On Sun, Oct 31, 2021 at 02:56:36PM +1100, Chris Angelico wrote:
> Current versions of the PEP do not use the term "default value" when
> referring to late binding (or at least, if I've made a mistake there,
> then please point it out so I can fix it). I'm using the term "default
> expression", or just "default" (to cover both values and expressions).
I was just thinking of suggesting that to you, so I'm glad to see you're
much faster on the uptake than I am!
Of course all parameters are syntactically an expression, including now:
# the status quo
def func(arg=CONFIG.get('key', NULL)):
The default expression is evaluated at function definition time, and the
result of that (an object, a.k.a. a value) is cached in the function
object for later use. With late-binding:
def func(@arg=CONFIG.get('key', NULL)):
the expression is stashed away somewhere (implementation details), in
some form (source code? byte-code? an AST?) rather than immediately
evaluated. At function call time, the expression is evaluated, and the
result (an object, a.k.a. a value) is bound to the parameter.
In neither case is it correct to say that the default value of arg is
the *expression* `CONFIG.get('key', NULL)`, it is in both the early and
late bound cases the *result* of *using* (evaluating) the expression to
generate a value.
https://en.wikipedia.org/wiki/Use%E2%80%93mention_distinction
I'm fairly confident that everyone understands that:
"the default value is CONFIG.get('key', NULL)"
is shorthand for the tediously long and pedantic explanation that it's
not the expression itself that is the default value, but the result of
evaluating the expression. Just like we understand it here:
if arg is None:
arg = CONFIG.get('key', NULL)
The only difference is when the expression is evaluated.
If we can understand that arg gets set to the result of the expression
in the second case (the None sentinel), we should be able to understand
it in if the syntax changes to late-bound parameters.
--
Steve
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/NPTK2TZLBL6KZVKNYKLUXVK3A3NULDLN/
Code of Conduct: http://python.org/psf/codeofconduct/
