I recently completed a discrete-event simulation model using SimPy. This was my first foray into Python programming and the first time I used a non-graphical discrete-event simulation package (most of my previous experience was using Witness).
The model tracks the utilisation of wagons on trains. Key constraints include availability of staff, availability of a path through the network for movement of trains and wagons, and availability of a spare locomotive to make ancillary wagon movements. Maintenance activities are simulated as are wagon faults and repairs.
The rail network is represented using a .dot file; our brilliant computer scientist, Loki, devised a mechanism to find a path through the network using the pygraph minmax module (pydot is used to parse the .dot file).
Although the learning curve was quite steep for me, I found that I really enjoy working with both Python and SimPy. It was liberating not fiddling with graphics which must be placed precisely on the screen or manipulated to get element iteractions to work perfectly. The use of a .dot file to represent the rail network means that it can be changed at will without requiring a vast amount of re-work to fix graphics (the initial network is a high-level representation of the actual network which will be refined in the future). As an added bonus, Graphviz can be used to view and modify the .dot files. SimPy doesn’t come loaded with pre-configured entities (like machines, staff, tracks, forklifts, etc.). However, you can create any entity you desire and define it’s behaviour using the Python programming language. Since SimPy is open source all code is freely available and can be modified to provide further functionality. This provides far greater flexibility than I’ve experienced with other packages since I was not limited by behaviours defined by the package developers and the use of a package-specific macro language. As with other simulation packages I’ve used, I created variables to track important information and then created both .txt and .csv output files on which extensive analysis can be performed.
While the cost of using Python and SimPy was a significant learning curve (for me) I feel that it was definitely worth it. We’ll be looking to use SimPy on future projects!
Special thanks to Klaus Muller for his patience in answering my questions and assistance!