[Python-ideas] Extend module objects to support properties

Thu Jan 31 09:56:16 CET 2013

On Thu, Jan 31, 2013 at 2:42 AM, Larry Hastings <larry at hastings.org> wrote:
> Of those four issues, the latter two are wontfix. Code that futzes with an
> object's __dict__ bypasses the property machinery but this is already viewed
> as acceptable.
>> Obviously the point of the proposal is to change the behavior of the first
> two. Whether this is manageable additional complexity, or fast enough,
> remains to be seen--which is why this is in ideas not dev.

Looking at the problem from a different direction:
Currently, modules are *instances* of a normal type
(types.ModuleType). Thus, anything stored in their global namespace is
like anything else stored in a normal instance dictionary: no
descriptor behaviour.
The request in this thread is basically for a way to:
1. Define a custom type
2. Put an instance of that type in sys.modules instead of the ordinary
module object
Now here's the thing: we already support this, because the import
system is designed to cope with modules replacing
"sys.modules[__name__]" while they're being loaded. The way this
happens is that, after we finish loading a module, we usually don't
trust what the loader gave us. Instead, we go look at what's in
sys.modules under the name being loaded.
So if, in your module code, you do this:
 import sys, types
 class MyPropertyUsingModule(types.ModuleType):
 def __init__(self, original):
 # Keep a reference to the original module to avoid the
 # destructive cleanup of the global namespace
 self._original = original
 @property
 def myglobal(self):
 return theglobal
 @myglobal.setter
 def myglobal(self, value):
 global theglobal
 theglobal = value
 sys.modules[__name__] = MyPropertyUsingModule(sys.modules[__name__])
Then what you end up with in sys.modules is a module with a global
property, "myglobal".
I'd prefer to upgrade this from "begrudged backwards compatibility
hack" to "supported feature", rather than doing anything more
complicated.
Cheers,
Nick.
-- 
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia