[Python-Dev] PEP 549: Instance Properties (aka: module properties)
larry at hastings.org
Thu Sep 7 18:49:32 EDT 2017
On 09/06/2017 09:45 AM, Guido van Rossum wrote:
> So we're looking for a competing PEP here. Shouldn't be long, just
> summarize the discussion about use cases and generality here.
I don't think it's necessarily a /competing/ PEP; in my opinion, they
serve slightly different use cases. After all, property (and
__getattribute__) were added long after __getattr__; if __getattr__ was
a reasonable solution for property's use cases, why did we bother adding
One guiding principle I use when writing Python: if I need to provide an
API, but there's conceptually only one of the thing, build it directly
into a module as opposed to writing a class and making users use an
instance. (For example: the random module, although these days it
provides both.) So I see the situation as symmetric between modules and
classes. What is the use case for property / __getattr__ /
__getattribute__ on a module? The same as the use case for property /
__getattr__ / __getattribute__ on a class.
Excluding Lib/test, there are 375 uses of "@property" in the stdlib in
trunk, 60 uses of __getattr__, and 34 of __getattribute__. Of course,
property is used once per member, whereas each instance of __getattr__
and __getattribute__ could be used for arbitrarily-many members. On the
other hand, it's also possible that some uses of __getattr__ are legacy
uses, and if property had been available it would have used that
instead. Anyway I assert that property is easily the most popular of
these three techniques.
TBH I forgot the specific use case that inspired this--it's on a project
I haven't touched in a while, in favor of devoting time to the
Gilectomy. But I can cite at least one place in the standard library
that would have been better if it'd been implemented as a module
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