Define fields that are not arguments to the constructor

[felix-hilden]

I'd like to define a field that is not available in the model arguments. So to define an instance attribute with either a default or a default factory, and no way to change it when constructing the model (assigning is fine). To essentially achieve this:

class Foo:
    def __init__(self, a: int, b: str = ""):
        self.a = a
        self.b = b
        self.c = generate_c()  # or without the call as just a default value

Does Pydantic support this? I didn't find relevant arguments to Field that would do it, or any information when searching online. I see that private (ignored) fields are available, but I'd like for this to be compatible with validation and inheritance. So I could write for example:

class Foo(BaseModel):
    a: int
    b: str = ""
    c: int

class Bar(Foo):
    c: int = Field(default_factory=list, no_init=True)

[adriangb]

We do not currently support this, but dataclasses do (via init=False) and it's a reasonable feature request for Pydantic.

[felix-hilden]

Thank you for the quick response! May I create an issue (or can this discussion be converted to one)?

[felix-hilden]

The support has been added in 2.6 it seems!

[lig]

Also, init specified in PEP 683 as a standard field parameter https://peps.python.org/pep-0681/#field-specifier-parameters.

[guilhem-fry]

I would also like init to be added to Pydantic as a boolean Field parameter.

[lig]

Pydantic uses Field(init_var: bool) for that. See https://docs.pydantic.dev/latest/usage/fields/#dataclass-constraints

[felix-hilden]

That's a very useful arg as well, but not quite what I was after. The new init does the trick though 🎉

[lautig32]

I'm in the same situation, I'm trying to make the attribute that I set the variable to not be considered in the initialization even if it's sent as a parameter, but I can't get it to work... Do you know if this possibility exists?