在 Jython 中自动处理/忽略 NameError

Question

I have a setup where I execute jython scripts from a Java application. The java application feed the jython script with variables, coming from the command line, so that a user can write the following code in it's jython script:

print("Hello, %s" % foobar)

And will call the java program with this:

$ java -jar myengine.jar script.py --foobar=baz
Hello, baz

My java application parse the command-line, and create a variable of that name with the given value to give to the jython scripting environment to consume. All is well so far.

My issue is that when the user does not provide the foobar command-line parameter, I'd like to be able to easily provide a fallback in my script. For now, the user needs to write that sort of code to handle the situation where the foobar parameter is missing from the command-line:

try: foobar
except NameError: foobar = "some default value"

But this is cumbersome, especially if the number of parameters is growing. Is there a way to handle that better from the script user point of view?

I was thinking of catching the jython NameError in the Java code, initializing the variable causing the exception to a default value if the variable causing the exception "looks like" a parameter (adding a naming convention is OK), and restarting where the exception occurred. Alternatively, I can require the script user to write code such as this:

parameter(foobar, "some default value")

Or something equivalent.

Answer 1

Well, this is one ugly workaround I found so far. Be careful , as this will call the script in loop many times, and is O(n^2).

private void callScriptLoop(String scriptfile) {
    PythonInterpreter pi = new PythonInterpreter();
    pi.set("env", someEnv);
    int nloop = 0;
    boolean shouldRestart;
    do {
        shouldRestart = false;
        try {
            pi.execfile(scriptfile);
        } catch (Throwable e) {
            if (e instanceof PyException) {
                PyException pe = (PyException) e;
                String typ = pe.type.toString();
                String val = pe.value.toString();
                Matcher m = Pattern.compile("^name '(.*)' is not defined")
                        .matcher(val);
                if (typ.equals("<type 'exceptions.NameError'>")
                        && m.find()) {
                    String varname = m.group(1);
                    pi.set(varname, Py.None);
                    System.out.println(
                            "Initializing missing parameter '"
                            + varname + "' to default value (None).");
                    shouldRestart = true;
                    nloop++;
                    if (nloop > 100)
                        throw new RuntimeException(
                                "NameError handler infinite loop detected: bailing-out.");
                }
            }
            if (!shouldRestart)
                throw e;
        }
    } while (shouldRestart);
}