Simpy

SimPy

Scope

Use this skill for process-based discrete-event models where active entities yield events and contend for resources: queues, production systems, logistics, networks, service operations, inventory, and other event-driven systems.

SimPy supplies an event scheduler and modeling primitives. It does not choose a scientifically valid conceptual model, input distribution, warm-up, run length, replication count, estimand, or causal interpretation. Treat those as simulation-study methodology, not SimPy API behavior.

Current release and installation

Reviewed 2026-10-01 against current official documentation and the released source:

• Latest stable: SimPy 4.1.2, released on PyPI 2026-05-24; source tag 4.1.2 points to commit f4381649.
• Package metadata requires Python >=3.8 and classifies CPython 3.8-3.14 plus PyPy. SimPy has no runtime dependencies.
• 4.1.2 adds Python 3.13/3.14 support and modern-interpreter test fixes.
• Upstream and this skill are MIT-licensed.

Create a reproducible environment:

uv venv --python 3.13
source .venv/bin/activate
uv pip install "simpy==4.1.2"
python -c "import importlib.metadata; print(importlib.metadata.version('simpy'))"

Do not silently substitute the latest documentation build: it may describe an unreleased development revision. Use the versioned 4.1.2 links in references/sources.md.

Model workflow

1. Define purpose and estimands. State the decision/question, system boundary, entities, resources, state, outputs, time units, and terminating event or steady-state target.
2. Write a conceptual model first. Record assumptions, distributions, routing, priorities, initial conditions, and omitted mechanisms.
3. Implement generators. A SimPy process is an event-yielding Python generator. Register the generator object with env.process(...).
4. Bound execution. Give every production run explicit time, entity, event, and replication caps. Never call env.run() on a model containing an endless process. A time horizon alone cannot stop an endless yield env.timeout(0) loop: events keep running at the same simulation time. Add an event-count or no-time-progress guard, and wait on state-change events instead of zero-delay busy polling.
5. Separate random streams. Use local RNG instances for logically distinct stochastic sources; retain a seed manifest.
6. Instrument deliberately. Observe state after the transition of interest, close time-weighted intervals at the horizon, and test that monitoring does not alter event order.
7. Verify and validate. Test deterministic edge cases, conservation identities, traces, queue discipline, and analytical benchmarks; compare against system or expert evidence for the stated purpose.
8. Run independent replications. Make intervals from replication-level estimates, not correlated entities within one run.
9. Report limitations. Include initialization, unfinished entities, run length, seeds/streams, precision, sensitivity, and validation evidence. Never convert simulation association into a causal claim.

Read references/simulation-methodology.md before making inferential claims.

Minimal bounded model

import random
import simpy

HORIZON = 480.0
arrival_rng = random.Random(101)
service_rng = random.Random(202)
env = simpy.Environment()
server = simpy.Resource(env, capacity=2)
completed = []

def customer(arrival):
    with server.request() as request:
        yield request
        wait = env.now - arrival
        yield env.timeout(service_rng.expovariate(1 / 6.0))
    completed.append((env.now, wait))

def arrivals():
    for _ in range(10_000):  # Entity cap.
        delay = arrival_rng.expovariate(1 / 4.0)
        if env.now + delay >= HORIZON:
            return
        yield env.timeout(delay)
        env.process(customer(env.now))

env.process(arrivals())
env.run(until=HORIZON)

The numeric horizon is half-open: normal events scheduled exactly at 480.0 are not processed. Report unfinished entities rather than silently treating them as completed observations.

Core semantics

Environment and deterministic ordering

Environment is single-threaded. The queue is ordered by simulation time, event priority, then a strictly increasing event ID. Same-time, same-priority events are therefore processed FIFO in scheduling order. Model processes may represent concurrency, but callbacks execute sequentially and deterministically.

This orders the events currently queued. A callback can insert an urgent event at the current time, so a valid processing trace need not have globally increasing priority or event IDs within a timestamp.

• env.now: unitless simulation clock; choose and document one unit.
• env.peek(): next event time or infinity.
• env.step(): process one event; raises EmptySchedule when empty.
• env.active_process: currently executing process, otherwise None.
• env.run(): drain the queue; unsafe with recurring or endless processes.

env.run(until=number) and env.run(until=event) are not interchangeable at boundaries:

• A numeric value schedules an urgent stop event and excludes ordinary events at that exact time.
• An Event criterion returns that event's value when its stop callback fires. Other same-time ordering depends on priority and scheduling order.
• In 4.1.2, Environment.step() preserves callbacks remaining after StopSimulation by rescheduling the target. Consequently, after env.run(until=target), target.processed can remain False until one more step()/run() even though its value was returned. Do not use processed as the sole post-run completion test.

See references/events.md and references/monitoring.md.

Event, Timeout, Process, and Condition

• An Event moves once through not-triggered -> triggered/scheduled -> processed. succeed(value) or fail(exception) triggers it once.
• A Timeout triggers when created, is scheduled for now + delay, and cannot be manually succeeded again.
• env.process(generator) creates a Process; the generator resumes with the yielded event value. Returning from the generator succeeds the Process with that return value. Uncaught exceptions fail it.
• AnyOf / a | b and AllOf / a & b yield a ConditionValue: an ordered, dict-like mapping from event objects to their values. Test membership using the original event objects; do not assume a scalar result.
• AnyOf does not cancel losing events. Explicitly cancel pending resource requests when abandoning them; ordinary timeouts remain scheduled.

Interrupts

process.interrupt(cause) schedules an urgent interruption that throws simpy.Interrupt into the target generator. Catch it around the yielded work that may be interrupted, inspect interrupt.cause, update remaining work, then either resume, re-yield the original event, or terminate.

Interrupting a process removes its resume callback from its current target; it does not cancel that target event. A process cannot interrupt itself or a terminated process. See references/process-interaction.md.

Shared resources

Use a request context manager:

def job(env, resource):
    with resource.request() as request:
        yield request
        yield env.timeout(3)

On exit it releases an acquired request or cancels a still-pending one, including during exception unwinding. For a manually retained pending put/get/request, call cancel() if an interrupt or timeout makes the process abandon it.

PreemptiveResource.request(priority=..., preempt=True) uses lower numbers as higher priority. The preempted process receives an Interrupt whose cause is a Preempted object: cause.by is the preempting Process, cause.usage_since is when use began, and cause.resource is the resource. Queued priority takes precedence over the preempt flag; mixing preempting and non-preempting requests needs explicit tests.

Read references/resources.md for blocked operations, queue rules, and examples.