How to build the simplest simulation in Python using Simpy?

Question

I have been trying to make this simulation using Simpy, but I just can't figure out how it works. If you have any tips on how to learn it from example code (starting at the bottom and going up through functions, or the other way around?), or any good sources that would already be of great help.

What I want to simulate: A bike rental service with S rental stations and T bikes at t=0. Customers arrivals and rental times are exponentially distributed. When a bike is rented, there is a given probability to go to any of the rental stations. For example, with S=2, the probabilities are [[0.9,0.1],[0.5,0.5]].

I tried to do it without simpy, but I don't know how to manage the number of bikes at the stations and manage arrivals while rentals are happening.

Any help is more than welcome as I am starting to get kind of desperate. Thank you!

Answer 1

Here is one way to do it

"""
Simple simulation of several bike rental stations

Stations are modeled with containers so bikes can be returned
to a station different from where it was rented from

programer:  Michael R. Gibbs
"""

import simpy
import random

# scenario attributes
station_names = ['A','B']
rent_probs = [.9,.1]
return_probs = [.5,.5]
bikes_per_station = 5

def rent_proc(env, id, station_names, rent_probs, return_probs, station_map):
    """
    Models the process of:
    selecting a station
    renting a bike
    using a bike
    returning a bike (can be different station)
    """

    #select a station
    name = random.choices(station_names,weights=rent_probs)
    name = name[0]
    station = station_map[name]

    print(f'{env.now}: id:{id} has arrived at station {name} q-len:{len(station.get_queue)} and {station.level} bikes')

    # get a bike
    yield station.get(1)

    print(f'{env.now}: id:{id} has rented bike at station {name} q-len:{len(station.get_queue)} and {station.level} bikes')

    # use bike
    yield env.timeout(random.triangular(1,5,3))

    # return bike
    name = random.choices(station_names,weights=return_probs)
    name = name[0]
    station = station_map[name]

    yield station.put(1)

    print(f'{env.now}: id:{id} has returned bike at station {name} q-len:{len(station.get_queue)} and {station.level} bikes')

def gen_arrivals(env, station_names, rent_probs, return_probs, station_map):
    """
    Generates arrivales to the rental stations
    """

    cnt = 0

    while True:
        yield env.timeout(random.expovariate(2.5))
        cnt += 1
        env.process(rent_proc(env,cnt,station_names,rent_probs,return_probs, station_map))

# set up
env = simpy.Environment()

# create station based on name list
cap = len(station_names) * bikes_per_station
station_map = {
    name: simpy.Container(env, init=bikes_per_station, capacity=cap)
    for name in station_names
}

# start generation arrivals
env.process(gen_arrivals(env, station_names, rent_probs, return_probs, station_map))

# start sim
env.run(100)