[Python-Dev] PEP 362: 4th edition

[Yury Selivanov]

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?

-
Yury