On 19/11/2015 18:19, BartC wrote:
On 19/11/2015 17:45, Chris Angelico wrote:
On Fri, Nov 20, 2015 at 4:30 AM, BartC <b...@freeuk.com> wrote:
The whole concept of 'mutable' default is alien to me. A default is
just a
convenient device to avoid having to write:
fn(0) or fn("") or fn([])
You just write fn() instead. But it shouldn't come at the cost of
completely
different semantics! Because then it can't really be called a default
value
at all.
Do you understand the concept of "mutable objects"? Don't even try to
discuss mutable function defaults until you do.
Yes. In the languages I create, pretty much everything is mutable,
provided it can supply an l-value. Constructs such as those for empty
lists ([] in Python, () in mine) aren't l-values.
But it doesn't apply default values. The language is very different
however, because the byte-code compiler always has full knowledge of
functions that are called directly. Then it knows when an argument is
omitted, and can simply substitute the expression used as the default.
If your language simply has no mutable objects, then sure, it's easy
to understand function defaults! There's no such thing as early or
late binding; in fact, you can even super-late-bind, where you don't
actually call a function until its *return value* is being used (as
long as that function has no side effects). But as soon as objects are
capable of retaining their identities while changing their values, you
need an object model that (a) distinguishes between identity and
value, and (b) allows some definition of early or late binding,
because it *will* matter.
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?
To summarize, it once again shows that you haven't got the faintest idea
what you're talking about. You're now in a very exclusive club with the
RUE and Nick the Greek, the world's leading webmaster.
--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.
Mark Lawrence
--
https://mail.python.org/mailman/listinfo/python-list
