Home Original page

call by reference (c module)

Michael Hudson mwh at python.net
Wed Nov 13 06:16:28 EST 2002 
Brian Lee <senux at senux.com> writes:

> Hi, I am newbie!
> 
> (1) call by reference
> 
> I am trying to make a simple C module. Follow is a brief example
> for my situation. How can I use `call by reference' to put argument 
> value to C module?
> 
> a = 1
> import some_c_module      
> some_c_module.function(a) # function change variable a to 2
> print a                   # I want a to be 2 (not 1)

Can't be done.  You could try reading one of these pages:

  http://effbot.org/guides/python-objects.htm
  http://starship.python.net/crew/mwh/hacks/objectthink.html

to get some understanding of why.

Actually, that's a slight lie; but

 ((PyIntObject*)ob)->ob_ival++;

is a *REALLY* bad idea: you would then have

>>> a = 1
>>> import some_c_module      
>>> some_c_module.function(a) # function change variable a to 2
>>> print a 
2
>>> 1
2 # oops!

don't even think of doing this -- I only mention this to discourage
you, so if you hadn't thought of this, forget the last few lines of my
post...

> (2) un-fixed size of arguments to a C module.
> 
> Is there any (easy) way to put un-fixed size of arguments to a
> C module? I tested PyArg_ParseTuple() but I don't know how to
> manage un-fixed size of values with it. 

Optional arguments are easy; just use the | character in the arglist
parameter to PyArg_ParseTuple.

For more flexible behaviour, noone says you have to call
PyArg_ParseTuple... it's just a convenience function.

Cheers,
M.

-- 
  My hat is lined with tinfoil for protection in the unlikely event
  that the droid gets his PowerPoint presentation working.
                               -- Alan W. Frame, alt.sysadmin.recovery
More information about the Python-list mailing list