[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[George Sakkis]

On Wed, May 13, 2009 at 10:41 PM, Bruce Leban <bruce at leapyear.org> wrote:
>
> On Wed, May 13, 2009 at 5:10 PM, George Sakkis <george.sakkis at gmail.com>
> wrote:
>>
>> On Wed, May 13, 2009 at 7:08 PM, Terry Reedy <tjreedy at udel.edu> wrote:
>>
>> > Bruce Leban wrote:
>> >> def myfunc(a, b, c = *lambda: expression):
>> >>  stuff
>>
>> > There is a proposal, which I thought was accepted in principle, to make
>> > '*
>> > seq' valid generally, not just in arg-lists. to mean 'unpack the
>> > sequence'.
>> > * (lambda:1,2)() would then be valid, and without the call, would be a
>> > runtime, not syntax error.
>>
>> Then how about putting the * before the parameter ?
>>
>>    def myfunc(a, b, *c = lambda: expression):
>>
>> It's currently a syntax error, although the fact that "*arg" and
>> "*arg=default" would mean something completely different is
>> problematic. Still the same idea can be applied for some other
>> operator (either valid already or not).
>
>
> I think similar syntax should do similar things. If *arg means one thing and
> &arg means something else, that's confusing.

Confusing?? It's certainly no more confusing than using *args for one
thing and **args for something else.

>> Subproposal (1): Get rid of the explicit lambda for dynamic arguments.
>> That is,
>>
>>    def myfunc(a, b, *x=[]):
>>
>> would be equivalent to what previous proposals would write as
>>
>>    def myfunc(a, b, *x=lambda: []):
>
> Explicit is better than implicit. There's a thunk getting created here,
> right? Don't you want that to be obvious? I do.

Explicitness is in the eye of the beholder, and also an acquired
taste. @decorator is less explicit than f = decorator(f) and yet it's
generally considered a successful addition these days, despite the
strong opposition it met initially.

>> Subproposal (2): If subproposal (1) is accepted, we could get for free
>> (in terms of syntax at least) dynamic args depending on previous ones.
>> That is,
>>
>>    def myfunc(a, b, *m=(a+b)/2):
>>
>> would mean
>>
>>    def myfunc(a, b, *m = lambda a,b: (a+b)/2):
>>
>> with the lambda being passed the values of a and b at runtime.
>
> It's not free and that adds quite a bit of complexity. Note that default
> parameters are evaluated in the context of where the function is defined,
> NOT in the middle of setting the other function parameters. (That's true for
> this new case too.)

Sure, but I'm not sure what's your point here. The compiler can
generate bytecode to the effect of:

    def myfunc(a, b, *m = (a+b)/2):
        if m is not passed:
            m = default_m(a,b)
        # actual body follows

Ideally an optimizer would further check whether it's safe to inline
the expression (which should be the typical case) to avoid the
function call overhead.

> Sure Lisp has let and let* but the proposal here is NOT to provide arbitrary
> computational ability in the parameter list but to provide a way to have
> defaults that are not static. We shouldn't over-engineer things just because
> we can.

Agreed, the primary target is to fix the common gotcha of mutable
defaults, and I'd rather see this handled than nothing at all. A
secondary goal that can be achieved here though is to reduce the
overusage of None (or other sentinels for that matter). There are
several articles to the effect of "Null considered harmful"; also SQL
as well as some statically typed languages provide both nullable and
non-nullable types and assume the latter by default. The only useful
operation to a sentinel is identity check, so you know that every `x =
sentinel` assignment should be followed by one or more `if x is
sentinel` checks. Fewer nulls means potentially less conditional logic
mental overhead. I'm not claiming we should get rid of None of course;
there are legitimate reasons for different behavior under different
conditions. Here however we're talking about a very specific pattern:
    def f(a, b=sentinel):
        if b is sentinel: b = <expression>

It may not seem such a big deal, but then again I don't think it's
less trivial than the case for the (eventually) accepted ternary
operator vs an if/else statement.

George