bytespec

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Typed Python models for sequential binary formats. Keep the wire layout explicit while working with ordinary Python objects; bytespec reads and writes the fields and tracks lengths and offsets.

from bytespec import ProtoModel


class User(ProtoModel):
    name: str
    active: bool


user = User(name="Anna", active=True)
encoded = user.encode()
decoded = User.decode(encoded)

assert decoded == user

The default header is used automatically. You only need to set __header__ when your format requires a different layout.

Existing wire formats

For an existing protocol, choose the header elements and field representations:

from bytespec import Constructor, Flags, PayloadLength, ProtoModel, field
from bytespec.types import UInt32


class Packet(ProtoModel):
    __constructor__ = 0x1234
    __header__ = (Constructor(2), Flags(1), PayloadLength(2))

    user_id: UInt32
    name: str | None = field(flag=0, prefix_length=1)


packet = Packet(user_id=42, name="Anna")
assert Packet.decode(packet.encode()) == packet

Constructor identifies the message, Flags marks present optional fields, and PayloadLength records the total byte length of all fields. bytespec computes the flags and lengths; UInt32 and field() define the field representation.

When to use bytespec

bytespec fits custom or existing sequential binary formats when you want typed models without a separate schema language or code generation. It supports lists, nested models, optional fields, IntEnum, UUID, datetime, numeric representations, and custom codecs.

Other tools serve different needs:

• Construct — a powerful binary parser/builder DSL.
• msgspec — standard formats such as JSON and MessagePack.
• Protobuf — a schema/compiler ecosystem with its own wire format.
• struct — small, fixed structures.

Why bytespec? explains the tradeoffs and shows the same packet in struct, Construct, and bytespec.

Installation

Python 3.10+:

pip install bytespec

Or uv add bytespec.

Documentation

The full guide and API reference are available at bytespec.pymax.org in Russian and English.

Take a known packet from your protocol, describe a few fields, and compare the re-encoded result with the original bytes. That is a useful first check of whether the library fits your format.

Community

Questions, ideas and feedback are welcome in the Telegram community.

MIT license.