On Mon, Oct 25, 2021 at 11:53 PM Marc-Andre Lemburg <m...@egenix.com> wrote:
>
> On 25.10.2021 14:26, Chris Angelico wrote:
> > On Mon, Oct 25, 2021 at 11:20 PM Marc-Andre Lemburg <m...@egenix.com> wrote:
> >>
> >> On 25.10.2021 13:53, Chris Angelico wrote:
> >>> On Mon, Oct 25, 2021 at 10:39 PM Marc-Andre Lemburg <m...@egenix.com> 
> >>> wrote:
> >>>> I would prefer to not go down this path.
> >>>>
> >>>> "Explicit is better than implicit" and this is too much "implicit"
> >>>> for my taste :-)
> >>>>
> >>>> For simple use cases, this may save a few lines of code, but as soon
> >>>> as you end up having to think whether the expression will evaluate to
> >>>> the right value at function call time, the scope it gets executed
> >>>> in, what to do with exceptions, etc., you're introducing too much
> >>>> confusion with this syntax.
> >>>
> >>> It's always possible to be more "explicit", as long as explicit means
> >>> "telling the computer precisely what to do". But Python has default
> >>> arguments for a reason. Instead of simply allowing arguments to be
> >>> optional, and then ALWAYS having code inside the function to provide
> >>> values when they are omitted, Python allows us to provide actual
> >>> default values that are visible to the caller (eg in help()). This is
> >>> a good thing. Is it "implicit"? Yes, in a sense. But it's very clear
> >>> what happens if the argument is omitted. The exact same thing is true
> >>> with these defaults; you can see what happens.
> >>>
> >>> The only difference is whether it is a *value* or an *expression* that
> >>> defines the default. Either way, if the argument is omitted, the given
> >>> default is used instead.
> >>
> >> I guess I wasn't clear enough. What I mean with "implicit" is that
> >> execution of the expression is delayed by simply adding a ">" to
> >> the keyword default parameter definition.
> >>
> >> Given that this alters the timing of evaluation, a single character
> >> does not create enough attention to make this choice explicit.
> >>
> >> If I instead write:
> >>
> >> def process_files(processor, files=deferred(os.listdir(DEFAULT_DIR))):
>
> def process_files(processor, files=deferred("os.listdir(DEFAULT_DIR)")):
>
> @deferred(files="os.listdir(DEFAULT_DIR)")

Ahhh, okay. Now your explanation makes sense :)

This does deal with the problem of function calls looking like
function calls. It comes at the price of using a string to represent
code, so unless it has compiler support, it's going to involve eval(),
which is quite inefficient. (And if it has compiler support, it should
have syntactic support too, otherwise you end up with weird magical
functions that don't do normal things.)

> > It's also extremely verbose, given that it's making a very small
> > difference to the behaviour - all it changes is when something is
> > calculated (and, for technical reasons, where; but I expect that
> > intuition will cover that).
>
> It is verbose indeed, which is why I still think that putting such
> code directly at the top of the function is the better way
> to go :-)

That's what I want to avoid though. Why go with the incredibly verbose
version that basically screams "don't use this"? Use something much
more akin to other argument defaults, and then it looks much more
useful.

> That's fair, but since the late binding code will have to sit at
> the top of the function definition anyway, you're not really saving
> much.
>
>     def add_item(item, target=>[]):
>
> vs.
>
>     def add_item(item, target=None):
>         if target is None: target = []

It doesn't always have to sit at the top of the function; it can be
anywhere in the function, including at the use site. More importantly,
this is completely opaque to introspection. Tools like help() can't
see that the default is a new empty list - they just see that the
default is None. That's not meaningful, that's not helpful.

It also pollutes the API with a fake argument value, such that one
might think that passing None is meaningful. If, in the future, you
change your API to have a unique sentinel object as the default,
people's code might break. Did you document that None was an
intentional parameter option, or did you intend for the default to be
"new empty list"? For technical reasons, the default currently has to
be a single value, but that value isn't really meaningful. A
function's header is its primary documentation and definition, and
things should only have perceived meaning when they also have real
meaning. (Case in point: positional-only args, where there is no
keyword argument that can masquerade as that positional arg. Being
forced to name every argument is a limitation.)

The purpose of late-evaluated argument defaults (I'm wondering if I
should call them LEADs, or if that's too cute) is to make the
function's signature truly meaningful. It shouldn't be necessary to
warp your code around a technical limitation.

ChrisA
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