"pass by reference?"
bokr at oz.net
Sat Feb 23 23:01:18 EST 2002
On Sat, 23 Feb 2002 15:34:01 +0700, Tripp Scott <tripps81 at yahoo.com> wrote:
>Suppose I want to make a function that modifies its argument:
> a = -1
> print a # = 1
>What is the Pythonic way of doing this? Wrap it in a list?
Well, if you really want to[**], I think you might want to use
the list wrapper a little differently:
>>> a = [-1]
>>> def make_abs(x):
... x = abs(x)
but be careful how you bind to mutable things, as all bindings
to the same thing will show a change to the thing bound to:
>>> a = b = [-1]
note the different result here:
>>> a = [-1]
>>> b = [-1]
You can also define your own mutable object with a class,
as mentioned elsewhere.
USING A CLASS IS PROBABLY A BETTER IDEA <<<< [**]notice ;-)
a=A(-1) and a.v = -1 or a.setval(-1) are probably less misleading than
a = -1 or a = [-1].
Though it might be nice Python had an object update operator, so you
wouldn't have to know method or attribute names to do a default
state update operation. E.g. (using a ':=' token for the operator),
a := -1 #read as 'a updated with -1'
could translate to a.__update__(-1), which you could define
or override as desired. Incidentally, this could be an expression
evaluating to a reference to the updated object by returning self
from the method, so it could be used flexibly.
Though it introduces ambiguities, I sometimes think I'd like
to be able to define a __getval__ method that would be called
by a simple reference to the object in an expression. E.g.,
x = y
might become x = y.__getval__() so instead of x binding to the
y object instance, it would get whatever y was defined to deliver
as a default value. It would mean that you couldn't bind to y
as an object, unless __getval__() returned self, or you provided
another method to do that, as in z = y.it_self() or such.
With __getval__, x.y and x .y would probably mean different things.
I.e., x.__getattr__('y') vs x.__getval__().__getattr__('y')
It could obviously be abused, but could be useful too.
More information about the Python-list