Java bridge

This library depends on the Python-to-Java bridge JPype. It therefore inherits its limitation, in that only one Java virtual machine can be managed within the same Python session, and thus only one Comsol client. If several simulations are to be run in parallel, distributed over independent processor cores in an effort to achieve maximum speed-up of a parameter sweep, they have to be started as separate Python (sub-)processes. This is a feasible work-around, but a limitation nonetheless.

For the same reason, the unit tests that come with MPh fail when collected and run via the testing framework pyTest. They must be run directly from the command line. Since they cannot be used for continuous integration testing anyway, given that they depend on Comsol being installed on the local machine, this is but a minor inconvenience.

Furthermore, there are some known, but unresolved issues with JPype’s shutdown of the Java virtual machine. Most notably, pressing Ctrl+C to interrupt an ongoing operation will usually crash out of the Python session. So do not rely on catching KeyboardInterrupt exceptions in application code.

(There is an alternative Java bridge, pyJNIus, which is not limited to one virtual machine, but then fails in another regard: A number of Java methods exposed by Comsol are inexplicably missing from the Python encapsulation.)

Creating models

The API is intentionally scope-limited to automating the simulation workflow, such as running parameter sweeps or optimization routines with customized, Python-powered post-processing. Exposing any and all Comsol features to create or alter every aspect of a model, and thus replicating the entire Java API, albeit in a more pythonic way, is out of scope.

Though users who do want to dig deeper may access the “pythonized” Java layer directly, via the .java attribute of Client instances (mapping to Comsol’s ModelUtil) as well as Model (mapping to Comsol’s model). Refer to section “Creating models” in chapter Demonstrations for an example and to Comsol’s Programming Reference Manual for further details.

This might even be worthwhile for Java developers of Comsol applications, as the interactive Python prompt provides easy introspection of the object hierarchy. In these circumstances, one might find the convenience function mph.inspect helpful, as it not only lists an object’s methods in a more readable way than Python’s built-in dir, but also displays the “properties” possibly defined on a model node.

In that event, it may equally help to call the returned Python model object something like pymodel and assign the name model to Then you can just copy-and-paste Java (or even Matlab) code from the Comsol Programming Manual or as exported from the Comsol front-end. Python will gracefully overlook gratuitous semicolons at the end of statements, so this approach would even work for entire blocks of code. Keep in mind, however, that JPype cannot perform all type conversions silently in the background. Occasionally, when there is ambiguity in overloaded methods, you will have to cast types explicitly. Refer to the JPype documentation for help.

Platform differences

The Comsol API offers two distinct ways to run a simulation session on the local machine. One may either start a “stand-alone” client, which does not require a Comsol server. Or one may start a server separately and have a “thin” client connect to it via a loop-back network socket. The first approach is more lightweight and arguably more robust, as it keeps everything inside the same process. The second approach is slower to start up and relies on the inter-process communication to work, but would also work across the network, i.e., for remote sessions where the client runs locally and delegates the heavy lifting to a server running on another machine. If we instantiate the Client class without providing a value for the host address and network port, it will create a stand-alone client. Otherwise it will run in client–server mode.

On Linux and macOS however, the stand-alone mode does not work out of the box. This is due to a limitation of Unix-like operating systems and explained in more detail in GitHub issue #8. On these platforms, if all you did was install MPh, starting the client in stand-alone mode will raise a java.lang.UnsatisfiedLinkError because required external libraries cannot be found. You would have to add the full paths of shared-library folders to an environment variable named LD_LIBRARY_PATH on Linux and DYLD_LIBRARY_PATH on macOS.

For example, for an installation of Comsol 5.6 on Ubuntu Linux, you would add the following lines at the end of the shell configuration file .bashrc:

# Help MPh find Comsol's shared libraries.

On macOS, the root folder is /Applications/COMSOL56/Multiphysics. The folder names in this example depend on the installed Comsol version and will have to be adapted accordingly.

Requiring this variable to be set correctly limits the possibility of selecting a specific Comsol version from within MPh, as adding multiple installations to that search path will lead to name collisions. One could work around the issue by wrapping a Python program that uses MPh in a shell script that sets the environment variable only for that one process. Or have the Python program start the Comsol session in a subprocess. However, none of this is ideal. Starting the client should work without any of these detours.

The function mph.start() exists to navigate these platform differences. On Windows, it starts a stand-alone client in order to profit from the better start-up performance. On Linux and macOS, it creates a local session in client–server mode so that no shell configuration is required up front. This behavior is reflected in the configuration option “session”, accessible via mph.option(), which is set to “platform-dependent” by default. It could also be set to “stand-alone” or “client-server” before calling mph.start() in order to override the default behavior.