You need to "Add framework support" / Facets

As long as you already have python installed, just follow this.

Right-click the directory/module/project where you have your python scripts, right at the top you will see "Add framework Support..."

https://www.jetbrains.com/help/idea/adding-support-for-frameworks-and-technologies.html#disable-framework-auto-detection

If I'm interpreting correctly, you want to use to use both interchangeably in the same file, so you'd end up with code like:

def main():
    int x = 5;
    print(x)

This is impossible, because there would be ambiguity when trying to interpret code if you allowed constructs from both languages. For example, "X" + 1 is allowed in java, and would give you the string "X1". In python, it would give you an error because you can't add an int to a string. This would mean that there would be no way to know what your code should do because it's runnable in both languages.

This is a problem that all of us face, where we like some parts of some languages and other parts of other languages. The solution is pretty much just to decide what's most important, choose one language based on that, and then put up with the parts you don't like.

You can keep things in separate projects without having to copy the .class files to the Python project provided that you make that project a PyDev project and add the place that contains the .class files (i.e.: bin folder) as a source folder in the PyDev configuration (and reference that project from your Python project).

Take a look at "Project reference for Jython users" (in the end of http://pydev.org/manual_101_project_conf2.html).

I think you could also do what you asked in your question: create a java project, set it as a PyDev project too (right click that project > pydev > set as pydev project) and configure the PyDev project properties setting the PYTHONPATH to the place where you have your Python files and add another entry to the bin folder (i.e.: where the .class files exist). Not sure if it'd be better to keep those as 2 separated projects though.

I had exactly the same need a week ago or so, and found a solution: Can Java and Python coexist in the same app?

EDIT: I accepted the first answer, but it happens that the second answer gave me best results, as i explain on the comments there.

EDIT 2: Well, it might not be exactly the same need, as i only needed to access the same datastore. If you want Java and Python running side-by-side, then i think Jython would be the best solution, as larsmans mentioned above.

Why not use Jython? The only downside I can immediately think of is if your library uses CPython native extensions.

EDIT: If you can use Jython now but think you may have problems with a later version of the library, I suggest you try to isolate the library from your app (e.g. some sort of adapter interface). Go with the simplest thing that works for the moment, then consider JNI/CPython/etc if and when you ever need to. There's little to be gained by going the (painful) JNI route unless you really have to.