[Python-Dev] PEP 549 v2: now titled Instance Descriptors (original) (raw)

Ethan Furman ethan at stoneleaf.us
Fri Sep 8 17:26:47 EDT 2017

On 09/08/2017 01:45 PM, Ethan Furman wrote:

I will admit I don't see what reassigning the class attribute on a module did for us.

Nathaniel Smith wrote:

If you have an existing package that doesn't replace itself in sys.modules, then it's difficult and risky to switch to that form -- don't think of toy examples, think of django/init.py or numpy/init.py. You have to rewrite the whole export logic, and you have to figure out what to do with things like submodules that import from the parent module before the swaparoo happens, you can get skew issues between the original module namespace and the replacement class namespace, etc. The advantage of the class assignment trick (as compared to what we had before) is that it lets you easily and safely retrofit this kind of magic onto existing packages.

Ah, that makes sense.

Larry Hastings wrote:

Assigning the module's class means you can otherwise initialize your module instance normally--all your class needs to do is declare your properties or magic methods. If you overwrite the sys.modules entry for your module with a new instance of a custom subclass, all your module initialization needs to be done within--or propagated to--the new class or instance.

So with the module level class we have to use two steps to get what we want, and PEP 549 takes it back to one step. Hmm. In theory it sounds cool; on the other hand, the existing class assignment method means all the magic is in one place in the module, not scattered around -- but that's a style issue.

Thanks for the info!

-- Ethan

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