By value or by reference?

Michael Hoffman m.h.3.9.1.without.dots.at.cam.ac.uk at example.com
Mon Oct 18 19:32:11 CEST 2004


Jonathan Ellis wrote:

>>By reference to an object....See the python tutorial.
> 
> Wrong.  Here is the difference between pass by reference and pass by
> value to CS types:

Actually it is a reference to an object being passed.

>>>>def foo(a): a = 1
> 
> ...
> 
>>>>i = 10
>>>>foo(i)
>>>>print i
> 
> With pass-by-reference, i would now be 1.  However, since python is
> pass-by-value, it remains 10.

 >>> def foo(a):
...     print id(a)
...
 >>> i = 10
 >>> id(i)
168377924
 >>> foo(i)
168377924

With pass-by-value, the memory location of a would be different than the 
memory location of i. However, since Python is pass-by-reference, it 
remains the same. <wink>

Alex is right that trying to shoehorn Python into a "pass-by-reference" 
or "pass-by-value" paradigm is misleading and probably not very helpful. 
In Python every variable assignment (even an assignment of a small 
integer) is an assignment of a reference. Every function call involves 
passing the values of those references.
-- 
Michael Hoffman



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