FAQ

What Python modules does Jep work with?

• Jep should work with any pure python modules. Unfortunately, CPython extensions and Cython modules may not work correctly, it depends on how they were coded. We have started a list of Package Compatibility. Please contribute to the list with your findings.

Is Jep a good fit for my project?

• That's a complex question. It depends on your project's requirements, but here's a simplification of the question. Do you want to run a Java process or a Python process?
  • Java
    • Do you need to use CPython modules (e.g. numpy)?
      • Yes: then Jep is a good fit for you.
      • No: consider Jep or Jython.
  • Python
    • Sinces Jep embeds CPython, it can run Python code just like other Python interpreters. But it cannot be launched from a Python process (we may add this in the future). If you want access to Java from a Python process, consider the following projects:
      • JPype
        • JPype forked
      • Pyjnius
      • Py4J

Does Jep work on Android?

• We haven't tried it yet. In theory it should possible, but there may be some challenges to overcome. If you try this, we'd love to hear about it and will gladly accept contributions to make Jep work well on more platforms.

What Java UI frameworks does Jep support?

• Jep has been run with Swing and SWT. We haven't tried JavaFX yet, but it should work.

How do I fix Unsatisfied Link Error: no jep in java.library.path?

• This error informs you that the Java process cannot find Jep's built C library. The name of the library is somewhat platform dependent, it is usually libjep.so on Linux, libjep.jnilib on OS X, and jep.dll on Windows. There are a few different ways to fix the problem:

  1. Place the shared library where Python has its other shared libraries. This is usually python/lib with *nix systems and python/dlls with Windows systems.
  2. Set an environment variable that tells Java the directory of the Jep shared library.
     • On Linux, set LD_LIBRARY_PATH.
     • On OS X, set DY_LD_LIBRARY_PATH.
     • On Windows, set PATH.
  3. Pass the argument -Djava.library.path to your Java process with the location of the Jep shared library.

• See http://stackoverflow.com/questions/20038789/default-java-library-path for more information.

Why does eval(String) return a boolean instead of an Object of the result?

• eval(String) was originally written to support interactive mode or non-interactive mode (multiple statements or a single statement, see javadoc), so the boolean return value is whether or not the statement was actually executed. There is also an overhead of always returning the result, especially if code is making multiple calls to eval(String) and doesn't need the results. To change the method signature now would potentially break compatibility with a number of applications.
• getValue(String) is almost identical in the C code to eval(String) and will return the result of the evaluation, and can be used instead of eval where desired. For example:

    // evaluates and discards the result
    jep.eval("2 + 4");

    // evaluates and places the results in x
    jep.eval("x = 2 + 4");
    Integer x = (Integer) jep.getValue("x");

    // evaluates and returns the results
    Integer y = (Integer) jep.getValue("2 + 4");

How do I fix Fatal Python Error when Jep starts up?

• If you see fatal python errors when first using Jep, that often implies the PATH or LD_LIBRARY_PATH environment variables are incorrect or inconsistent. This is often seen if multiple versions of python are installed and/or Jep was built with a different version of Python than it is running with.