protodantic

Bidirectional bridge between Protocol Buffers and Pydantic.

Point it at your .proto files and it generates plain pydantic v2 models — with full validation — where every model round-trips losslessly to and from real protobuf messages, wire bytes, and proto JSON. The pydantic → proto direction is a first-class citizen: to_proto_bytes() produces genuine wire-format output that any protobuf consumer in any language can parse.

Install

uv add protodantic-py   # or: pip install protodantic-py

The distribution is named protodantic-py (the plain name is squatted on PyPI); the import stays protodantic:

import protodantic

Usage

Given demo.proto:

syntax = "proto3";
package demo;

message Address {
  string street = 1;
  string city = 2;
}

message User {
  int64 id = 1;
  string name = 2;
  Address address = 3;
  repeated string tags = 4;
  optional string nickname = 5;
}

Generate models:

protodantic generate demo.proto -o models.py

Then:

from models import User, Address

user = User(id=7, name="kory", address=Address(city="Warsaw"), tags=["a", "b"])

# pydantic -> proto: real wire format, readable by any protobuf runtime
data: bytes = user.to_proto_bytes()
msg = user.to_proto()          # a live protobuf Message
json_str = user.to_proto_json()  # canonical proto JSON

# proto -> pydantic: parse + validate in one step
restored = User.from_proto_bytes(data)
assert restored == user

Or drive it from Python:

from protodantic import compile_fdset, generate_source

source = generate_source(compile_fdset(["demo.proto"]))

Type mapping

proto	pydantic
int32/64, uint32/64, sint, fixed	range-validated int (out-of-range fails at construction)
float, double	float
string / bytes / bool	str / bytes / bool
enum	generated OpenEnum (IntEnum that preserves unknown wire values — proto3 enums are open)
message	generated ProtoModel (nested types flattened as Outer_Inner)
repeated T / map<K, V>	list[T] / dict[K, V]
optional, oneof members, singular messages	T | None (presence-aware: None ⇄ unset)
oneof groups	mutual exclusion enforced by a model validator
google.protobuf.Timestamp	datetime.datetime (UTC; naive input treated as UTC)
google.protobuf.Duration	datetime.timedelta
google.protobuf.*Value wrappers	T | None
google.protobuf.Struct / Value / ListValue	dict[str, Any] / Any / list[Any]
google.protobuf.Any	typing.Any — accepts any ProtoModel; packed/unpacked via the model registry

Field names that collide with python keywords or pydantic internals (class, from, model_config, ...) get a trailing underscore (class_) with the proto name kept as a populate alias. The same rule applies to message/enum type names and enum members that are python keywords or would shadow generated code (message list → class list_) — the proto full name stays the source of truth. Same-named messages in different packages get package-qualified class names; every model is also reachable via protodantic.model_for("pkg.Message").

Semantics worth knowing:

• Validation on mutation is on by default (validate_assignment=True): assigning a second oneof member or an out-of-range int raises immediately. Opt out per-model with standard pydantic config on a subclass.
• protodantic.NULL expresses an explicit JSON null in a google.protobuf.Value field (None means unset). In model_dump_json() it serializes as real null; python-mode dumps keep the sentinel.
• Subclassing a generated model does not affect parsing: from_proto/model_for keep resolving to the generated class. To make your subclass the resolution target (e.g. to add custom validators applied on parse), re-declare __proto_full_name__ in its body — explicit opt-in.

Interop with existing _pb2 code

Already consuming a centralized proto package as protoc-generated _pb2 modules? Generated models interoperate directly:

user = User.from_proto(their_pb2_user_instance)   # accepts _pb2 messages
their_msg = their_pb2.User.FromString(user.to_proto_bytes())  # canonical bytes

How it works

protoc (bundled via grpcio-tools) compiles your protos to a FileDescriptorSet, which codegen embeds in the generated module. At runtime, ProtoModel builds dynamic protobuf message classes from those descriptors — no _pb2.py files needed, and no protobuf internals leak into your models.

If several imported generated modules define the same proto type, the registry behind model_for() / nested-message resolution is last-import-wins.

Status & roadmap

Requires Python ≥ 3.11. proto3 only by design (proto2 input is rejected with a clear error). The full supported-behavior spec lives in tests/ — every test documents one use case. Documented policies: unknown fields are dropped when a model re-serializes (the model is the source of truth), and naive datetimes are interpreted as UTC.

• 0.1.0 (current) — greenfield: .proto → pydantic codegen with lossless bidirectional round-trips, plus the semantics future drops build on.
• 0.2.0 — brownfield — reverse schema codegen (pydantic models → .proto), generating from installed _pb2 packages by descriptor reflection, to_proto(into=TheirPb2Class).
• 0.3.0 — performance — benchmark suite (vs json.loads+pydantic, raw _pb2, betterproto), then cached field plans and trusted-construction fast paths.

gRPC service stubs are out of scope: protodantic is a message layer.