Allow use of `object.__setattr__` to modify `frozen` instances of `Struct`

[wikiped]

Description

Docs for built-in dataclass mention using object.__setattr__ to modify frozen instances.

When the same approach is used to modify frozen Struct (for example in __post_init__) this leads to error:

TypeError: can't apply this __setattr__ to <StructName> object

At the same time Struct api docs say:

frozen (bool, default False) – Whether instances of this type are pseudo-immutable. If true, attribute assignment is disabled ...

So, pseudo-immutable in the docs 'hints' that there is a way to modify the struct, but it is unclear how exactly this could be done, since Struct.__setattr__ is hardcoded to unconditionally throw error unlike builtin dataclass.

[jcrist]

Here "pseudo-immutable" was only intended to indicate that struct attributes couldn't be altered after creation, but if the attribute values themselves weren't immutable they could still be mutated. For example:

class Demo(Struct, frozen=True):
    x: Any

obj = Demo([])

# Can't change x; this will error
obj.x = 2

# Can mutate the value of x if x itself is mutable; this won't error
obj.x.append(2)

I agree that the docs here aren't being specific in what we mean by that, I'd be happy to update them (or accept a PR to update them) to be more clear.

When the same approach is used to modify frozen Struct (for example in __post_init__) this leads to error:

The error you're seeing is raised by the cpython VM since due to layout differences object.__setattr__ can't apply to struct instances. Like anything in cpython I'm sure there's some way to hack around the immutability of a struct (with ctypes anything is possible), but we don't provide or document a specific method to do so. Messing with immutability guarantees can lead to weird unexpected behavior, i'd rather not enable it unless necessary. Can you explain your use case here?

[wikiped]

The error you're seeing is raised by the cpython VM since due to layout differences object.setattr can't apply to struct instances.

I understand the origin of the error, but was hoping for a "way-around" this "limitation".

Can you explain your use case here?

class A(msgspec.Struct, frozen=False):
    name: str
    alias: str | None = None

    def __post_init__(self):
        if not self.alias:
            self.alias = self.name

What it basically comes down to - is setting some value for a field, depending on the value of the other field of the struct.

Of course, this logic could be applied at a consumer end. But the same could, probably, be said about everything else linked to validation.

This isn't a 'deal breaker', but a nice-to-have feature for 'conscenting adults'.

[jcrist]

I've added msgspec.structs.force_setattr to handle this task. It works the same as setattr, but will still work on a frozen struct. This does violate the frozen guarantees, so should only be used when you know what you're doing. Fixed in #600.

[wikiped]

Thank you very much for adding this feature!