Always require optional field in python target

[kanghyojun]

record foo (
  text? optional-data
);

Above foo record compiled to

class Foo:
    ...

    def __init__(self, optional_data: typing.Optional[str]) -> None:
        ...

It requires optional_data as a argument to initiate class Foo, although optional_data is optional.

[dahlia]

Should we treat it as a bug? I'm not sure. Think of the following example:

record bar (
    text a,
    text? b,
    text c,
    text? d,
);

It consists of two required fields and two optional fields, and they appear by turns. So should it be compiled to __init__(self, a: str, b: Optional[str]=None, c: str, d: Optional[str]=None)? As you probably are aware of, it is not a valid Python syntax since Python doesn't allow optional argument followed by required argument.

[dahlia]

There could be some ways. One way is making __init__() method to take only keyword arguments (e.g. __init__(self, *, a: str, b: Optional[str]=None, c: str, d: Optional[str]=None)).

[dahlia]

Now it becomes harder to be keyword-only than before, since now Python target has Python 2 generator as well. Whereas Python 3 has a operator for keyword-only parameters (PEP 3102), Python 2 does not. In order to implement keyword-only parameters in Python 2, we need some tricks:

def __init__(self, **kwargs):
    invalid_params = frozenset(kwargs) - {'a', 'b', 'c', 'd'}
    if invalid_params:
        raise TypeError('unexpected keywords: ' + ', '.join(invalid_params))
    ...

[earlbread]

👀