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

[CTO]

On May 15, 3:30 am, Carl Johnson <cmjohnson.mailingl... at gmail.com>
wrote:
> Bruce Leban wrote:
> > I'll use an @dynamic decorator-like syntax to illustrate. These would be
> > valid:
>
> > def foo(a, b = @dynamic lambda: []):
> > def foo(a, b = @dynamic lambda: list()):
> > def foo(a, b = @dynamic list):
> > def foo(a, b = @dynamic random.random):
>
> > and this would not:
>
> > def foo(a, b = @dynamic [])
> > def foo(a, b = @dynamic 5)
>
> > because @dynamic says that the thing that follows is called to generate a
> > dynamic default parameter value and you can't call [] or 5.
>
> Hmm, very interesting, but in your example what is "dynamic" doing?
> Are you proposing it as a keyword to signal "here comes a dynamic
> default"? Do we really need it? Why not something like this:
>
> def five_appender(x=@list):
>     x.append(5)
>     return x
>
> >>> five_appender()
> [5]
> >>> five_appender()
>
> [5]
>
> The idea is that @ is a magic sigil meaning, "call this if no argument
> is passed in." So, as per your prior example @[] or @5 would be result
> in a runtime error, since they're not callable. If for some reason you
> want a fresh 5 (I can't think of why, since it's immutable, but
> whatever), you would need to use a lambda:
>
> def n_appender(n=@lambda: 5, x=@list):
>     x.append(n)
>     return x
>
> Do y'all think this is enough inline with how @ is already used to
> make sense? Or is it too different from the existing use of @?
>
> -- Carl Johnson

If we're making magical objects, why not just make a magical object
that gives you the ability to defer the execution of a block of code
until an operation is performed on it? That way at least it makes
sense if you've learned the rest of the language.

Geremy Condra