A simulation environment manages the simulation time as well as the scheduling and processing of events. It also provides means to step through or execute the simulation.
The base class for all environments is
“Normal” simulations usually use its subclass
Environment. For real-time simulations, SimPy provides a
RealtimeEnvironment (more on that in
SimPy is very flexible in terms of simulation execution. You can run your simulation until there are no more events, until a certain simulation time is reached, or until a certain event is triggered. You can also step through the simulation event by event. Furthermore, you can mix these things as you like.
For example, you could run your simulation until an interesting event occurs. You could then step through the simulation event by event for a while; and finally run the simulation until there are no more events left and your processes have all terminated.
The most important method here is
If you call it without any argument (
env.run()), it steps through the simulation until there are no more events left.
If your processes run forever (
while True: yield env.timeout(1)), this method will never terminate (unless you kill your script by e.g., pressing
In most cases it is advisable to stop your simulation when it reaches a certain simulation time. Therefore, you can pass the desired time via the until parameter, e.g.:
The simulation will then stop when the internal clock reaches 10 but will not process any events scheduled for time 10. This is similar to a new environment where the clock is 0 but (obviously) no events have yet been processed.
If you want to integrate your simulation in a GUI and want to draw a process bar, you can repeatedly call this function with increasing until values and update your progress bar after each call:
for i in range(100): env.run(until=i) progressbar.update(i)
Instead of passing a number to
run(), you can also pass any event to it.
run()will then return when the event has been processed.
Assuming that the current time is 0,
env.run(until=env.timeout(5))is equivalent to
You can also pass other types of events (remember, that a
Processis an event, too):
>>> import simpy >>> >>> def my_proc(env): ... yield env.timeout(1) ... return 'Monty Python’s Flying Circus' >>> >>> env = simpy.Environment() >>> proc = env.process(my_proc(env)) >>> env.run(until=proc) 'Monty Python’s Flying Circus'
peek() returns the time of the next scheduled event or infinity
float('inf')) if no future events are scheduled.
step() processes the next scheduled event. It raises an
EmptySchedule exception if no event is available.
In a typical use case, you use these methods in a loop like:
until = 10 while env.peek() < until: env.step()
now starts at 0, but you can pass an
initial_time to the
Environment to use something else.
Although the simulation time is technically unitless, you can pretend that
it is, for example, in seconds and use it like a timestamp returned by
time.time() to calculate a date or the day of the week.
Environment.active_process is comparable to
os.getpid() and is either
None or pointing at the currently active
Process. A process is active when its process function
is being executed. It becomes inactive (or suspended) when it yields an
Thus, it only makes sense to access this property from within a process function or a function that is called by your process function:
>>> def subfunc(env): ... print(env.active_process) # will print "p1" >>> >>> def my_proc(env): ... while True: ... print(env.active_process) # will print "p1" ... subfunc(env) ... yield env.timeout(1) >>> >>> env = simpy.Environment() >>> p1 = env.process(my_proc(env)) >>> env.active_process # None >>> env.step() <Process(my_proc) object at 0x...> <Process(my_proc) object at 0x...> >>> env.active_process # None
An exemplary use case for this is the resource system: If a process function
request() to request
a resource, the resource determines the requesting process via
env.active_process. Take a look at the code to see how we do this :-).
To create events, you normally have to import
the event class and pass a reference to the environment to it. To reduce the
amount of typing, the
Environment provides some shortcuts for event
creation. For example,
Environment.event() is equivalent to
Other shortcuts are:
More details on what the events do can be found in the guide to events.
Since Python 3.3, a generator function can have a return value:
def my_proc(env): yield env.timeout(1) return 42
In SimPy, this can be used to provide return values for processes that can be used by other processes:
def other_proc(env): ret_val = yield env.process(my_proc(env)) assert ret_val == 42
Internally, Python passes the return value as parameter to the
StopIteration exception that it raises when a generator is exhausted. So
in Python 2.7 and 3.2 you could replace the
return 42 with a
StopIteration(42) to achieve the same result.
To keep your code more readable, the environment provides the method
exit() to do exactly this:
def my_proc(env): yield env.timeout(1) env.exit(42) # Py2 equivalent to "return 42"