What is a function parameter =[] for?

[BartC]

On 20/11/2015 12:28, Marko Rauhamaa wrote:
> BartC <bc at freeuk.com>:

>> We're arguing at cross-purposes then since you are obviously
>> interested in these esoteric aspects,
>
> The Ackermann function really is an esoteric example, but the other
> example that has been discussed here can make practical use of the
> default-value semantics:

And you go on to give a couple of slightly less esoteric examples! (I 
don't think I've ever written a lambda function in my life.)

> Even more appropriately, you may expressly want a mutable, singleton
> object to be the default:
>
>     def initiate_query(query, database=global_database):

Finally, a down-to-earth example. Here it probably doesn't matter at 
what point 'global_database' gets bound. You know it will always refer 
to the current state of global_database, and you know that it is a data 
structure external to the function even if it is updated from inside.

That is a bit different from an empty list that later is not empty.

>     def asklist(caption, data, n=omitted, rows=omitted, width=omitted,
>                 flags=omitted, buttons=omitted, tablist=omitted,
>                 heading=omitted):
>
> but that would be rather pedantic in most circumstances.

Yes, that's along the lines of what I used do before I had proper 
default values!

Then I had code inside the function to test a parameter and set it as 
needed. I still do this when the default isn't straightforward (where it 
depends on other parameters for example).

-- 
Bartc