[Python-ideas] Dataclasses, keyword args, and inheritance

[George Leslie-Waksman]

The proposed implementation of dataclasses prevents defining fields with
defaults before fields without defaults. This can create limitations on
logical grouping of fields and on inheritance.

Take, for example, the case:

@dataclass
class Foo:
    some_default: dict = field(default_factory=dict)

@dataclass
class Bar(Foo):
    other_field: int

this results in the error:

      5 @dataclass
----> 6 class Bar(Foo):
      7     other_field: int
      8

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py
in dataclass(_cls, init, repr, eq, order, hash, frozen)
    751
    752     # We're called as @dataclass, with a class.
--> 753     return wrap(_cls)
    754
    755

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py
in wrap(cls)
    743
    744     def wrap(cls):
--> 745         return _process_class(cls, repr, eq, order, hash, init,
frozen)
    746
    747     # See if we're being called as @dataclass or @dataclass().

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py
in _process_class(cls, repr, eq, order, hash, init, frozen)
    675                                 #  in __init__.  Use "self" if
possible.
    676                                 '__dataclass_self__' if 'self' in
fields
--> 677                                     else 'self',
    678                                 ))
    679     if repr:

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py
in _init_fn(fields, frozen, has_post_init, self_name)
    422                 seen_default = True
    423             elif seen_default:
--> 424                 raise TypeError(f'non-default argument {f.name!r} '
    425                                 'follows default argument')
    426

TypeError: non-default argument 'other_field' follows default argument

I understand that this is a limitation of positional arguments because the
effective __init__ signature is:

def __init__(self, some_default: dict = <something>, other_field: int):

However, keyword only arguments allow an entirely reasonable solution to
this problem:

def __init__(self, *, some_default: dict = <something>, other_field: int):

And have the added benefit of making the fields in the __init__ call
entirely explicit.

So, I propose the addition of a keyword_only flag to the @dataclass
decorator that renders the __init__ method using keyword only arguments:

@dataclass(keyword_only=True)
class Bar(Foo):
    other_field: int

--George Leslie-Waksman
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