[Python-Dev] Updated PEP 362 (Function Signature Object)

Thu Jun 7 16:45:34 CEST 2012

On 2012年6月07日 07:00:29 -0700, Larry Hastings <larry at hastings.org> wrote:
> On 06/06/2012 11:56 PM, Nick Coghlan wrote:
> > I'd say return a copy in the first case to be safe against accidental
> > modification. If someone actually wants in-place modification, they
> > can access __signature__ directly.
>> I really don't understand this anxiety about mutable Signature objects. 
> Can you give a plausible example of "accidental modification" of a 
> Signature object? I for one--as clumsy as I am--cannot recall ever 
> "accidentally" modifying an object.

Maybe it would make more sense if you read that as "naively" rather than
"accidentally"?
In the 3.3 email extension I made a similar decision, although there I
went even further and made the objects read only. My logic for doing
this is that a naive user would...naively...try to set the attributes
and expect the object they got it from to change, but that object (a
string subclass) is inherently read-only.
I am thinking that in fact we may ultimately want to return copies of
these objects that are mutable, because that might be useful, but I'm
starting with read-only because it is easy to make them mutable later
but pretty much impossible (backward compatibility wise) to make them
immutable if they start mutable.
I see the signature object as a very parallel case to this, except that
it is already obvious that having them be a mutable copy is useful.
--David