On Tue, 13 Dec 2005, Mike Meyer wrote:
> You can show the same difference in behavior between Python and C (for
> example) without using a function call.
Really? You certainly don't do that with the code below.
> Here's C:
>
> #include <assert.h>
>
> main() {
> int i, *ref ;
> i = 1 ;
> ref = &i ; /* Save identity of i */
Here, ref is a reference to a variable.
> i = 2 ;
> assert(ref == &i) ;
Here, you're comparing the addresses of variables.
> }
>
> This runs just fine; i is the same object throughout the program.
>
> On the other hand, the equivalent Python:
>
>>>> i = 1
>>>> ref = id(i) # Save the identity of i
Here, ref is a reference to a value.
>>>> i = 2
>>>> assert ref == id(i)
Here, you're comparing values.
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> AssertionError
>>>>
>
> Blows up - i is no longer the same object.
Right, because the two bits of code are doing quite different things.
> Python does call by reference, which means that it passes pointers to
> objects by value.
That's not what call by reference is - call by reference is passing
pointers to *variables* by value.
> C is call by value, faking call by reference by passing reference
> values. The real difference is that in C, you can get a reference to a
> variable to pass, allowing you to change the variable. In python, you
> can't get a reference to a name (one of the reasons we call them "names"
> instead of "variables"), so you can't pass a value that will let the
> called function change it.
Kinda. Here's a python translation of Steven's incrementing function
example:
def increment(n):
"""Add one to the argument changing it in place."""
# python (rightly) doesn't have references to variables
# so i will use a 2-tuple (namespace, name) to fake them
# n should be such a 2-tuple
n_namespace = n[0]
n_name = n[1]
n_namespace[n_name] += 1
x = 1
increment((locals(), "x"))
assert x == 2
This is an evil, festering, bletcherous hack, but it is a direct
translation of the use of pass-by-reference in C.
As a bonus, here's a similarly literal python translation of your C
program:
>>> i = 1
>>> ref = "i"
>>> i = 2
>>> assert ref == "i"
tom
--
The sky above the port was the colour of television, tuned to a dead
channel
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