Replace '+pya' with a bit smarter app class

bin/mkx

             ;;
 
         pya)
-            echo '#!/usr/bin/python'
+            echo '#!/usr/bin/python3'
             echo '"""'
             echo 'A simple application'
             echo '"""'
             echo ''
             echo 'import sys'
+            echo 'import traceback'
             echo ''
             echo ''
-            echo 'class App(object):'
+            echo 'class UsageError(ValueError):'
+            echo '    pass'
+            echo ''
+            echo ''
+            echo 'class AppError(RuntimeError):'
+            echo '    pass'
+            echo ''
+            echo ''
+            echo 'class BaseApp:'
             echo '    """'
             echo '    Main application class'
             echo '    """'
             echo ''
-            echo '    def __init__(self, argv):'
-            echo '        self.argv = argv'
-            echo '        # agument decoding and further initializations'
+            echo '    @classmethod'
+            echo '    def main(cls, factory=None):'
+            echo '        factory = factory if factory else cls.from_argv'
+            echo '        try:'
+            echo '            app = factory()'
+            echo '            es = app.run()'
+            echo '        except UsageError as e:'
+            echo '            print(e, file=sys.stderr)'
+            echo '            sys.exit(2)'
+            echo '        except AppError as e:'
+            echo '            print(e, file=sys.stderr)'
+            echo '            sys.exit(3)'
+            echo '        except Exception:'
+            echo '            sys.stderr.write(traceback.format_exc())'
+            echo '            sys.exit(4)'
+            echo '        if type(es) is not int:'
+            echo '            msg = ('
+            echo '                "%s.run() did not return proper exit status: %r is not int"'
+            echo '                % (app.__class__.__name__, es)'
+            echo '            )'
+            echo '            BaseApp.WARN(msg)'
+            echo '            es = 4'
+            echo '        sys.exit(es)'
             echo ''
             echo '    @classmethod'
-            echo '    def main(cls, argv):'
+            echo '    def from_argv(cls):'
+            echo '        return cls('
+            echo '            name=sys.argv[0],'
+            echo '            args=sys.argv[1:]'
+            echo '        )'
+            echo ''
+            echo '    def __init__(self, name, args):'
+            echo '        self.name = name'
+            echo '        self.args = args'
+            echo ''
+            echo '    def _throw_usage(self, pattern=None):'
             echo '        """'
-            echo '        Initialize and launch'
+            echo '        Throw UsageError with pattern *pattern*.'
             echo '        """'
-            echo '        app = cls(argv)'
-            echo '        app.run()'
+            echo '        parts = ["usage:", self.name]'
+            echo '        if pattern:'
+            echo '            parts.append(pattern)'
+            echo '        raise UsageError(" ".join(parts))'
+            echo ''
+            echo '    @staticmethod'
+            echo '    def WARN(msg):'
+            echo '        """'
+            echo '        Warn user with message *msg*.  *msg* can be string or a list of lines.'
+            echo '        """'
+            echo '        if type(msg) is list:'
+            echo '            lines = msg'
+            echo '        else:'
+            echo '            lines = [str(msg)]'
+            echo '        for line in lines:'
+            echo '            print(line, file=sys.stderr)'
+            echo ''
+            echo '    def _validate(self):'
+            echo '        """'
+            echo '        Validate *self.args*; throw UsageError if needed.'
+            echo '        """'
+            echo '        if not self.args:'
+            echo '            self._throw_usage("ARG..")'
             echo ''
             echo '    def run(self):'
             echo '        """'
-            echo '        Run the application'
+            echo '        Run the application; return integer exit status'
             echo '        """'
-            echo '        # actual action (calling other methods)'
-            echo '        print self.argv'
-            echo '        print __doc__'
+            echo '        self._validate()'
+            echo '        if self.args[0] == "--fail":'
+            echo '            raise AppError("you asked for it")'
+            echo '        if self.args[0] == "--fail-unexpectedly":'
+            echo '            raise RuntimeError("you asked for it ANyway")'
+            echo '        if self.args[0] == "--warn":'
+            echo '            BaseApp.WARN("you asked for a warning")'
+            echo '            return 0'
+            echo '        if self.args[0] == "--badret":'
+            echo '            return object()'
+            echo '        return 0'
+            echo ''
+            echo ''
+            echo 'class App(BaseApp):'
+            echo ''
+            echo '    def _validate(self):'
+            echo '        """'
+            echo '        Validate *self.args*; throw UsageError as needed.'
             echo ''
+            echo '        Prefer using self._throw_usage().'
+            echo '        """'
+            echo '        App.WARN("You need to rewrite App._validate()!!!")'
+            echo '        super()._validate()'
             echo ''
-            echo 'if __name__ == '__main__':'
-            echo '    App.main(sys.argv)'
+            echo '    def run(self):'
+            echo '        """'
+            echo '        Run the application; return integer exit status'
+            echo '        """'
+            echo '        App.WARN("You need to rewrite App.run()")'
+            echo '        super().run()'
+            echo ''
+            echo ''
+            echo 'if __name__ == "__main__":'
+            echo '    App.main()'
             ;;
 
         shellfu)