Mutable defaults

[Chris Angelico]

On Wed, Feb 10, 2021 at 2:19 AM Antoon Pardon <antoon.pardon at vub.be> wrote:
>
> Most of us know of the perils of mutable default values. So I came up with the following proof of concept:
>
> def copy_defaults(f):
>
>      def wrapper(*args):
>          return f(*newargs)
>      return wrapper
>
> @copy_defaults
> def prepare(value, lst = []):
>      for vl in range(value):
>          lst.append(vl)
>      return lst
>
> print(prepare(2))
> print(prepare(3))
>
> Running the above will produce:
>
> [0, 1]
> [0, 1, 2]
>

Nice idea, but you've limited your functions to positional-only
parameters. Extending this to support keyword arguments is a lot
harder, and is why it's generally easier to use standard idioms like
defaulting to None (or to a unique object) and then "if lst is None:
lst = []" at the start of the function.

Have you considered inspect.signature and its various methods? It may
be useful here.

ChrisA