What is a function parameter =[] for?

[BartC]

On 20/11/2015 12:12, Ned Batchelder wrote:
> On Friday, November 20, 2015 at 6:59:54 AM UTC-5, BartC wrote:
>> On 20/11/2015 01:05, Steven D'Aprano wrote:
>>> On Fri, 20 Nov 2015 04:30 am, BartC wrote:
>>>
>>>> On 19/11/2015 16:01, Steven D'Aprano 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([])
>>>
>>> Says who?
>>
>> People who want to avoid having to write:
>>
>>        fn(0) or fn("") or fn([])
>
> I think we all understand by now that you are unhappy with what happens
> in Python with mutable defaults.  We get it.  You are not alone. Lots of
> people are surprised by this.  But it is how Python works.
>
> I'm not sure what your goal is at this point.  Are you:
>
>    1) still unsure what the behavior is, or
>    2) trying to understand why it behaves that way, or
>    3) hoping to change Python, or
>    4) trying to convince us that your language is better, or
>    5) something else?

Mostly 5. (With a bit of 4. However that language is not something I'm 
selling or pushing. It's just something different used as a contrast.).

What's bothering me is:

* The large amount of mumbo-jumbo used to describe what's going on

* The insistence (I think largely from Steven) that the way this feature 
works is good rather than bad

* The refusal to acknowledge that the def fn(a=[]) syntax is misleading. 
(What value will a have when you call fn()? The true answer is that you 
can't tell.)

* The persistent nonsense that somehow [] is mutable (what happens is 
that [] is assigned to a variable, and /that/ is mutable) (And I will 
probably get some flak now because 'assign' and 'variable' are 
meaningless in Python!)

* And above, where I lay out /why/ people want to use the feature, so 
that the function can take care of some details, even that is refuted.

-- 
Bartc