[Python-ideas] Positional-only parameters

[Abe Dillon]

That looks great to me! I also think the '/' syntax looks fine and the pun
works. If part of the motivation for position-only arguments was better
performance and that motivation still holds water, then it makes sense to
allow Python to support that optimization, but I would be happy with just a
decorator too.

I definitely DON'T like the double-underscore. On top of all the other
complaints, I think it's more prone to break code. It's also more ugly than
'/' IMHO.

‪On Thu, Mar 2, 2017 at 5:10 AM ‫אלעזר‬‎ <elazarg at gmail.com> wrote:‬

> Here's a proof-of-concept for the decorator. It does not address the issue
> of passing aliases to positional arguments to **kwargs - I guess this
> requires changes in the CPython's core.
>
> (Sorry about the coloring, that's how it's pasted)
>
> from inspect import signature, Parameter
> from functools import wraps
>
>
> def positional_only(n):
>     def wrap(f):
>         s = signature(f)
>         params = list(s.parameters.values())
>         for i in range(n):
>             if params[i].kind != Parameter.POSITIONAL_OR_KEYWORD:
>                 raise TypeError('{} has less than {} positional arguments'.format(f.__name__, n))
>             params[i] = params[i].replace(kind=Parameter.POSITIONAL_ONLY)
>         f.__signature__ = s.replace(parameters=params)
>         @wraps(f)
>         def inner(*args, **kwargs):
>             if len(args) < n:
>                 raise TypeError('{} takes at least {} positional arguments'.format(f.__name__, n))
>             return f(*args, **kwargs)
>         return inner
>     return wrap
>
>
> @positional_only(2)
> def f(a, b, c):
>     print(a, b, c)
>
>
> help(f)
> # f(a, b, /, c, **kwargs)
>
> f(1, 2, c=2)
>
> # f(1, b=2, c=3)
> # TypeError: f takes at least 2 positional arguments
>
>
> @positional_only(3)
> def g(a, b, *, c):
>     print(a, b, c)
>
> # TypeError: g has less than 3 positional arguments
>
> Elazar
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