[Python-Dev] What is the design purpose of metaclasses vs code generating decorators? (was Re: PEP 557: Data Classes)

Koos Zevenhoven k7hoven at gmail.com
Fri Oct 13 07:06:14 EDT 2017

While I really can't continue to be active in this discussion now [*], here
are some thoughts based on observations I made:

These three PEPs are all essentially solving an occurrence of the same

PEP 549 Instance descriptors
PEP 562 Module __getattr__
PEP 560 Core support for generic types (the __class_getitem__ part)


PEPs 549 and 562 want an instance of ModuleType (i.e. modules) to define
something on itself that looks like there was something defined on the
class. For PEP 549 this is a property and for 562 it's a __getattr__ method.

PEP 560 wants a __class_getitem__ method, defined on a class (instance of a
metaclass), to look like there was a __getitem__ on the metaclass.

PEP 560 is thus an attempt at a more fine-grained definition of a
metaclass-like feature, where conflicts are less likely or can potentially
be better dealth with. While PEPs 549 and 562 are doing a very similar
thing to PEP 560 in theory, these use cases do not fall nicely into Nick's
classification of uses for metaclasses in the email below.

PEP 560 is trying to avoid a metaclass (a subclass of type) as an
additional base class specifically for one class object). PEPs 549 and 562
are trying to avoid an additional class (a subclass of ModuleType) as an
additional base class specifically for this one module.

Whether or not fine-grainedness is the answer, it might make sense to list
more different related use cases. Probably even the peps repo has more
examples than the three I listed above.


​[*] I'll try to be able to do what's nee½ded for the PEP 555 discussion —
no1/3w still on python-ideas.

On Fri, Oct 13, 2017 at 9:30 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 13 October 2017 at 04:21, Martin Teichmann <lkb.teichmann at gmail.com>
> wrote:
>> For me, the dataclasses were a typical example for inheritance, to be
>> more precise, for metaclasses. I was astonished to see them
>> implemented using decorators, and I was not the only one, citing
>> Guido:
>> > I think it would be useful to write 1-2 sentences about the problem with
>> > inheritance -- in that case you pretty much have to use a metaclass,
>> and the
>> > use of a metaclass makes life harder for people who want to use their
>> own
>> > metaclass (since metaclasses don't combine without some manual
>> > intervention).
>> Python is at a weird point here. At about every new release of Python,
>> a new idea shows up that could be easily solved using metaclasses, yet
>> every time we hesitate to use them, because of said necessary manual
>> intervention for metaclass combination.
> Metaclasses currently tend to serve two distinct purposes:
> 1. Actually altering the runtime behaviour of a class and its children in
> non-standard ways (e.g. enums, ABCs, ORMs)
> 2. Boilerplate reduction in class definitions, reducing the amount of code
> you need to write as the author of that class
> Nobody has a problem with using metaclasses for the first purpose - that's
> what they're for.
> It's the second use case where they're problematic, as the fact that
> they're preserved on the class becomes a leaky implementation detail, and
> the lack of a JIT in CPython means they can also end up being expensive
> from a runtime performance perspective.
> Mixin classes have the same problem: something that the author may want to
> handle as an internal implementation detail leaks through to the runtime
> state of the class object.
> Code generating decorators like functools.total_ordering and
> dataclasses.dataclass (aka attr.s) instead aim at the boilerplate reduction
> problem directly: they let you declare in the class body the parts that you
> need to specify as the class designer, and then fill in at class definition
> time the parts that can be inferred from that base.
> If all you have access to is the runtime class, it behaves almost exactly
> as if you had written out all the autogenerated methods by hand (there may
> be subtle differences in the method metadata, such as the values of
> `__qualname__` and `__globals__`).
> Such decorators also do more work at class definition time in order to
> reduce the amount of runtime overhead introduced by reliance on chained
> method calls in a non-JITted Python runtime.
> As such, the code generating decorators have a clear domain of
> applicability: boilerplate reduction for class definitions without
> impacting the way instances behave (other than attribute and method
> injection), and without implicitly impacting subclass definitions (other
> than through regular inheritance behaviour).
> As far as the dataclass interaction with `__slots__` goes, that's a
> problem largely specific to slots (and `__metaclass__` before it), in that
> they're the only characteristics of a class definition that affect how
> CPython allocates memory for the class object itself (the descriptors for
> the slots are stored as a pointer array after the class struct, rather than
> only in the class dict).
> Given PEP 526 variable annotations, __slots__ could potentially benefit
> from a __metaclass__ style makeover, allowing an "infer_slots=True" keyword
> argument to type.__new__ to request that the list of slots be inferred from
> __annotations__ (Slot inference would conflict with setting class level
> default values, but that's a real conflict, as you'd be trying to use the
> same name on the class object for both the slot descriptor and the default
> value)
> Cheers,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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