Make PythonScriptRunner logic less tightly coupled

Now, if there is any java.util.function.Function named
PythonScriptRunner in the ObjectService, we try to use its apply
function, rather than requiring the runner to be of a special type.

In this way, the scyjava library can gain a new function
enable_python_scripting(context) that injects such a function
(implemented on the Python side via @JImplements, of course, in the same
way that the PythonScriptRunner was with the previous design iteration),
without needing to guard against the possibility that scripting-python
is unavailable at runtime when the @JImplements decorator executes;
the only class needed (Function) is part of the Java standard library.

Python scripts wanting to launch a Java capable of making use of
this PythonScriptEngine should now perform the following steps:

1. Put scripting-python on the classpath, either via the
   org.scijava:scripting-python Maven coordinate string in an endpoint,
   or locally via scyjava.config.add_classpath;

2. Construct a SciJava context; and

3. Invoke the scyjava.enable_scijava_scripting(context) function.

After which point, using the context's ScriptService to execute Python
scripts should start working! \^_^/

Author: ctrueden

src/main/java/org/scijava/plugins/scripting/python/PythonScriptEngine.java

@@ -34,9 +34,12 @@
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
+import java.util.function.Function;
 
 import javax.script.Bindings;
+import javax.script.ScriptContext;
 import javax.script.ScriptEngine;
 import javax.script.ScriptException;
 
@@ -45,11 +48,10 @@
 import org.scijava.object.ObjectService;
 import org.scijava.plugin.Parameter;
 import org.scijava.script.AbstractScriptEngine;
-import org.scijava.ui.DialogPrompt;
-import org.scijava.ui.UIService;
 
 /**
- * A script engine for Python (PyImageJ).
+ * A script engine for Python (CPython, not Jython!), backed by the
+ * <a href="https://github.com/scijava/scyjava">scyjava</a> library.
  *
  * @author Curtis Rueden
  * @author Karl Duderstadt
@@ -58,29 +60,30 @@
 public class PythonScriptEngine extends AbstractScriptEngine {
 
 	@Parameter
-	ObjectService objectService;
+	private ObjectService objectService;
 
 	@Parameter
-	LogService logService;
+	private LogService logService;
 
-	@Parameter
-	UIService uiService;
-
-	public PythonScriptEngine(Context context) {
+	public PythonScriptEngine(final Context context) {
 		context.inject(this);
 		setLogService(logService);
 		engineScopeBindings = new ScriptBindings();
 	}
 
 	@Override
-	public Object eval(String script) throws ScriptException {
-		if (objectService.getObjects(PythonScriptRunner.class).stream().count() > 0)
-			return objectService.getObjects(PythonScriptRunner.class).get(0).run(script, engineScopeBindings, scriptContext);
-		
-		uiService.showDialog("The PythonScriptRunner could not be found in the ObjectService. To use the\n" +
-			                   "Conda Python 3 script engine Fiji must be launched from python inside a conda\n " +
-			                   "environment and a PythonScriptRunner must be added to the ObjectService.\n", DialogPrompt.MessageType.ERROR_MESSAGE);
-		return null;
+	public Object eval(final String script) throws ScriptException {
+		final Optional<Function> pythonScriptRunner = //
+			objectService.getObjects(Function.class).stream()//
+				.filter(obj -> "PythonScriptRunner".equals(objectService.getName(obj)))//
+				.findFirst();
+		if (!pythonScriptRunner.isPresent()) {
+			throw new IllegalStateException(//
+				"The PythonScriptRunner could not be found in the ObjectService. To use the\n" +
+					"Python script engine, you must call scyjava.enable_scijava_scripting(context)\n" +
+					"with this script engine's associated SciJava context before using it.");
+		}
+		return pythonScriptRunner.get().apply(new Args(script, engineScopeBindings, scriptContext));
 	}
 
 	@Override
@@ -105,10 +108,10 @@ public Bindings createBindings() {
 	}
 
 	//Somehow just type casting did not work...
-	class ScriptBindings implements Bindings {
-		
-		private Map<String, Object> bindingsMap; 
-		
+	private static class ScriptBindings implements Bindings {
+
+		private Map<String, Object> bindingsMap;
+
 		ScriptBindings() {
 			bindingsMap = new HashMap<String, Object>();
 		}
@@ -173,4 +176,16 @@ public Object remove(Object key) {
 			return bindingsMap.remove(key);
 		}
 	}
+
+	private static class Args {
+		public final String script;
+		public final Map<String, Object> vars;
+		public final ScriptContext scriptContext;
+
+		public Args(final String script, final Map<String, Object> vars, final ScriptContext scriptContext) {
+			this.script = script;
+			this.vars = vars;
+			this.scriptContext = scriptContext;
+		}
+	}
 }