Drop Python 3.6 support?

[cschramm]

#1341 tries to make PluginManager generic (again) and as it's also a GObject and Generic is of type GenericMeta in Python 3.6, it fails in Python 3.6 without a dedicated meta class as proposed in #1130 (comment). However, that solution does not work for me either as Python 3.6 gives me that:

Traceback (most recent call last):
  File "blueman/main/PluginManager.py", line 35, in <module>
    class PluginManager(GObject.GObject, Generic[_T], metaclass=PluginManagerMeta):
  File "/usr/local/lib/python3.6/typing.py", line 978, in __new__
    self = super().__new__(cls, name, bases, namespace, _root=True)
  File "/usr/local/lib/python3.6/typing.py", line 137, in __new__
    return super().__new__(cls, name, bases, namespace)
  File "/usr/local/lib/python3.6/abc.py", line 133, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/gi/types.py", line 253, in mro
    return mro(cls)
  File "/usr/local/lib/python3.6/site-packages/gi/types.py", line 311, in mro
    not_head = [s for s in bases_of_subclasses if candidate in s[1:]]
  File "/usr/local/lib/python3.6/site-packages/gi/types.py", line 311, in <listcomp>
    not_head = [s for s in bases_of_subclasses if candidate in s[1:]]
  File "/usr/local/lib/python3.6/typing.py", line 1092, in __eq__
    if self.__origin__ is None or other.__origin__ is None:
AttributeError: type object 'PluginManager' has no attribute '__origin__'

I might be doing something really stupid there, not sure. Anyway, as this blocks me a little bit, I was thinking about just dropping support for Python 3.6. I checked the "rolling repositories" that have blueman 2.1.3 according to https://repology.org/project/blueman/versions and all of them currently have Python 3.8 or are even migrating to 3.9 already. So basically the only group targeted by support for Python 3.6 and 3.7 is people installing blueman from source using those versions for whatever reason and that sounds reeaally limited.

[infirit]

How about "find /usr/ -name typing.py -exec rm {} ;". What I understood is that the typing ppl broken api going from 3.6 to 3.7 by removing genericmeta. We got to keep the broken bits and left to deal with it. The SO may be of help, https://stackoverflow.com/questions/49171189/whats-the-correct-way-to-check-if-an-object-is-a-typing-generic

[cschramm]

The thing is that in Python 3.6 we need a joint metaclass to be able to define a generic GObject and that does not seem trivial, independent of the fact that it does not work for later versions anyway.

While this trivial case works fine:

from typing import Generic, GenericMeta, TypeVar

class AMeta(type):
    pass

class A(metaclass=AMeta):
    pass

T = TypeVar("T")

class GenericAMeta(AMeta, GenericMeta):
    pass

class GenericA(A, Generic[T], metaclass=GenericAMeta):
    pass

This fails with the above stacktrace:

from typing import Generic, GenericMeta, TypeVar

from gi.repository.GObject import GObject
from gi.types import GObjectMeta

T = TypeVar("T")

class GenericObjectMeta(GObjectMeta, GenericMeta):
    pass

class GenericObject(GObject, Generic[T], metaclass=GenericObjectMeta):
    pass

With GenericObject(Generic[T], GObject, metaclass=GenericObjectMeta) it's pretty much the same except that in place of

  File "/usr/local/lib/python3.6/site-packages/gi/types.py", line 311, in mro
    not_head = [s for s in bases_of_subclasses if candidate in s[1:]]
  File "/usr/local/lib/python3.6/site-packages/gi/types.py", line 311, in <listcomp>
    not_head = [s for s in bases_of_subclasses if candidate in s[1:]]

the stacktrace contains

  File "/usr/lib/python3.6/site-packages/gi/types.py", line 324, in mro
    if subclass_bases and subclass_bases[0] == candidate:

So it seems like a rather sophisticated GenericObjectMeta is required to make that work and I'm note sure if keeping Python 3.6 support is really worth such things.

[cschramm]

So... I found a solution that seems to be working in

blueman/blueman/main/PluginManager.py

Line 26 in d36b7cd

class PluginManagerMeta(GenericMeta, GObjectMeta):

. I have no idea what side effects that hack might have, though... Also, from typing import GenericMeta # type: ignore will need some thoughts as mypy reports an unused ignore here when run in Python 3.6.

[infirit]

tbh, all of this goes way over my head. So if this works and it does not break the working of blueman 👍

[cschramm]

Not saying that we should not consider dropping support for Python 3.6 anymore but I found a way around the generic in #1341, so my major pain point with it is gone. 🙂