[Python-3000] Draft pre-PEP: function annotations
Phillip J. Eby
pje at telecommunity.com
Fri Aug 11 21:34:01 CEST 2006
At 09:04 AM 8/11/2006 -0700, Josiah Carlson wrote:
>I think you misunderstood Talin. While it was a pain for him to work
>his way through implementing all of the loading/etc. protocols, I
>believe his point was that if we allow any and all arbitrary metadata to
>be placed on arguments to and from functions, then invariably there will
>be multiple methods of doing as much. That isn't a problem unto itself,
>but when there ends up being multiple metadata formats, with multiple
>interpretations of them, and a user decides that they want to combine
>the functionality of two metadata formats, they may be stuck due to
I was giving him the benefit of the doubt by assuming he was bringing up a
*new* objection that I hadn't already answered. This "incompatibility"
argument has already been addressed; it is trivially solved by overloaded
functions (e.g. pickle.dump(), str(), iter(), etc.).
>This method may or may not be good. But, if we don't define a standard
>method for metadata to be combined from multiple protocols, etc., then
>we could end up with incompatabilities.
Not if you use overloaded functions to define the operations you're going
to perform. You and Talin are proposing a problem here that is not only
hypothetical, it's non-existent.
Remember, PEAK already does this kind of openly-extensible metadata for
attributes, using a single-dispatch overloaded function (analagous to
pickle.dump). If you want to show that it's really possible to create
"incompatible" annotations, try creating some for attributes in PEAK.
But, you'll quickly find that the only "meaning" that metadata has is
*operational*. That is, either some behavior is influenced by the
metadata, or no behavior is. If no behavior is involved, then there can be
no incompatibility. If there is behavior, there is an operation to be
performed, and that operation can be based on the type of the metadata.
Ergo, using an overloadable function for the operation to be performed
allows a meaning to be defined for the specific combination of operation
and type. Therefore, there is no problem - every piece of metadata may be
assigned a meaning that is relevant for each operation that needs to be
Now, it is of course possible that two pieces of metadata may be
contradictory, redundant, overlapping, etc. However, this has nothing to
do with whether the semantics of metadata are predefined. Any
sufficiently-useful annotation scheme will include these possibilities, and
the operations to be performed are going to have to have some defined
semantics for them. This is entirely independent of whether there is more
than one metadata framework in existence.
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