Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Oscar Benjamin <oscar.j.benjamin@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: What is a function parameter =[] for?
Date: Thu, 19 Nov 2015 18:44:50 +0000
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On 19 November 2015 at 18:19, BartC <bc@freeuk.com> wrote:
>>>
>>> if you write A=B then something of B needs to have been copied into
>>> A, even if it's just the reference that B contains. Otherwise it would be
>>> difficult to get A to refer to the same object as B.
>>
>> Please, PLEASE, go and read/watch Ned's PyCon talk (the one I linked
>> you to earlier). Don't post another word on this subject until you
>> comprehend what he is saying.
>
> I looked through the long article (I don't remember seeing a link to a
> video), and followed it up to about 3/4 of the way through, then it got a
> bit heavy.
>
> But what is it about my remarks above that isn't right?

It's not necessarily incorrect (just focussing on the remark quoted
above) but you'll find it easier to understand the model if you adopt
the same terminology that is usually used. When talking of "copying"
in a Python context something different is implied than what happens
with A=B.

The statement A=B (assuming A and B are names and not expressions)
simply binds the name A to the same object that the name B is bound
to. In the implementation of CPython the names are associated with
pointers and the value of the pointer associated with B is copied to
the pointer associated with A. Another implementation may not use
pointers but there will in some way be a "reference" that is "copied"
over if you like to view it that way.

However in a Python context when someone talks of "copying" they will
mean something different: creating a new object which is distinct
from, but has the same value as, some previously existing object. It
is important in Python to distinguish between operations that
create/mutate objects and operations that rebind names. This
distinction has semantic implications that are not hard to understand
if you think about them carefully and consistent use of terminology
really helps.

--
Oscar