just curious

[Elaine Jackson]

I'm new to Python, and I've noticed the following:
[color=blue][color=green][color=darkred]
>>> def f(a,b):[/color][/color][/color]
a+=b[color=blue][color=green][color=darkred]
>>> def g(a,b):[/color][/color][/color]
a=a+b[color=blue][color=green][color=darkred]
>>> p=[1,2,3]
>>> q=[4,5,6]
>>> r=[7,8,9]
>>> s=[10,11,12]
>>> f(p,q)
>>> p[/color][/color][/color]
[1, 2, 3, 4, 5, 6][color=blue][color=green][color=darkred]
>>> g(r,s)
>>> r[/color][/color][/color]
[7, 8, 9]

Any deep reason for this, or "just because"? TIA.

Peace,
EJ

[Erik Max Francis]

Re: just curious

Elaine Jackson wrote:
[color=blue]
> Any deep reason for this, or "just because"? TIA.[/color]

It's because the augmented assignment operators, like +=, are supposed
to efficiently mutate objects when they have the opportunity (at least
for the builtin classes). So a = a + b always creates a new object, but
a += b might just mutate a preexisting object if that option is
available.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
__ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
/ \ The great floodgates of the wonder-world swung open.
\__/ Herman Melville

[Michael Peuser]

Re: just curious


"Elaine Jackson" <elainejackson7 355@home.com> schrieb im Newsbeitrag
news:rnd6b.9049 24$ro6.18233500 @news2.calgary. shaw.ca...[color=blue]
> I'm new to Python, and I've noticed the following:[/color]


(1) Python has a quite strict "call by value" similar to C. This means - as
in C) you can use formal parameters as if they were local variables. (And
there are some useful tricks with that..)
So A = A + B will have no outside impact.

(2) A += B for lists is a shortcut for
A.extend(B)
which means that it changes something A is bound to (or "points to" as you
would say in C). This is somewhat awkward because it sometimes works counter
intuitive as in your case. Just keep in mind: A+=B is *not* A=A+B but
behaves as if in most cases.... ;-)

Kindly
Michael P




[color=blue]
>[color=green][color=darkred]
> >>> def f(a,b):[/color][/color]
> a+=b[color=green][color=darkred]
> >>> def g(a,b):[/color][/color]
> a=a+b[color=green][color=darkred]
> >>> p=[1,2,3]
> >>> q=[4,5,6]
> >>> r=[7,8,9]
> >>> s=[10,11,12]
> >>> f(p,q)
> >>> p[/color][/color]
> [1, 2, 3, 4, 5, 6][color=green][color=darkred]
> >>> g(r,s)
> >>> r[/color][/color]
> [7, 8, 9]
>
> Any deep reason for this, or "just because"? TIA.
>
> Peace,
> EJ
>
>[/color]

[News M Claveau /Hamster-P]

Re: just curious

Hi !

In
def g(a,b):
a=a+b

a new object "a" is create ; but it's a local object, who are not the "a"
global.
In python, variables are "pointer to object".

If you try :
def g(a,b):
global a
a=a+b
You obtain : "name 'a' is global and local"

@-salutations
--
Michel Claveau