How to initialize an instance where all fields are default values?

[wencan]

How to initialize an instance where all fields are default values?

I don't want to add a default value to each field, and I want each field to automatically get a zero value by default.

This would make dataclass much like struct in the go language.

[hynek]

Python doesn't have the concept of zero values, so this is icky.

But I suspect you could get this done with field hooks @sscherfke?

[wencan]

@hynek

I have implemented it via field hooks.

def field_transformer(cls, fields):
    new_fields = []
    for field in fields:
        if field.default is attrs.NOTHING:
            new_fields.append(field.evolve(default=attrs.Factory(field.type)))
        else:
            new_fields.append(field)
    return new_fields

@attrs.define(kw_only=True, field_transformer=field_transformer)
class Name:
    first_name: str = attrs.field(default='')
    last_name: str 

However, in the above example, I had to specify kw_only as true, otherwise I would get the following error:

ValueError: No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: Attribute(name='city', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=<class 'str'>, converter=None, kw_only=False, inherited=False, on_setattr=None, alias=None)

I read the attrs code and found that the field_transformer is executed later than the default value checks.

    # Mandatory vs non-mandatory attr order only matters when they are part of
    # the __init__ signature and when they aren't kw_only (which are moved to
    # the end and can be mandatory or non-mandatory in any order, as they will
    # be specified as keyword args anyway). Check the order of those attrs:
    had_default = False
    for a in (a for a in attrs if a.init is not False and a.kw_only is False):
        if had_default is True and a.default is NOTHING:
            raise ValueError(
                "No mandatory attributes allowed after an attribute with a "
                f"default value or factory.  Attribute in question: {a!r}"
            )

        if had_default is False and a.default is not NOTHING:
            had_default = True

    if field_transformer is not None:
        attrs = field_transformer(cls, attrs)

[klezm]

Running over a similar issue. Would it be possible to use attrs.field() with a context manager? Something like that:

import typing
import attrs

@attrs.define
class MyClass:
    a: int = 1
    with attrs.field(converter = typing.Self):
        b: str
        c: list

typing.Self should indicate to use field.type. Alternatively one could use a lambda expression: lambda f: f.type.

The benefits I see:

1. This would also allow for assigning the same (more complex) field to multiple attributes.
2. Better readability
3. Reusability principle