[Tutor] Confused about functions
Titu Kim
kimtitu@yahoo.com
Thu, 1 Nov 2001 14:31:49 -0800 (PST)
So, can i conclude that in local namespace of
changer(), object reference of 'x' is altered to point
to integer 1, while object reference of 'X' in global
namespace still points to integer 2? If i define my
changer as 'changer(X,Y)', changer() still behaves the
same way because 'X' in changer() and 'X' in global
reside at different momory location(although same
name) but they point to same memory location of
integer '2'. Once i changer laters object reference of
local 'X' to point to integer '1', then at this point
two 'X's refer to different memory locations. Am
thinking in right direction? Please correct me.
Regards,
Kim Titu.
--- Andrew Wilkins <toodles@yifan.net> wrote:
> > 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.
>
> In section 4.6 Defining Functions of the Python
> tutorial:
> "The actual parameters (arguments) to a function
> call are introduced in the
> local symbol table of the called function when it is
> called; thus, arguments
> are passed using call by value (where the value is
> always an object
> reference, not the value of the object)."
>
> The objects are both passed in by reference. The
> difference is (as Remco,
> Kalle and Andrei pointed out) in the use of the
> assignment operator and the
> fact that "x" refers to a name and y[0] refers
> directly to an object.
>
> In the case of
> x=1
> the integer 1 is given an alias of "x", but only in
> the local namespace.
> Think of it this way: "X" is in the global
> namespace, referring to the
> integer 2. When X is passed to changer, a reference
> is created and bound to
> the alias "x" in the local namespace. When "x=1" is
> evaluated, rather than
> assigning 1 to the value of "x" it is rebinds "x" to
> refer to the integer 1.
> Given that the object reference was changed and the
> value of the object "x"
> was originally referring to wasn't changed, the
> value of "X" will remain the
> same in the global namespace.
>
> In the case of
> y[0]='spam'
> because y[0] is not a name but an object, the value
> of object that y and Y
> will change.
>
> ERGH that sounds really bad I hope you can
> understand me...
>
> Regards,
> Andrew
>
> > --- 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
> > > >
> > > >
>
=== message truncated ===
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