On 04/12/2021 01:06, Chris Angelico wrote:
On Sat, Dec 4, 2021 at 11:59 AM Rob Cliffe via Python-ideas
<python-ideas@python.org> wrote:
On 03/12/2021 22:38, Chris Angelico wrote:
On Sat, Dec 4, 2021 at 8:18 AM Rob Cliffe via Python-ideas
<python-ideas@python.org> wrote:
On 03/12/2021 19:32, Adam Johnson wrote:
The first unwelcome surprise was:
>>> def func(a=>[]):
... return a
...
>>> import inspect
>>> inspect.signature(func).parameters['a'].default
Ellipsis
Here the current behaviour of returning `Ellipsis` is very unfortunate,
and I think could lead to a lot of head scratching — people wondering
why they are getting ellipses in their code, seemingly from nowhere.
Sure, it can be noted in the official documentation that `Ellipsis` is
used as the indicator of late bound defaults, but third-party resources
which aim to explain the uses of `Ellipsis` would (with their current
content) leave someone clueless.
+1.
This may be a very naive question, apologies if it's nonsense.
Instead of Ellipsis, would it be possible to have a built-in LateBound
class and use instances of that class instead of Ellipsis?
The __str__ method of the inspect.Parameter class could be modified to
return something like
"a=>[]"
(or whatever syntax is adopted for specifying late-bound defaults) in
such cases.
The __repr__ and __str__ methods of a LateBound object could return
something like, respectively,
"LateBound('[]')"
"[]"
I am sure there is code that uses inspect.signature that would be
broken, but isn't that inevitable anyway?
That's a possibility for the inspect module. For the core language -
and therefore for anything that directly inspects the function's
dunders - it's much more efficient to use a well-known object.
OK.
I'm guessing that by "well-known" you mean pre-existing. Python has a
number of built-in singleton objects (None, True, False, Ellipsis).
Mainly by "well-known" I mean "not private to any particular module",
so those four you mention are all well-known, but a singleton as part
of the inspect module would be a pain, since the core language would
have to import that (or it would have to be magically created).
What about adding a new one called LateBound (or other bikeshed colour)?
What would be gained? You would still be able to use LateBound as an
early-bound default, so you would still need the same dual check.
I'm struggling here. Yes you could use LateBound as an early-bound
default (or as a parameter value to explicitly pass to a function) but
ISTM that such usages would be perverse.
I've a gut feeling that a solution can be found (to avoid the "Ellipsis
from nowhere" problem) but I can't put my finger on it. Maybe explicitly
specifying LateBound could be an error, perhaps even a SyntaxError?
Help, please!
With inspect.Parameter, you already have a repr and str that show that
it's late-bound, exactly like you suggest. What advantage is there
from using a dedicated sentinel instead of Ellipsis?
It wouldn't be too hard to change the inspect module, but I'd need to know why.
ChrisA
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