Pass by reference

alex goretoy aleksandr.goretoy at gmail.com
Thu Jan 1 10:25:16 CET 2009


I recently conquered this pass by ref thing. This is how I did it.

Python 2.4.3 (#1, Apr  3 2006, 14:02:53)
[GCC 3.4.6] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def f1(v):
...     v="asdf"
...
>>> def f2(v):
...     v=["asdf"]
...
>>> def f3(v):
...     v.append("asdf")
...
>>> a="fdsa"
>>> b=["fdsa"]
>>> f1(a)
>>> a
'fdsa'
>>> f1(b)
>>> b
['fdsa']
>>> f2(a)
>>> a
'fdsa'
>>> f2(b)
>>> b
['fdsa']
>>> f3(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in f3
AttributeError: 'str' object has no attribute 'append'
>>> f3(b)
>>> b
['fdsa', 'asdf']
>>>
if you only want one value in there, you can pop off the first value and
then append.
def f4(v):
    _old=v.pop()
    v.append("asdf")

you can then also turn it back into a string by doing this:

b[0]

I was very glad to have learned this way to do by ref. Thanks.

On Wed, Dec 31, 2008 at 11:32 PM, Aaron Brady <castironpi at gmail.com> wrote:

> On Dec 31, 5:30 am, iu2 <isra... at elbit.co.il> wrote:
> > Hi,
> >
> > Is it possible somehow to change a varible by passing it to a
> > function?
> >
> > I tried this:
> >
> > def change_var(dict0, varname, val):
> >   dict0[varname] = val
> >
> > def test():
> >   a = 100
> >   change_var(locals(), 'a', 3)
> >   print a
> >
> > But test() didn't work, the value a remains 100.
> >
> > I have several variables initialized to None.
> > I need to convert each one of them an object only if it is None.
> > something like:
> >
> > if not var1: var1 = MyObject()
> >
> > I want this to be a function, that is:
> >
> > def create_obj(var):
> >   if not var: var = MyObj()
> >   # set properties of var
> >
> > Now, I know I can achieve this by functional programming,
> >
> > def create_obj(var):
> >   if not var:
> >     x = MyObj()
> >     # set properties of x
> >     return x
> >   return var
> >
> > and then
> >
> > var = creaet_obj(var)
> >
> > Is there another way?
> >
> > Thanks
>
> A practical way is to use a container.  Some people use lists; I like
> an object.
>
> thingref= Ref( thing )
> f( thingref )
> print thingref() #or thingref.get() or w'ver.
>
> Then 'f' can assign like this:
>
> def f( aref ):
>  # blah blah
>  aref( newthing ) #or aref.set( newthing )
>
> But the short answer is no.  A function receives the contents of a
> variable, not a variable.
> --
> http://mail.python.org/mailman/listinfo/python-list
>



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