pydantic-projections

Elegant projection of Pydantic BaseModels through Python Protocols — serialise and deserialise only the fields a Protocol declares, nothing more.

Install

uv add pydantic-projections
# or
pip install pydantic-projections

For the optional FastAPI integration (ProjectedResponse), install the extra:

pip install pydantic-projections[fastapi]

Why

You have a fat BaseModel for internal use, and you want to expose only a subset of its fields over an API, to a logging system, or to a downstream consumer. Pydantic already lets you do this with model_dump(include=...), but that's stringly-typed and type-unsafe. A Protocol describes the shape you want; pydantic-projections turns that Protocol into a real BaseModel at runtime, cached per (protocol, frozen, config) triple.

Usage

from typing import Protocol

from pydantic import BaseModel
from pydantic_projections import project, projection


class User(BaseModel):
    id: int
    name: str
    email: str
    password_hash: str


class UserSummary(Protocol):
    id: int
    name: str


user = User(id=1, name="Alice", email="a@b.c", password_hash="secret")

# One-shot: project an instance, get a BaseModel typed as UserSummary
summary = project(user, UserSummary)
summary.model_dump_json()
# -> '{"id":1,"name":"Alice"}'

# Get the reusable class (cached): useful for response_model, schema export, etc.
SummaryModel = projection(UserSummary)
SummaryModel.model_validate_json('{"id":1,"name":"Alice","extra":"ignored"}')
# -> extra fields are ignored
SummaryModel.model_json_schema()
# -> standard pydantic JSON schema

Field metadata

A Protocol member can carry pydantic metadata through Annotated[T, Field(...)]. Constraints are enforced and schema hints reach the JSON schema:

from typing import Annotated, Protocol

from pydantic import Field


class UserSummary(Protocol):
    id: int
    name: Annotated[str, Field(min_length=3, examples=["Alice"])]


projection(UserSummary).model_json_schema()["properties"]["name"]
# -> {'examples': ['Alice'], 'minLength': 3, 'title': 'Name', 'type': 'string'}

project(User(id=1, name="Al", email="a@b.c", password_hash="x"), UserSummary)
# -> raises ProjectionError: name is shorter than min_length

@property members can be annotated the same way, and a Protocol nested inside the annotation (Annotated[AddressSummary, Field(description=...)]) is still substituted. A default= in the metadata does not make the field optional — projected fields stay required.

Nested protocols and containers

Protocols can reference other Protocols. The projection is built recursively, so list[P], dict[str, P], tuple[P, ...], P | None, Union[P, ...], and plain P all work:

class AddressSummary(Protocol):
    street: str
    zip_code: str


class UserWithAddresses(Protocol):
    id: int
    name: str
    address: AddressSummary
    past_addresses: list[AddressSummary]
    shipping: AddressSummary | None

@property-style Protocols

Protocols that declare fields as properties are also supported — the property's return type is used:

class UserDisplay(Protocol):
    @property
    def display_name(self) -> str: ...


project(user, UserDisplay).display_name

Computed / derived fields on the source

@computed_field / @property declarations on the source model are readable through the projection, because validation runs with from_attributes=True:

class User(BaseModel):
    id: int
    name: str

    @computed_field
    @property
    def display_name(self) -> str:
        return f"User: {self.name}"


class UserDisplay(Protocol):
    display_name: str


project(user, UserDisplay).display_name  # -> "User: Alice"

Deriving a projection from a model (pick)

Hand-written response models drift. Thirteen FastAPI routes handing SQLModel table=True classes straight to response_model publish every raw DB column — password_hash today, whatever the next migration adds tomorrow. The usual fix is thirteen hand-written response models, each of which has to be kept in step with its table by hand. pick(Model, *names) replaces each one with a single line whose types are read from the source, so they cannot drift from it:

from datetime import datetime

from pydantic import BaseModel
from pydantic_projections import pick, project


class User(BaseModel):
    id: int
    email: str
    created_at: datetime
    password_hash: str


UserView = pick(User, "id", "email", "created_at")

user = User(
    id=1,
    email="alice@example.com",
    created_at=datetime(2026, 1, 1),
    password_hash="secret",
)

project(user, UserView).model_dump_json()
# -> '{"id":1,"email":"alice@example.com","created_at":"2026-01-01T00:00:00"}'

pick returns an ordinary Protocol, so the rest of this README applies to it unchanged — project(), projection(..., config=, frozen=), nesting it inside another Protocol, and both FastAPI patterns described below:

@app.get("/users/{id}", response_model=projection(UserView))
def get_user(id: int) -> User:
    return db.get_user(id)


@app.get("/fast/users/{id}", responses={200: openapi_response(UserView)})
def get_user_fast(id: int) -> Response:
    return ProjectedResponse(db.get_user(id), UserView)

Both return {"id":1,"email":"alice@example.com","created_at":"2026-01-01T00:00:00"}, and the second advertises $ref: '#/components/schemas/UserViewProjection' in the spec.

pick is an allowlist: a column added to User later stays private until someone opts it in. There is deliberately no omit() counterpart — a denylist publishes every new column by default, which is the failure mode pick exists to remove.

• Every picked field is required, whatever default the source declares.
• Constraints (min_length, gt, …) and schema documentation (title, description, examples, json_schema_extra, deprecated) carry over. Pass metadata=False for bare annotations.
• Aliases are dropped by default: a validation alias makes pydantic look the attribute up under a name the source instance does not have, which breaks from_attributes. Pass aliases=True to carry the source's serialization alias, so a dump with by_alias=True matches the source's wire format while validation still reads the attribute name.
• @computed_field members are pickable; their return type is used.
• name= overrides the generated Protocol's name (default f"{Model.__name__}View", so UserView projects to UserViewProjection). Set it when two picks of the same model would otherwise collide in an OpenAPI schema.
• Unknown names raise at pick() call time — import time in practice, not on the first request:

pick(User, "id", "emial")
# -> ValueError: User has no field 'emial'; available: created_at, email, id, password_hash

A source that isn't a BaseModel subclass raises TypeError. Results are cached, so equal calls return the same Protocol and therefore the same projection class; cache_clear() clears it.

Narrowing a picked field

The allowlist is one level deep: a picked field whose type is another BaseModel publishes that model whole. To narrow it — or to tighten an annotation the source declares as nullable — subclass the picked Protocol and restate only the fields you are changing:

class Order(BaseModel):
    id: int | None = None  # SQLModel-style: nullable until the row is flushed
    total: float
    address: Address       # a full model, published whole by a bare pick


class OrderView(pick(Order, "id", "total", "address"), Protocol):
    id: int                    # narrowed: no longer nullable
    address: AddressSummary    # the Protocol from above, projected recursively


project(order, OrderView).model_dump_json()
# -> '{"id":7,"total":9.5,"address":{"street":"1 Main St","zip_code":"75001"}}'

The base class is picked, so the fields you don't restate still can't drift. And because this is a real class statement, OrderView is valid as a type — owner: OrderView type-checks, unlike the bare pick(...) result. Type checkers do flag the dynamic base class, so it needs a # type: ignore[misc].

Typing at the call site

project(instance, Proto) is typed to return Proto, so summary.name resolves to str in mypy/pyright without a cast. At runtime the object is a BaseModel subclass that structurally satisfies the Protocol.

FastAPI integration

Two patterns, in order of speed:

Drop-in response_model. projection(Proto) returns a real BaseModel class, so it plugs into FastAPI's response_model unchanged — the endpoint's output is pruned to the Protocol's fields and the OpenAPI schema matches:

from fastapi import FastAPI
from pydantic_projections import projection

app = FastAPI()


@app.get("/users/{id}", response_model=projection(UserSummary))
def get_user(id: int) -> User:
    return db.get_user(id)  # returns the fat User; caller sees only UserSummary's fields

This path still goes through FastAPI's full serialize_response + jsonable_encoder + json.dumps chain every request. Fine for most endpoints.

High-throughput: ProjectedResponse. For hot paths, return a ProjectedResponse instead. It bypasses serialize_response/jsonable_encoder entirely and emits JSON bytes via two Rust-backed calls (validate, then serialize) on the projection class's __pydantic_validator__ and __pydantic_serializer__, with no jsonable_encoder / json.dumps step in between:

from fastapi import FastAPI
from fastapi.responses import Response
from pydantic_projections import ProjectedResponse

app = FastAPI()


@app.get("/users/{id}")
def get_user(id: int) -> Response:
    return ProjectedResponse(db.get_user(id), UserSummary)

Don't set response_model when using ProjectedResponse — FastAPI would run validation + serialization again and defeat the purpose. ProjectedResponse(...) validates at construction time, so a source that doesn't satisfy the Protocol raises ProjectionError from the handler (catchable via a FastAPI exception handler). Install with pip install pydantic-projections[fastapi].

Extra serializer kwargs (by_alias=True, exclude_none=True, indent=2, …) are forwarded to the projection's __pydantic_serializer__.to_json, so a project using a camelCase alias_generator in its projection() config can do ProjectedResponse(user, UserSummary, by_alias=True).

OpenAPI schema. Because response_model is unset, FastAPI cannot derive a 200 response schema for the endpoint — the OpenAPI spec will show an empty schema. Use openapi_response(Protocol) to advertise the projection's schema via responses=:

from fastapi import FastAPI
from fastapi.responses import Response
from pydantic_projections import ProjectedResponse, openapi_response

app = FastAPI()


@app.get("/users/{id}", responses={200: openapi_response(UserSummary)})
def get_user(id: int) -> Response:
    return ProjectedResponse(db.get_user(id), UserSummary)

This advertises the projection's schema in the spec ($ref: '#/components/schemas/UserSummaryProjection') without re-running serialization on the response path. openapi_response() returns a {"model": ...} entry, so it composes naturally with other status codes: responses={200: openapi_response(UserSummary), 404: {"model": NotFound}}.

See benches/test_render_bench.py for the comparison; in our measurements ProjectedResponse is roughly 2–4× faster than the response_model=projection(...) path on raw ser/deser work, depending on FastAPI version and response shape. Note that FastAPI's TestClient is a poor way to measure this — its per-call transport setup dominates — use uvicorn + an external HTTP benchmark tool (wrk, hey, oha) for end-to-end numbers.

Config pass-through and frozen

Projections are immutable by default (frozen=True): a projection is a derived view of its source, so attempting instance.x = ... raises ValidationError. Opt back into mutation with frozen=False if you need it. Merge additional ConfigDict options (e.g. alias generator for camelCase output) via config=:

from pydantic import ConfigDict
from pydantic.alias_generators import to_camel

CamelSummary = projection(
    UserSummary,
    config=ConfigDict(alias_generator=to_camel, populate_by_name=True),
)

MutableSummary = projection(UserSummary, frozen=False)

frozen and config propagate into every Protocol reachable from the outer one, so an alias generator applied at the top level also camelCases nested projections. extra="ignore" and from_attributes=True are hard invariants — user-supplied ConfigDict cannot override them.

Classes are cached per (protocol, config, frozen) triple; config values must be hashable.

Error handling

project() wraps pydantic's ValidationError in a ProjectionError that carries the protocol, source type, and original validation error:

from pydantic_projections import ProjectionError

try:
    project(partial_user, UserSummary)
except ProjectionError as e:
    e.protocol           # the Protocol class
    e.source_type        # type(instance)
    e.validation_error   # the underlying pydantic ValidationError

JSON shortcut

from pydantic_projections import project_json, project_json_bytes

project_json(user, UserSummary)                 # str
project_json(user, UserSummary, indent=2)       # forwards **kwargs to the projection's serializer
project_json_bytes(user, UserSummary)           # bytes — skip the str intermediate

Prefer project_json_bytes when writing to a socket or HTTP response: it calls the projection class's Rust-backed serializer directly and avoids the bytes→str→bytes round-trip.

Cache management

from pydantic_projections import cache_clear
cache_clear()  # useful in test fixtures or hot-reload workflows

Semantics

• Extras are ignored on deserialisation (extra="ignore"). This is a hard invariant — passing extra="forbid" via config= does not override it.
• from_attributes=True — accepts dicts, JSON, or arbitrary objects that expose the Protocol's members. Also a hard invariant.
• Projections are immutable by default (frozen=True). Pass frozen=False for a mutable variant.
• frozen and config= propagate to nested projections — an alias generator or frozen flag applied at the top level also applies to every Protocol reachable through containers and unions.
• Optional widening is allowed: source name: str is accepted by a Protocol declaring name: str | None.
• Narrowing is not: if the source value is None for a Protocol field typed str, validation raises.
• Classes are cached per (protocol, config, frozen) via functools.cache.

Performance

• project() and project_json() invoke the projection class's __pydantic_validator__ directly, skipping BaseModel.model_validate's Python wrapper. Observable behaviour is unchanged; per-call cost is ~1.3–1.5× lower.
• project_json_bytes() emits bytes via __pydantic_serializer__.to_json directly, avoiding model_dump_json().encode()'s bytes→str→bytes round-trip.
• ProjectedResponse (FastAPI) skips serialize_response + jsonable_encoder + json.dumps and goes straight from source → validated projection → JSON bytes via two Rust-backed calls (validate_python, then to_json) with no jsonable_encoder / json.dumps step in between. In our benches (benches/test_render_bench.py) the fast path runs roughly 2–4× faster than the response_model=projection(...) baseline, depending on FastAPI version and response shape. Run locally with uv run pytest benches/ --benchmark-only — numbers vary by machine, so compare relative columns.

Limitations

• Cyclic Protocols (a Protocol that references itself transitively) are not supported and will recurse.
• Generic Protocols (Protocol[T]) with unresolved TypeVars are not supported.
• Config values passed via config= must be hashable for caching.
• pick() synthesizes its Protocol at runtime, so type checkers see Any. UserView = pick(...) is a variable, not a class statement: using it as an annotation (owner: UserView) is a mypy error (Variable "UserView" is not valid as a type) even though it works at runtime. The field names are strings, so typos and renames are not caught statically either — pick() validates them eagerly at call time to compensate. Subclassing the result gets you back a real class statement that is valid as a type, at the cost of one # type: ignore[misc] for the dynamic base class.
• pick() cannot invent non-nullability. A SQLModel table=True class annotates its primary key id: int | None, so pick(Model, "id") yields int | None — required, but still nullable in the schema. pick fixes required-ness, not the annotation; subclass the picked Protocol and restate that one field to narrow it.
• pick()'s allowlist is one level deep. A picked field typed as another BaseModel publishes that model whole, including columns you would not have picked — narrow it with a Protocol-typed override.

Development

uv sync
uv run pytest
uv run python scripts/validate_tests.py
uv run ruff check src/ tests/ benches/ scripts/
uv run mypy src/
uv run coverage run -m pytest && uv run coverage report
uv run pytest benches/ --benchmark-only    # perf micro-benches

Tests use pytest-describe (describe_/when_/with_/it_). See CLAUDE.md for conventions.