uow-lib 0.1.2

Generic Unit of Work pattern implementation for Python

uow-lib

Generic, backend-agnostic implementation of the Unit of Work pattern for Python 3.12+.

The library tracks entity lifecycle, automatically detects mutations, and coordinates persistence through user-defined data mappers — no ORM required.

Installation

pip install uow-lib

Quick start

from dataclasses import dataclass, field
from uow import (
    Connection,
    EntityConfig,
    GenericDataMapper,
    InstrumentationRegistry,
    ListOf,
    UnitOfWork,
)


# 1. Define your entities
@dataclass
class OrderItem:
    id: int | None
    product: str
    qty: int


@dataclass
class Order:
    id: int | None
    customer: str
    items: list[OrderItem] = field(default_factory=list)


# 2. Implement data mappers (one per entity type)
class OrderMapper:
    def __init__(self, conn: Connection) -> None:
        self.conn = conn

    async def save(self, entities):
        ...  # INSERT into the database

    async def update(self, entities):
        ...  # UPDATE in the database

    async def delete(self, entities):
        ...  # DELETE from the database


class OrderItemMapper:
    def __init__(self, conn: Connection) -> None:
        self.conn = conn

    async def save(self, entities): ...
    async def update(self, entities): ...
    async def delete(self, entities): ...


# 3. Register entity configurations
registry = InstrumentationRegistry()
registry.register(EntityConfig(
    entity_type=Order,
    identity_key=("id",),
    mapper_type=OrderMapper,
    children={"items": ListOf(OrderItem)},
))
registry.register(EntityConfig(
    entity_type=OrderItem,
    identity_key=("id",),
    mapper_type=OrderItemMapper,
    depends_on=[Order],
))

# 4. Use the Unit of Work
async def create_order(conn: Connection) -> None:
    uow = UnitOfWork(conn, registry)

    order = Order(id=None, customer="Alice", items=[
        OrderItem(id=None, product="Widget", qty=3),
    ])
    uow.register_new(order)       # order + items tracked as NEW
    await uow.commit()            # calls OrderMapper.save, then OrderItemMapper.save

async def update_order(conn: Connection, order: Order) -> None:
    uow = UnitOfWork(conn, registry)
    uow.register_clean(order)     # track existing entity

    order.customer = "Bob"        # change detected automatically
    order.items.append(           # new child auto-registered as NEW
        OrderItem(id=None, product="Gadget", qty=1),
    )
    await uow.commit()            # UPDATE order, INSERT new item

Features

Automatic change tracking

After register_clean, the library instruments entity classes to intercept __setattr__. Any mutation to a tracked attribute marks the entity as dirty — no manual flags needed.

uow.register_clean(order)
order.customer = "Bob"   # automatically detected, will trigger UPDATE on flush

Child relationship types

Describe entity graphs declaratively via children in EntityConfig:

Spec	Description
ListOf(ChildType)	One-to-many list, wrapped in TrackedList
SetOf(ChildType)	One-to-many set, wrapped in TrackedSet
SingleOf(ChildType)	One-to-one reference
EmbeddedOf(VOType)	Immutable value object (frozen dataclass)
CollectionOfEmbedded(VOType)	List of immutable value objects

Entity children (ListOf, SetOf, SingleOf) are tracked and persisted individually. Adding a child to a tracked collection registers it as NEW; removing one marks it as DELETED. Replacing a SingleOf reference deletes the old child and inserts the new one.

Embedded value objects (EmbeddedOf, CollectionOfEmbedded) are not separate entities. Changes to them mark the parent entity as dirty. EmbeddedOf requires a frozen dataclass:

from dataclasses import dataclass
from uow import EmbeddedOf

@dataclass(frozen=True)
class Address:
    street: str
    city: str

registry.register(EntityConfig(
    entity_type=Customer,
    identity_key=("id",),
    mapper_type=CustomerMapper,
    children={"address": EmbeddedOf(Address)},
))

Dirty primitive collections

Plain list, set, and dict attributes that aren't entity children are automatically wrapped in mutation-aware proxies (DirtyList, DirtySet, DirtyDict). Mutations mark the parent entity as dirty:

profile.tags.append("new-tag")       # DirtyList  -> parent marked dirty
profile.roles.add("editor")          # DirtySet   -> parent marked dirty
profile.metadata["key"] = "value"    # DirtyDict  -> parent marked dirty

Lazy child materialization

Collections registered via register_clean use lazy materialization — children are not registered with the UoW until the collection is first accessed. This avoids unnecessary work when loading large entity graphs.

Dependency-aware flush ordering

Specify depends_on in EntityConfig to control persistence order. The library uses topological sort (Kahn's algorithm) to ensure:

• Inserts: parents before children (by dependency depth)
• Deletes: children before parents (reversed)
• Updates: stable registration order

Circular dependencies raise CyclicDependencyError.

Identity map

The built-in IdentityMap guarantees at most one in-memory instance per entity identity (type, key). Attempting to register two different objects with the same identity raises DuplicateEntityError.

Transactional semantics

Method	Behavior
flush()	Detect changes and call mapper operations; rollback on error
commit()	Flush + connection.commit(); rollback on error
rollback()	connection.rollback() and detach all entities

Backend agnostic

Persistence is defined through two protocols — implement them for any database:

class Connection(Protocol):
    async def commit(self) -> None: ...
    async def rollback(self) -> None: ...

class GenericDataMapper[T](Protocol):
    async def save(self, entities: Iterable[T]) -> None: ...
    async def update(self, entities: Iterable[T]) -> None: ...
    async def delete(self, entities: Iterable[T]) -> None: ...

Works with asyncpg, aiosqlite, databases, or any async connection that satisfies the Connection protocol.

Excluding fields from tracking

Use exclude_from_tracking to prevent internal attributes (e.g., domain events) from triggering updates:

EntityConfig(
    entity_type=Aggregate,
    identity_key=("id",),
    mapper_type=AggregateMapper,
    exclude_from_tracking=frozenset({"_events"}),
)

API reference

Core classes

• UnitOfWork(connection, registry) — main entry point. Methods: register_new, register_clean, register_deleted, flush, commit, rollback.
• InstrumentationRegistry — registry for EntityConfig objects. Call register(config) for each entity type.
• EntityConfig — declares entity type, identity key, mapper type, children, dependencies, and excluded fields.

Child specs

ListOf, SetOf, SingleOf, EmbeddedOf, CollectionOfEmbedded

Collections

TrackedList, TrackedSet — collection wrappers that fire callbacks on add/remove.

Protocols

Connection, GenericDataMapper[T]

Exceptions

Exception	When
UoWError	Base exception
UnregisteredEntityError	Entity type has no registered EntityConfig
DuplicateEntityError	Two objects share the same identity
UntrackedEntityError	Operation on entity not tracked by this UoW
CyclicDependencyError	depends_on graph contains a cycle

License

MIT