[Python-Dev] PEP 520: Ordered Class Definition Namespace (round 3) (original) (raw)

Émanuel Barry vgr255 at live.ca
Sat Jun 11 22:51:29 EDT 2016

From: Eric Snow Sent: Saturday, June 11, 2016 10:37 PM To: Python-Dev; Guido van Rossum Subject: [Python-Dev] PEP 520: Ordered Class Definition Namespace (round 3)

The only change to the original proposal has been that a manually set definitionorder must be a tuple of identifiers or None (rather that using the value as-is).

1. if _definitionorder_ is defined in the class body then it must be a tuple of identifiers or None; any other value will result in TypeError

Why not just any arbitrary iterable, which get converted to a tuple at runtime? slots allows any arbitrary iterable:

def g(): ... yield "foo" ... yield "bar" ... yield "baz" class C: ... slots = g() C.slots <generator object g at 0x0074A9F0> C.slots.girunning False dir(C) [, 'bar', 'baz', 'foo']

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 ``definitionorder`` is set, the_ content and order of the value won't be changing. Thus we use a type that communicates that state of immutability.

Typo: missing leading underscores in definition_order

Compatibility =============

This PEP does not break backward compatibility, except in the case that someone relies strictly on dict as the class definition namespace. This shouldn't be a problem.

Perhaps add a mention that isinstance(namespace, dict) will still be true, so users don't get unnecessarily confused.

.dict as OrderedDict -------------------------------

looks weird to me. I tend to use cls (although klass isn't uncommon). C might also not be a bad choice.

Thanks! -Emanuel

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