Support jsonschema's const keyword.

Feature Request

https://json-schema.org/latest/json-schema-validation.html#rfc.section.6.1.3

This is a very useful feature for when structural subtyping just doesn’t quite cut it and you need to differentiate two semantically different but structurally identical types.

Some ways that this could be implemented:

class Foo(pydantic.BaseModel):
    bar: pydantic.Constant = "baz"

class Foo(pydantic.BaseModel):
    bar: pydantic.constant("baz")

Which would each generate the following schema:

{
    "title": "Foo",
    "type": "object",
    "properties": {
        "bar": {
            "title": "Bar",
            "const": "baz"
        }
    },
    "required": ["bar"]
}

I’m happy to put some work in for this, if it’s something that you’re willing to support.

[tiangolo]

Sounds good!

I can imagine something like:

class Foo(BaseModel):
    bar: str = Schema("baz", const=True)

[Qazzquimby]

That doesn’t set the constant value to ‘bar’ as I had in my example, unless I misunderstand you. foo: typing.Literal['bar'] seems to be what I was looking for.