[Python-ideas] fixing mutable default argument values

Roman Susi rnd at onego.ru
Tue Jan 30 20:29:34 CET 2007

Jan Kanis wrote:
> On Mon, 29 Jan 2007 08:38:39 +0100, Roman Susi <rnd at onego.ru> wrote:
>> This is what
>> incremental dynamic semantics is about. So, the suggestion is good only
>> as separated feature, but is IMHO wrong
>> if considered in the language design as a whole.
> wtf is incremental dynamic semantics, in this context? I did some
> googling  but all I found referred to techniques related to programming 
> environments. This proposal is just about a change in the language.

Oh, I thought this is very fundamental and well-known as it is the
second line of Guido's definition of Python:

"Python is an interpreted, object-oriented, high-level programming
language with dynamic semantics. ..."

As I understand it, it means that statements are read sequencially and
the semantics of names depends on the flow control.

The proposal (as it seems to me) wants to make change to the language
inconsistent with nature dynamic semantics. Its like put implicit:

    do this
    do that

for the same line of code (def statement's first line).

> [snip]

> Well, it's good to be clear on where the disagreements lie. However I'm 
> not yet ready to let it rest at that without some more arguments.
> As Chris pointed out in his first mail, this 'wart' is mentioned on 
> several lists of python misfeatures: [0][1][2]. I'd like to add to this 
> that even the python documentation finds this issue severe enough to
> issue  an "Important warning"[4].
> It seems clear that this behaviour is a gotcha, at least for newbies.
> This  could be excused if there is a good reason to spend the additional
> time  learning this behaviour, but some of the links state, and my

But as somebody already said the alternative is even worse...
Its quite easier to mention 3-5 Python warts up front to newbies than to
introduce subtle exception for semantics and noise words such as "new"
which do not have any other use elsewhere.

> assumption  is, that there are very few situations where re-evaluating
> causes a  problem and which isn't easily fixable.
> The semantics which I'd like to have are even easier than the current 
> semantics: everything in a function, be it before or after the colon,
> is  executed when the function is called.
> Of course, as Collin Winters pointed out, the burden of proof of
> showing  that these semantics aren't going to be a problem is still on
> the pep  proponents.

> So, are there any _other_ arguments in favour of the current semantics??

Repeat the mantra:

    Python's dynamic semantics will not change
    Python's dynamic semantics will not change
    Python's dynamic semantics will not change


> - Jan


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