(x, y):
>   z = y[:]  # make a copy of y list
>   z[0] = "spam"
> 
> But, you know, I'm not sure what I said here is
> exactly right.. if you know
> otherwise, please correct me!
> 
>   
> > 
> > Also, does anyone have an opinion on the Python
> Essential Referance from
> > New Riders? How does it compare to O'Reilly's
> Programming Python or their
> > Standard Library book?
> > 
> > Thanks!
> > Kit
> > 
> > -- 
> >  * Bring to me a handful of stardust,  *    Kit
> O'Connell
> > * that I can cast into my darkest night *  
> vulpine@pobox.com
> >  *	-Dream Trybe, "Come Down"      *   
> http://www.dreamspeakers.net/
> > *       http://www.dreamtrybe.com/      *  
> Austin, Texas, Planet Earth
> > 			'finger vulpine@io.com' for PGP key
> > 
> > 
> > _______________________________________________
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> 
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> 
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