[Python-Dev] PEP 549: Instance Properties (aka: module properties) (original) (raw)

Larry Hastings 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 property?

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 property: os.stat_float_times().

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