a wierd parameter passing behavior

[Terry Reedy]

"Torvo Sollazzo" <mpradella at yahoo.it> wrote in message
news:5ca99eab.0306040529.2d6f85 at posting.google.com...
> I found a strange parameter passing behavior in Python (v. 2.2.2).
> Look at this piece of code:
> def p(l) :
>     l += [3]
>     l[0] = 2
>
> def p1(l) :
>     l = l + [3]
>     l[0] = 2
...
> Why p1() is different from p()?  The list l should be passed by
> reference in both cases,

We just had a long discussion on another thread about 'pass by value'
versus 'pass by reference' versus what Python does, which is to bind
local parameter names to actual arguments.  I strongly recommend
sticking with the binding view.

> while it seems that "l = l + [3]" creates a new local variable, also
called l.

Better put: both functions have one local name (variable) 'l'.  The
first mutates the object bound to l.  The second leave the original
object alone and rebinds l to a new object.  In this case, the new
object has a different value, but that is not essential.  The new
object is not a new 'variable'.

Terry J. Reedy