var or inout parm?

[Steven D'Aprano]

On Sun, 07 Dec 2008 08:54:46 +0000, mh wrote:

> How can I make a "var" parm, where the called function can modify the
> value of the parameter in the caller?

By using another language.

> def f(x):
>     x = x + 1
> 
> n = 1
> f(n)
> # n should now be 2

Python doesn't work like that. You should read this:

http://effbot.org/zone/python-objects.htm


Some work arounds, in order from worst-to-best:


(1) Use a hard-coded global name:


x = 1

def f():
    global x
    x = x + 1

f()
assert x == 2


(2) Wrap the value you want to change in a list, then modify the list in 
place.

n = [1]

def f(alist):
    alist[0] = alist[0] + 1

f(n)
assert n[0] == 2


(4) Just use an ordinary function. Functions can return multiple values.

n = 1

def f(x):
    return (x+1, 99)

n, y = f(n)
assert y == 99
assert n == 2


(5) Find another way to solve your problem.

Why do you think you need var parameters? What problem are you hoping to 
solve by using them? As a former Pascal programmer, I missed var 
parameters at first, but now I don't.




-- 
Steven