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Date: Sun, 6 Jan 2013 00:27:19 +1100
Subject: Re: pygame - importing GL - very bad...
From: Chris Angelico <rosuav@gmail.com>
To: python-list@python.org
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On Sun, Jan 6, 2013 at 12:06 AM, someone <newsboost@gmail.com> wrote:
> On 01/05/2013 12:47 PM, Chris Angelico wrote:
>> You can find good references on the subject in various
>> places, but call-by-reference as implemented in Pascal simply doesn't
>> exist in most modern languages, because its semantics are way
>> confusing. The name given to this technique varies; here's a couple of
>> links:
>>
>> http://effbot.org/zone/call-by-object.htm
>> http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing
>
> If you don't like calling it "pass-by-reference", perhaps you prefer call=
ing
> it: =93call by object reference=93...  From:
> http://effbot.org/zone/call-by-object.htm
>
> In any case I think we understand each other.

That's one of the links I just posted :) It's not just a naming
difference, though. With Pascal's pass-by-reference semantics, this
code would act differently:

def foo(x):
  x =3D 5

a =3D 2
foo(a)
print(a)

Python prints 2, because the assignment to x just rebinds the name
inside foo. True pass-by-reference actually changes the caller's
variable. C can achieve this by means of pointers; in Python, you can
pass and mutate a list, thus:

def foo(x):
  x[0] =3D 5 # Dereference the pointer, kinda

x=3D[None] # Declare a pointer variable, ish
x[0] =3D 2
foo(x) # Don't forget to drop the [0] when passing the pointer to
another function
print(x[0]) # Prints 5. See? We have pass-by-reference!

But otherwise, rebinding names in the function has no effect on
anything outside. Among other things, this guarantees that, in any
situation, a name referencing an object can be perfectly substituted
for any other name referencing the same object, or any other way of
accessing the object.

def foo(lst):
  lst[0]=3Dlen(lst)

x =3D [10,20,30]
y =3D x
foo(x) # These two...
foo(y) # ... are identical!

This is a philosophy that extends through the rest of the language. A
function returning a list can be directly dereferenced, as can a list
literal:

def foo():
  return [0,1,4,9,16]

print( ["Hello","world!","Testing","Testing","One","Two","Three"][foo()[2]]=
 )

This is a flexibility and power that just doesn't exist in many older
languages (C and PHP, I'm looking at you). Object semantics make more
sense than any other system for a modern high level language, which is
why many of them do things that way.

ChrisA