Add sort keys option for dataclasses inheriting from mixin

[mishamsk]

Hi,

Thought this is small enough to directly open a PR and see if you would support this feature.

What: add a sort_keys option for BaseConfig which would efficiently generate serialization code such that keys are sorted on creation (which in many cases will make much heavier encoder approach unnecessary).

What's missing:

• I've only added the feature to Mixin subclasses for now, but not for arbitrary dataclasses
• I haven't added any docs, since there is no guarantee you would approve this PR
• I've only added one test, maybe you would want additional ones

Rationale:
I am using key sort (via __post_serialize__ hook) a lot in order to maintain stable serialized view of my objects (those pesky trees). This would make it more efficient (depending on how classes are defined of course).

[Fatal1ty]

There is one minor but everything else is fine.

I've only added the feature to Mixin subclasses for now, but not for arbitrary dataclasses

It will also work for arbitrary dataclasses:

@dataclass
class Foo:
    b: int
    a: int

@dataclass
class Bar(DataClassDictMixin):
    z: int
    foo: Foo

bar = Bar(z=3, foo=Foo(b=2, a=1))
print(bar.to_dict())  # {'foo': {'a': 1, 'b': 2}, 'z': 3}

Would you like to add such a case to the tests since you mentioned it?

[mishamsk]

Would you like to add such a case to the tests since you mentioned it?
forgot to pull latest changes, so naturally it didn't work for dataclasses in my branch.

now pulled and rebased. added a test + added a README

[Fatal1ty]

@mishamsk Thank you for contribution, I'm merging it now.

[mishamsk]

glad to help!

[Fatal1ty]

@mishamsk

I just realised that I was wrong. Config options are not propagated to inner dataclasses by design, and the tests didn't show it because two dictionaries with different key order are equal. I created the pull request to fix the tests.

[mishamsk]

@Fatal1ty ouch. sorry. my bad!