[Python-Dev] Preserving the definition order of class namespaces. (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon May 25 00:52:57 CEST 2015

On 25 May 2015 07:26, "Guido van Rossum" <guido at python.org> wrote:

On Sun, May 24, 2015 at 1:36 PM, Eric Snow <ericsnowcurrently at gmail.com> wrote:

My premise for storing the definition order on the class is that Guido was okay with using OrderedDict for cls.dict, which is a bigger change. Regardless, there are two reasons why it makes sense: * If it makes sense to use OrderedDict by default for class definition then it makes sense to preserve the extra information OrderedDict provides. * As I noted at the beginning of the thread, you could still preserve that info manually, but that makes it less convenient for library authors.

It occurs to me that even the basic change makes it possible to provide initialisation helpers that accept locals() from a currently running class definition and return a definition ordered list of fields (perhaps restricted to values of a certain type, such as database column definitions, or webform field definitions).

If you still think that's not enough justification then we can table definition_order for now.

Let's table it. It's hard to compare alternatives on a single dimension of "which is a bigger change".

Right, it isn't that I think definition_order is necessarily a bad idea, I just suspect it's redundant if we end up going ahead with init_subclass (which would allow a base class to opt in to preserving the definition order, either of all fields or selected ones), and the latter change is definitely out of scope for 3.5 at this point.

There are also other open questions, like whether or not dir() should respect the order when reporting attribute names, or if dict_proxy should respect the order when iterating.

Regards, Nick.

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