runsheet 0.1.4

Type-safe, composable business logic pipelines for Python

runsheet

Type-safe, composable business logic pipelines for Python.

from pydantic import BaseModel
from runsheet import Pipeline, step

class OrderInput(BaseModel):
    order_id: str
    amount: float

class ValidatedOrder(BaseModel):
    validated: bool

class ChargeOutput(BaseModel):
    charge_id: str

@step(requires=OrderInput, provides=ValidatedOrder)
async def validate_order(ctx: OrderInput) -> ValidatedOrder:
    if ctx.amount <= 0:
        raise ValueError("Invalid amount")
    return ValidatedOrder(validated=True)

@step(requires=OrderInput, provides=ChargeOutput)
async def charge_payment(ctx: OrderInput) -> ChargeOutput:
    charge = await stripe.charges.create(amount=ctx.amount)
    return ChargeOutput(charge_id=charge.id)

@charge_payment.rollback
async def undo_charge(ctx: OrderInput, output: ChargeOutput) -> None:
    await stripe.refunds.create(charge=output.charge_id)

checkout = Pipeline(
    name="checkout",
    steps=[validate_order, charge_payment, send_confirmation],
)

result = await checkout.run(OrderInput(order_id="123", amount=50.0))
# result.data["charge_id"] — accumulated context, validated at every boundary

Why runsheet

Business logic has a way of growing into tangled, hard-to-test code. A checkout flow starts as one function, then gains validation, payment processing, inventory reservation, email notifications — each with its own failure modes and cleanup logic. Before long you're staring at a 300-line function with nested try/except blocks and no clear way to reuse any of it.

runsheet gives that logic structure. You break work into small, focused steps with explicit inputs and outputs, then compose them into pipelines. Each step is independently testable. The pipeline handles context accumulation — args persist, outputs merge — along with rollback on failure and Pydantic validation at step boundaries. Immutable context boundaries mean steps can't accidentally interfere with each other.

runsheet is for in-process business logic orchestration — multi-step flows inside an application service. It is not a distributed workflow engine, job queue, or durable execution runtime. That said, the two are complementary: a runsheet pipeline works well as the business logic inside a Temporal activity or an Inngest function handler.

The name takes its inspiration from the world of stage productions and live broadcast events. A runsheet is the document that sequences every cue, handoff, and contingency so the show runs smoothly. Same idea here: you define the steps, and runsheet makes sure they execute in order with clear contracts between them.

When to use it

Good fit:

• Multi-step business flows — checkout, onboarding, provisioning, data import
• Operations that need compensating actions (rollback) on failure
• Reusable orchestration shared across handlers, jobs, and routes
• Logic where schema-checked boundaries between steps add confidence
• Anywhere you'd otherwise write a long imperative function with a growing number of intermediate variables and try/except blocks

Not the right tool:

• Trivial one-off functions that don't benefit from step decomposition
• Long-running durable workflows that survive process restarts — use Temporal or Inngest
• Cross-service event choreography — use a message bus
• Simple CRUD handlers with no orchestration complexity

What you get

Typed accumulated context

Args persist and outputs accumulate. Initial arguments flow through the entire pipeline, each step's output merges into the context, and every step sees the full picture of everything before it.

Immutable step boundaries

Context is frozen (MappingProxyType) at every step boundary — this is a guarantee, not an implementation detail. Each step receives a read-only snapshot and returns only what it adds. Steps cannot mutate shared pipeline state; the pipeline engine manages accumulation. This eliminates a class of bugs where step B accidentally corrupts step A's data, and it makes rollback reliable because each handler receives the exact snapshot from before the step ran.

Pydantic validation at boundaries

Each step declares requires and provides as Pydantic models. Pydantic validates context against requires before each step and validates output against provides after. Mismatches fail fast with clear errors. Your IDE shows exact input and output shapes.

Rollback with snapshots

On failure, rollback handlers execute in reverse order. Each handler receives the pre-step context snapshot and the step's output, so it knows exactly what to undo. Rollback is best-effort: if a handler raises, remaining rollbacks still execute.

Compared to alternatives

Capability	Plain functions	Ad hoc orchestration	runsheet
Reusable business steps	Manual	Manual	Built-in
Typed accumulated context	—	Manual	Built-in
Rollback / compensation	Manual	Manual	Automatic, snapshot-based
Schema validation at boundaries	—	Manual	Pydantic at every step
Middleware (logging, timing)	Manual	Manual	Built-in
Control-flow combinators	Manual	Manual	Built-in
Composable (nest pipelines)	Manual	Difficult	Pipelines are steps
Immutable context boundaries	—	Rarely	Always (MappingProxyType)

Install

pip install runsheet
# or
uv add runsheet

Quick start

Define steps

Each step declares what it reads from context (requires) and what it adds (provides). The @step decorator wraps a function with metadata for pipeline execution.

Step functions can be sync or async — both are supported.

from pydantic import BaseModel
from runsheet import step, RetryPolicy


class OrderInput(BaseModel):
    order_id: str
    amount: float


class ValidatedOrder(BaseModel):
    validated: bool


class ChargeOutput(BaseModel):
    charge_id: str


@step(requires=OrderInput, provides=ValidatedOrder)
async def validate_order(ctx: OrderInput) -> ValidatedOrder:
    if ctx.amount <= 0:
        raise ValueError("Invalid amount")
    return ValidatedOrder(validated=True)


@step(
    requires=OrderInput,
    provides=ChargeOutput,
    retry=RetryPolicy(count=3, delay=0.2, backoff="exponential"),
    timeout=5.0,
)
async def charge_payment(ctx: OrderInput) -> ChargeOutput:
    charge = await stripe.charges.create(amount=ctx.amount)
    return ChargeOutput(charge_id=charge.id)


@charge_payment.rollback
async def undo_charge(ctx: OrderInput, output: ChargeOutput) -> None:
    await stripe.refunds.create(charge=output.charge_id)

Build and run a pipeline

import asyncio
from runsheet import Pipeline, AggregateSuccess, AggregateFailure

checkout = Pipeline(
    name="checkout",
    steps=[validate_order, charge_payment, send_confirmation],
    args_schema=OrderInput,
)

result = await checkout.run(OrderInput(order_id="123", amount=50.0))

if result.success:
    print(result.data["charge_id"])   # accumulated context
    print(result.data["email_sent"])
else:
    print(result.error)               # what went wrong
    print(result.rollback)            # RollbackReport(completed=..., failed=...)

pipeline.run() never raises. It always returns a result — either AggregateSuccess or AggregateFailure.

A second example: workspace onboarding

Steps compose across domains. The same patterns — typed inputs, rollback on failure, accumulated context — apply to any multi-step flow:

from pydantic import BaseModel
from runsheet import Pipeline, step


class OnboardInput(BaseModel):
    owner_email: str
    plan: str


class WorkspaceOutput(BaseModel):
    workspace_id: str


class ResourceOutput(BaseModel):
    bucket_arn: str
    db_url: str


@step(requires=OnboardInput, provides=WorkspaceOutput)
async def create_workspace(ctx: OnboardInput) -> WorkspaceOutput:
    ws = await db.workspaces.create(owner=ctx.owner_email, plan=ctx.plan)
    return WorkspaceOutput(workspace_id=ws.id)


@create_workspace.rollback
async def undo_workspace(ctx: OnboardInput, output: WorkspaceOutput) -> None:
    await db.workspaces.delete(output.workspace_id)


@step(provides=ResourceOutput)
async def provision_resources(ctx: dict) -> ResourceOutput:
    infra = await provisioner.create(ctx["workspace_id"], ctx["plan"])
    return ResourceOutput(bucket_arn=infra.bucket_arn, db_url=infra.db_url)


@provision_resources.rollback
async def undo_resources(ctx: dict, output: ResourceOutput) -> None:
    await provisioner.teardown(output.bucket_arn, output.db_url)


onboard = Pipeline(
    name="onboard_workspace",
    steps=[create_workspace, provision_resources, send_welcome_email],
)

result = await onboard.run(OnboardInput(owner_email="[e-mail]", plan="team"))

If send_welcome_email fails, provisioned resources are torn down and the workspace is deleted — automatically, in reverse order.

Pipeline composition

Pipelines are steps — use one pipeline as a step in another:

checkout = Pipeline(
    name="checkout",
    steps=[validate_order, charge_payment, send_confirmation],
)

full_flow = Pipeline(
    name="full_flow",
    steps=[checkout, ship_order, notify_warehouse],
)

Retry and timeout

Steps can declare retry policies and timeouts directly:

@step(
    provides=ApiResponse,
    retry=RetryPolicy(count=3, delay=0.2, backoff="exponential"),
    timeout=5.0,  # seconds
)
async def call_external_api(ctx) -> ApiResponse:
    ...

Retry re-executes the step on failure. The retry_if predicate lets you inspect errors and decide whether to retry:

RetryPolicy(
    count=3,
    retry_if=lambda errors: any("ECONNRESET" in str(e) for e in errors),
)

Timeout cancels run via asyncio.timeout. If the step exceeds the limit, it fails with a TimeoutError. When both are set, each retry attempt gets its own timeout.

Conditional steps

from runsheet import when

checkout = Pipeline(
    name="checkout",
    steps=[
        validate_order,
        charge_payment,
        when(lambda ctx: ctx.get("amount", 0) > 10000, notify_manager),
        send_confirmation,
    ],
)

Skipped steps produce no snapshot, no rollback entry, and do not appear in result.meta.steps_executed.

Parallel steps

Run steps concurrently with parallel(). Outputs merge in array order:

from runsheet import parallel

checkout = Pipeline(
    name="checkout",
    steps=[
        validate_order,
        parallel(reserve_inventory, charge_payment),
        send_confirmation,
    ],
)

On partial failure, succeeded inner steps are rolled back before the error propagates. Inner steps retain their own requires/provides validation, retry, and timeout behavior. Conditional steps (via when()) work inside parallel().

Choice (branching)

Execute the first branch whose predicate returns True — like an AWS Step Functions Choice state:

from runsheet import choice

checkout = Pipeline(
    name="checkout",
    steps=[
        validate_order,
        choice(
            (lambda ctx: ctx.get("method") == "card", charge_card),
            (lambda ctx: ctx.get("method") == "bank", charge_bank),
            charge_default,  # bare step = default
        ),
        send_confirmation,
    ],
)

Predicates are evaluated in order — first match wins. A bare step (without a tuple) can be passed as the last argument to serve as a default. If no predicate matches, the step fails with a CHOICE_NO_MATCH error. Only the matched branch participates in rollback.

Collection combinators

Distribute (collection distribution)

Fan out execution over one or more context collections, binding each item to the step's named scalar inputs. With multiple collections, distribute computes the cross product and runs the step once per combination.

from runsheet import distribute

# Single collection — run send_email once per account_id
pipeline = Pipeline(
    name="notify",
    steps=[
        distribute("emails", {"account_ids": "account_id"}, send_email),
    ],
)
# Context: {"org_id": "org-1", "account_ids": ["a1", "a2"]}
# Output:  {"emails": [{"email_id": "email-a1"}, {"email_id": "email-a2"}]}

# Cross product — run once per (account_id, region_id) pair
pipeline = Pipeline(
    name="reports",
    steps=[
        distribute(
            "reports",
            {"account_ids": "account_id", "region_ids": "region_id"},
            generate_report,
        ),
    ],
)
# 2 accounts x 3 regions = 6 concurrent executions

The mapping dict connects context array keys to the step's scalar input keys. All non-mapped context keys pass through unchanged. Items run concurrently and support partial-failure rollback.

Map (iteration)

Iterate over a collection and run a function or step per item, concurrently:

from runsheet import map_step

# Function form — mapper(item, ctx) -> result
pipeline = Pipeline(
    name="notify",
    steps=[
        map_step(
            "emails",
            lambda ctx: ctx.get("users", []),
            lambda user, ctx: {
                "email": user["email"],
                "sent_at": str(send_email(user["email"])),
            },
        ),
    ],
)

# Step form — reuse existing steps
pipeline = Pipeline(
    name="process",
    steps=[map_step("results", lambda ctx: ctx.get("items", []), process_item)],
)

Items run concurrently via asyncio.gather. Results are collected into a list under the given key. In step form, each item is spread into the pipeline context ({**ctx, **item}) so the step sees both pipeline-level and per-item values. On partial failure, succeeded items are rolled back (step form only).

Filter (collection filtering)

from runsheet import filter_step

pipeline = Pipeline(
    name="notify",
    steps=[
        filter_step(
            "eligible",
            lambda ctx: ctx.get("users", []),
            lambda user, ctx: user["opted_in"],
        ),
        map_step("emails", lambda ctx: ctx.get("eligible", []), send_email),
    ],
)

Predicates receive (item, ctx) and run concurrently. Original order is preserved. If any predicate raises, the step fails. No rollback (filtering is a pure operation).

FlatMap (collection expansion)

from runsheet import flat_map

pipeline = Pipeline(
    name="process",
    steps=[
        flat_map(
            "line_items",
            lambda ctx: ctx.get("orders", []),
            lambda order, ctx: order["items"],
        ),
    ],
)

Maps each item to a list, then flattens one level. Callbacks receive (item, ctx) and run concurrently. No rollback (pure operation).

Dependency injection

No special mechanism needed — pass dependencies as pipeline args and they're available to every step through the accumulated context:

from pydantic import BaseModel, ConfigDict


class Dependencies(BaseModel):
    model_config = ConfigDict(arbitrary_types_allowed=True)

    order_id: str
    stripe: StripeClient
    db: Database


@step(requires=Dependencies, provides=ChargeOutput)
async def charge_payment(ctx: Dependencies) -> ChargeOutput:
    charge = await ctx.stripe.charges.create(amount=ctx.order.total)
    return ChargeOutput(charge_id=charge.id)


pipeline = Pipeline(
    name="checkout",
    steps=[validate_order, charge_payment, send_confirmation],
    args_schema=Dependencies,
)

await pipeline.run(Dependencies(
    order_id="123",
    stripe=stripe_client,
    db=db_client,
))

Args persist through the entire pipeline without any step needing to provides them. Pydantic validates at every boundary that each step's requires are satisfied by the accumulated context. For testing, swap in mocks at the call site.

Rollback

When a step fails, rollback handlers for all previously completed steps execute in reverse order. Each handler receives the pre-step context snapshot and the step's output:

@step(requires=OrderInput, provides=ReservationOutput)
async def reserve_inventory(ctx: OrderInput) -> ReservationOutput:
    reservation = await inventory.reserve(ctx.order_items)
    return ReservationOutput(reservation_id=reservation.id)


@reserve_inventory.rollback
async def undo_reservation(ctx: OrderInput, output: ReservationOutput) -> None:
    await inventory.release(output.reservation_id)

Rollback is best-effort: if a rollback handler raises, remaining rollbacks still execute. The result includes a structured report:

if not result.success:
    result.rollback.completed   # ("charge_payment", "reserve_inventory")
    result.rollback.failed      # (RollbackFailure(step="send_notification", error=...),)

Step result

Every run() returns a result with execution metadata:

# Success (AggregateSuccess)
result.success          # True
result.data             # dict — accumulated context
result.meta.name        # "checkout"
result.meta.args        # Mapping — original args snapshot
result.meta.steps_executed  # ("validate_order", "charge_payment", "send_confirmation")

# Failure (AggregateFailure)
result.success          # False
result.error            # Exception
result.failed_step      # "charge_payment"
result.rollback         # RollbackReport(completed=..., failed=...)
result.meta             # same execution metadata

Middleware

Middleware wraps the full step lifecycle including schema validation:

import time
from runsheet import StepInfo, StepMiddleware


async def timing(step_info: StepInfo, next_fn, ctx):
    start = time.perf_counter()
    result = await next_fn(ctx)
    elapsed = time.perf_counter() - start
    print(f"{step_info.name}: {elapsed:.3f}s")
    return result


async def logging_mw(step_info: StepInfo, next_fn, ctx):
    print(f"-> {step_info.name}")
    result = await next_fn(ctx)
    print(f"<- {step_info.name}")
    return result


pipeline = Pipeline(
    name="checkout",
    steps=[validate_order, charge_payment, send_confirmation],
    middleware=[logging_mw, timing],
)

Error hierarchy

All library errors are RunsheetError subclasses with a code discriminator. Application exceptions pass through as-is.

from runsheet import RunsheetError, TimeoutError, RetryExhaustedError

if not result.success:
    error = result.error
    if isinstance(error, TimeoutError):
        print(f"Timed out after {error.timeout_seconds}s")
    elif isinstance(error, RetryExhaustedError):
        print(f"Failed after {error.attempts} attempts")
    elif isinstance(error, RunsheetError):
        print(f"Library error: {error.code}")
    else:
        print(f"Application error: {error}")

Strict mode

Detect provides key collisions at build time:

pipeline = Pipeline(
    name="checkout",
    steps=[step_a, step_b],  # raises StrictOverlapError if both provide "charge_id"
    strict=True,
)

API reference

@step(requires, provides, name, retry, timeout)

Decorator to create a pipeline step. Returns a Step object with a .rollback sub-decorator.

Option	Type	Description
requires	type[BaseModel]	Optional Pydantic model for required context keys
provides	type[BaseModel]	Optional Pydantic model for provided context keys
name	str	Step name (defaults to function name)
retry	RetryPolicy	Optional retry policy for transient failures
timeout	float	Optional max duration in seconds

Pipeline(name, steps, args_schema, middleware, strict)

Construct a pipeline from a list of steps.

Option	Type	Description
name	str	Pipeline name
steps	Sequence[Runnable]	Steps to execute in order
args_schema	type[BaseModel]	Optional Pydantic model for pipeline input validation
middleware	list[StepMiddleware]	Optional middleware
strict	bool	Optional — raises at build time if two steps provide the same key

when(predicate, step)

Conditional execution. The step only runs when the predicate returns True.

parallel(*steps)

Run steps concurrently and merge outputs. On partial failure, succeeded inner steps are rolled back.

choice(*branches)

Execute the first branch whose predicate returns True. Each branch is a (predicate, step) tuple. A bare step as the last argument serves as a default.

distribute(key, mapping, step)

Distribute collections from context across a step. The mapping dict connects context array keys to the step's scalar input keys. Runs the step once per item (single mapping) or once per cross-product combination (multiple mappings). Non-mapped context keys pass through. Supports per-item rollback on partial and external failure.

map_step(key, collection_fn, mapper)

Iterate over a collection. Mapper receives (item, ctx). Accepts a function or a Step (items spread into context). Step form supports rollback on partial failure.

filter_step(key, collection_fn, predicate)

Filter a collection. Predicate receives (item, ctx). Keeps items where predicate returns True. Concurrent. No rollback.

flat_map(key, collection_fn, mapper)

Map each item to a list, then flatten one level. Mapper receives (item, ctx). Concurrent. No rollback.

StepMiddleware

async def my_middleware(step_info: StepInfo, next_fn, ctx: dict) -> StepResult:
    result = await next_fn(ctx)
    return result

Origins

runsheet draws from sunny/actor (Ruby) for the service-object pattern with declared I/O, sequential composition, and rollback, and @fieldguide/pipeline (TypeScript) for the typed context flow and middleware design. The TypeScript original is runsheet-js.

LLM-friendly docs

See llms.txt for a complete API reference designed for LLM consumption.

License

MIT