[Python-Dev] PEP 362: 4th edition

[Ethan Furman]

Yury Selivanov wrote:
> On 2012-06-18, at 4:25 PM, Guido van Rossum wrote:
> 
>> On Mon, Jun 18, 2012 at 11:09 AM, Yury Selivanov
>> <yselivanov.ml at gmail.com> wrote:
>>> The rationale is that sometimes you need to modify signatures.
>>> For instance, in decorators.
>> A decorator should make a modified copy, not modify it in place (since
>> the signature of the decorated function does not change, and you have
>> no guarantee that that function is no longer accessible.)
> 
> 
> 
> It seems that we have the following options for 'signature(obj)':
> 
> 1. If 'obj' has a '__signature__' attribute - return a copy of it,
> if not - create a new one.
> 
> 2. If 'obj' has a '__signature__' attribute - return it,
> if not - create a new one.
> 
> 3. Same as '2', but Signature is also immutable.
> 
> 
> The first option is the one currently implemented.  Its advantage
> is consistency - we always have a Signature we can safely modify.
> 
> The second option has a design flaw - sometimes the result Signature
> is safe to modify, sometimes not, you never know.
> 
> The third option is hard to work with.
> Instead of:
> 
>     sig = signature(wrapper)
>     sig.parameters.popitem(last=False)
>     decorator.__signature__ = sig
> 
> We will have (because Signature is immutable):
> 
>     sig = signature(wrapper)
>     params = OrderedDict(sig.parameters.items())
>     params.popitem(last=False)
> 	
>     attrs = {'parameters': params}
>     try:
>         ra = sig.return_annotation
>     except AttributeError:
>         pass
>     else:
>         attrs['return_annotation'] = ra
> 
>     decorator.__signature__ = Signature.from_attrs(**attrs)
> 
> It looks like a total overkill (unless we can come up with a nicer
> API).
> 
> So it was decided to go with the first option, as it has the least
> complications.  Plus, the copying itself should be fast, as
> Signatures contain little information.
> 
> What do you think?

Option 1 makes sense to me -- we already know we'll have cases where we 
want to modify a given signature, so why make it hard on ourselves?

~Ethan~