Python quickstart

Ten minutes. You will start a workflow, kill the process in the middle of it, and watch it carry on from where it was.

Before you begin, follow the Python installation instructions. If you already started the sample there, continue at Run it.

1. A projectCopy Anchor Link

mkdir hello && cd hello
fusee init --lang python

fusee init writes three files:

• workflows.py: the workflow, a plain value apart from any engine.
• app.py: opens an engine, starts one run, and keeps working.
• test_workflows.py: a test with fusee.testing.

It also writes AGENTS.md (and a CLAUDE.md that imports it): the rules a coding assistant needs to change the workflows safely.

The workflow composes a greeting in a step, pauses five seconds, then waits for an approval signal:

@fusee.workflow("greet")
async def greet(ctx: fusee.Ctx, name: str) -> str:
    greeting = await ctx.step("compose", lambda: f"hello {name}")
    await ctx.sleep("pause", "5s")
    ok = await ctx.wait_for(approve, timeout="1h")
    return greeting if ok else f"{greeting} (not approved)"

A step's result is recorded once it runs. When the code runs again after a crash, compose returns its recorded result instead of running again; the sleep and the wait pick up where they were.

2. Run itCopy Anchor Link

fusee dev

fusee dev runs app.py under the dev profile and prints:

fusee: dev profile (/path/to/hello/.fusee/dev): every run's history is kept, each run is replayed against the code when it parks or ends. Not for production.
fusee: UI and HTTP API at http://127.0.0.1:7601
started greet-1; approve it with: fusee runs signal greet-1 approve true

Open http://127.0.0.1:7601: the run greet-1 is there, with its compose step done and its pause running. fusee dev restarts the app when a file changes or the app exits.

3. Kill itCopy Anchor Link

Within the five seconds of the pause, in another terminal:

pkill -9 -f app.py

That is a crash: no shutdown, no flush. fusee dev starts the app again, and the run continues from its journal. The UI shows compose once; it did not run a second time.

4. Approve itCopy Anchor Link

fusee runs signal greet-1 approve true

The app prints hello world. fusee runs ls lists the run as ok; fusee runs show greet-1 prints its timeline.

5. Test itCopy Anchor Link

python test_workflows.py

The test opens a TestEngine (in memory, under the dev profile), starts the run, signals it, and checks the result, the steps it recorded, and that the code replays against its journal (t.check()). See testing your workflows.

NextCopy Anchor Link

• Errors and retries: every error code and what to do.
• Features: steps, signals, queries, updates, children, compensation, schedules, objects, streams, limits.
• Operations and deploying: a server, replicated groups, monitoring.