Add default to JSON schema when const is True

[aminalaee]

Change Summary

Right now for the following Model:

from pydantic import BaseModel, Field

class MyModel(BaseModel):
    x: int = Field(default=0, const=True)

The JSON schema is:

{
  "title": "MyModel",
  "type": "object",
  "properties": {
    "x": {
      "title": "X",
      "const": 0,
      "type": "integer"
    }
  }
}

Which will include the default:

{
  "title": "MyModel",
  "type": "object",
  "properties": {
    "x": {
      "title": "X",
      "const": 0,
      "default": 0,
      "type": "integer"
    }
  }
}

I couldn't find the reason why it was introduced in #2094 but seems like the main purpose was something else.

Related issue number

Fixes #4031

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[aminalaee]

please review

[hramezani]

I couldn't find the reason why it was introduced in #2094 but seems like the main purpose was something else.

The and not field.field_info.const was introduced originally in Implement const keyword in Schema and just reformatted in #2094. IMO, seems the original implementation was wrong.

[PrettyWood]

IMO the const keyword should be deprecated in favor of Literal. With Literal, is the behaviour the expected one?
edit: I don't say we should deprecate it now or that this PR should not be merged! :P

[aminalaee]

@PrettyWood

Not sure if I understand correctly but this will not include a default either:

class Test(BaseModel):
    x: Literal[10]

{"title": "Test", "type": "object", "properties": {"x": {"title": "X", "enum": [10], "type": "integer"}}, "required": ["x"]}

But this will which I'm not sure if is correct:

class Test(BaseModel):
    x: Literal[10] = Field(default=10)

{"title": "Test", "type": "object", "properties": {"x": {"title": "X", "default": 10, "enum": [10], "type": "integer"}}}

[aminalaee]

And about the const deprecation, const=True will produce "const": 10 in schema but Literal[10] will produce "enum": [10] which should serve the same purpose but they are both possible according the the json schema, I could not find any references to the difference though.

[samuelcolvin]

IMO the const keyword should be deprecated in favor of Literal. With Literal, is the behaviour the expected one? edit: I don't say we should deprecate it now or that this PR should not be merged! :P

I think what @PrettyWood means is that Literal[10] (just one option in the literal), should result in "const": 10 in the schema? I think he just means we should get rid of the kwarg from Field (at least that's what I think he means 🤨)

[samuelcolvin]

thanks so much for this.