What is a function parameter =[] for?

[Terry Reedy]

On 11/24/2015 9:34 AM, Antoon Pardon wrote:

I agree that the tutorial should talk about default argument objects 
(which have values) instead of conflating 'object' with 'value'.

> Op 20-11-15 om 13:12 schreef Ned Batchelder:

>> 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?
>
> Maybe just have us recognize that some aspects of python indeed are bizarre.

The current behavior under discussion is: default argument expressions 
are evaluated once at function creation time to produce default argument 
objects that are bound to the function object.  I do not see this as 
bizarre.

> That there is nothing wrong with him thinking so.

I do recognize that some would prefer that default argument expressions 
be treated differently, that

def f(a=expression): pass

be compiled to operate as the following does now

def f(a=None):
     a = a if a is not None else expression

perhaps with None replaced with a hidden object() instance.

I find the continuing fuss over the choice that was made to be the 
bizarre thing here.

-- 
Terry Jan Reedy