[Python-Dev] PEP 549: Instance Properties (aka: module properties) (original) (raw)
Nathaniel Smith njs at pobox.com
Tue Sep 5 18:52:31 EDT 2017
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On Tue, Sep 5, 2017 at 3:03 PM, Larry Hastings <larry at hastings.org> wrote:
I've written a PEP proposing a language change: https://www.python.org/dev/peps/pep-0549/ The TL;DR summary: add support for property objects to modules. I've already posted a prototype.
Interesting idea! It's definitely less arcane than the class assignment support that was added in 3.5. I guess the question is whether to add another language feature here, or to provide better documentation/helpers for the existing feature.
If anyone's curious what the class trick looks like in practice, here's some simple deprecation machinery: https://github.com/njsmith/trio/blob/ee8d909e34a2b28d55b5c6137707e8861eee3234/trio/_deprecate.py#L102-L138
And here's what it looks like in use: https://github.com/njsmith/trio/blob/ee8d909e34a2b28d55b5c6137707e8861eee3234/trio/init.py#L91-L115
Advantages of PEP 549:
- easier to explain and use
Advantages of the class trick:
faster (no need to add an extra step to the normal attribute lookup fast path); only those who need the feature pay for it
exposes the full power of Python's class model. Notice that the above code overrides getattr but not dir, so the attributes are accessible via direct lookup but not listed in dir(mod). This is on purpose, for two reasons: (a) tab completion shouldn't be suggesting deprecated attributes, (b) when I did expose them in dir, I had trouble with test runners that iterated through dir(mod) looking for tests, and ended up spewing tons of spurious deprecation warnings. (This is especially bad when you have a policy of running your tests with DeprecationWarnings converted to errors.) I don't think there's any way to do this with PEP 549.
already supported in CPython 3.5+ and PyPy3, and with a bit of care can be faked on all older CPython releases (including, crucially, CPython 2). PEP 549 OTOH AFAICT will only be usable for packages that have 3.7 as their minimum supported version.
I don't imagine that I would actually use PEP 549 any time in the foreseeable future, due to the inability to override dir and the minimum version requirement. If you only need to support CPython 3.5+ and PyPy3 5.9+, then you can effectively get PEP 549's functionality at the cost of 3 lines of code and a block indent:
import sys, types class _MyModuleType(types.ModuleType): @property def ...
@property
def ...sys.modules[name].class = _MyModuleType
It's definitely true though that they're not the most obvious lines of code :-)
-n
-- Nathaniel J. Smith -- https://vorpus.org
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