The idea is to have a `default_factory` like argument (either in the `field` function, or a new function entirely) that takes a function as an argument, and that function, with the value provided by `__init__`, is called and the return value is used as the value for the respective field. For example:
x: str = field(init_fn=chr)
f = Foo(65)
f.x # "A"
The `chr` function is called, given the value `65` and `x` is set to its return value of `"A"`. I understand that there is both `__init__` and `__post_init__` which can be used for this purpose, but sometimes it isn't ideal to override them. If you overrided `__init__`, and were using `__post_init__`, you would need to manually call it, and in my case, `__post_init__` is implemented on a base class, which all other classes inherit, and so overloading it would require re-implementing the logic from it (and that's ignoring the fact that you also need to type the field with `InitVar` to even have it passed to `__post_init__` in the first place).
I've created a proof of concept, shown below:
def initfn(fn, default=None):
def __set_name__(_, owner_cls, owner_name):
old_setattr = getattr(owner_cls, "__setattr__")
def __setattr__(self, attr_name, value):
if attr_name == owner_name:
# Bypass `__setattr__`
self.__dict__[attr_name] = fac(value)
old_setattr(self, attr_name, value)
setattr(owner_cls, "__setattr__", __setattr__)
if isinstance(value, Inner):
It makes use of the fact that providing `default` as an argument to `field` means it checks the value for a `__set_name__` function, and calls it with the class and field name as arguments. Overriding `__setattr__` is just used to catch when a value is being assigned to a field, and if that field's name matches the name given to `__set_name__`, it calls the function on the value, at sets the field to that instead.
It can be used like so:
x: str = initfn(fn=chr, default="Z")
f = Foo(65)
f2 = Foo()
f.x # "A"
f2.x # "Z"
It adds a little overhead, especially with having to override `__setattr__` however, I believe it would have very little overhead if directly implemented in the dataclass library.
Even in the case of being able to override one of the init functions, I still think it would be nice to have as a quality of life feature as I feel calling a function is too simple to want to override the functions, if that makes sense.