Add PatchSchema

[adriaanwm]

Disclaimer: The code in this PR was written entirely with AI. Based on my tests it is working. However, I am not very familiar with Pydantic.

Suppose you have the following schema:

class MyProfileIn(Schema):
    bio: str
    email: Optional[str]

If you use PatchDict[MyProfileIn], then the following patch body passes validation:

{ "bio": null, "email": "[email protected]" }

This should return a 422, because if bio is included, it should be a string. This will cause issues when updating models where non-nullable fields are being set to null.

For this reason I made PatchSchema (PatchSchema[MyProfileIn]) which would result in the following:

{ "bio": null, "email": "[email protected]" }  # 422
{ "email": null }  # 200
{ "bio": "Example" }  # 200
{ "bio": "Example", "email": null }  # 200

[vitalik]

@adriaanwm - we already have PatchDict

[adriaanwm]

@vitalik PatchDict allows null to be provided for a field where null is not a valid value.

if you have

class MyProfileIn(Schema):
    bio: str

then PatchDict[MyProfileIn] means that this payload passes validation:

{ 'bio': null }

Ideally this should return a 422 because null is not a valid value for bio (if it is included, it should be a string). This is why I made PatchSchema.

[vitalik]

I would say this is questinoable/undefinded behaviour - sending { 'bio': null } via PATCH request clearly tells that client wants to set some value as None

anyway - I would look into extending PatchDict with some option instead of introducing class with very similar name and a bit different behaviour