[Python-Dev] PEP 520: Ordered Class Definition Namespace (original) (raw)

Eric Snow ericsnowcurrently at gmail.com
Tue Jun 7 23:17:16 EDT 2016

On Tue, Jun 7, 2016 at 6:20 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

On 06/07/2016 05:50 PM, Eric Snow wrote:

definitionorder = tuple(k for k in locals() if (!k.startswith('') or_ !k.endswith('')))_

Still mixing C and Python! ;)

I knew I was missing something!

Why a tuple? ------------ Use of a tuple reflects the fact that we are exposing the order in which attributes on the class were defined. Since the definition is already complete by the time ``definitionorderis set, the_ _content and order of the value won't be changing. Thus we use a type_ _that communicates that state of immutability._ _Why a read-only attribute?_ _--------------------------_ _As with the use of tuple, makingdefinitionordera read-only_ _attribute communicates the fact that the information it represents is_ _complete. Since it represents the state of a particular one-time event_ _(execution of the class definition body), allowing the value to be_ _replaced would reduce confidence that the attribute corresponds to the_ _original class body._ _If a use case for a writable (or mutable)definitionorder``_ arises, the restriction may be loosened later. Presently this seems unlikely and furthermore it is usually best to go immutable-by-default. If definitionorder is supposed to be immutable as well as read-only then we should convert non-tuples to tuples. No point in letting that user bug slip through.

Do you mean if a class explicitly defines definition_order? If so, I'm not clear on how that would work. It could be set to anything, including None or a value that does not iterate into a definition order. If someone explicitly set definition_order then I think it should be used as-is.

Why ignore "dunder" names? -------------------------- Names starting and ending with "" are reserved for use by the_ interpreter. In practice they should not be relevant to the users of _definitionorder_. Instead, for early everyone they would only s/early/nearly

fixed

Why is definitionorder even necessary? -------------------------------------------

Since the definition order is not preserved in _dict_, it would be lost once class definition execution completes. Classes could explicitly set the attribute as the last thing in the body. However, then independent decorators could only make use of classes that had done so. Instead, _definitionorder_ preserves this one bit of info from the class body so that it is universally available. s/would be/is

fixed

Thanks!

-eric

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