Allow use of object.__setattr__ to modify frozen instances of Struct #588

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1. jcrist commented on Nov 28, 2023

   

   jcrist

   on Nov 28, 2023

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   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?

   👍React with 👍1Reacted by Michael
2. wikiped commented on Dec 3, 2023

   

   wikiped

   on Dec 3, 2023

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   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'.
3. jcrist

   closed this as completedin #600on Dec 4, 2023
4. jcrist commented on Dec 4, 2023

   

   jcrist

   on Dec 4, 2023

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   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.

   👍React with 👍2Reacted by wikiped and Umar Butler
5. wikiped commented on Dec 4, 2023

   

   wikiped

   on Dec 4, 2023

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   Thank you very much for adding this feature!