if __name__=="main": in standalone Python script for QGIS

Question

I am working on a standalone Python script for loading shapefile layers into an application window with my central widget being canvas. The previous code I had written is almost identical to the code pasted below except for the last bit. if name=="main" was substituted with a def main():. The previous code that used with def main(): successfully pulled up the main window and displayed my layers and tool bar accordingly; however, some of the buttons on my tool bar weren't working. I'm assuming this is because this application shouldn't be running on def main():, but rather, should run in a "main loop".

Once I've substituted def main(): with this loop, my program no longer pulls up an application window or anything (I also don't receive any errors). I'm unfamiliar with the usage of if__name__=="main", am I employing it incorrectly?

Here is an abridged version of my code:

---

class MyWnd(QMainWindow):
    def __init__(self):
        QMainWindow.__init__(self)

        # supply path to qgis install location
        QgsApplication.setPrefixPath("/usr", True)

        # initialize application
        QgsApplication.initQgis()

        self.canvas_layers = []

        # set the title for the application
        self.setWindowTitle("ENC Viewer")

        # create the canvas
        self.canvas = QgsMapCanvas()
        self.canvas.setCanvasColor(Qt.white)
        self.canvas.enableAntiAliasing(True)

        # add layers
        vlayer = QgsVectorLayer("/home/cassandra/desktop/file_formats/Shapefiles/US6SP10M-BUAARE.shp", "BUAARE Layer", "ogr")
        vlayer2 = QgsVectorLayer("/home/cassandra/desktop/file_formats/Shapefiles/US6SP10M-LNDARE.shp", "LNDARE Layer", "ogr")
        vlayer3 =  QgsVectorLayer("/home/cassandra/desktop/file_formats/Shapefiles/US6SP10M-SOUNDG.shp", "SOUNDG Layer", "ogr")
        layers = [vlayer, vlayer2, vlayer3]

        # create rectangle object
        extent = QgsRectangle()
        extent.setMinimal()
        extent.scale(1.1)

        # traverse each layer in the list of layers, may be one layer
        for layer in layers:

        # register each layer
        QgsMapLayerRegistry.instance().addMapLayer(layer)

        # combine the extent of each layer to the rectangle object
        extent.combineExtentWith(layer.extent())
        self.canvas_layers.append(QgsMapCanvasLayer(layer))

    # set the extent of the canvas
    self.canvas.setExtent(extent)

    # provide set of layers to display on canvas
    self.canvas.setLayerSet(self.canvas_layers)

    # set centra lwidget to the canvas app
    self.setCentralWidget(self.canvas)

    actionPoint = QAction("Point", self)
    actionPoly = QAction("Polygon", self)

    actionPoint.setCheckable(True)
    actionPoly.setCheckable(True)

    self.connect(actionPoint, SIGNAL("triggered()"), self.point)
    self.connect(actionPoly, SIGNAL("triggered()"), self.poly)

    self.toolbar.addAction(actionPoint)
    self.toolbar.addAction(actionPoly)

    # create the map tools
    self.toolPan = QgsMapToolPan(self.canvas)
    self.toolPan.setAction(actionPan)

    self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in
    self.toolZoomIn.setAction(actionZoomIn)

    self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out
    self.toolZoomOut.setAction(actionZoomOut)

    self.toolPoint = PointMapTool(self.canvas)
    self.toolPoint.setAction(actionPoint)

    self.toolPoly = PolyMapTool(self.canvas)
    self.toolPoly.setAction(actionPoly)

    self.point()
    self.poly()

    def point(self):
      self.canvas.setMapTool(self.toolPoint)

    def poly(self):
        self.canvas.setMapTool(self.toolPoly)


class PolyMapTool(QgsMapToolEmitPoint):
    def __init__(self, canvas):
        self.canvas = canvas

        QgsMapToolEmitPoint.__init__(self, self.canvas)
        self.rubberband = QgsRubberBand(self.canvas, QGis.Polygon)
        self.rubberband.setColor(Qt.red)
        self.rubberband.setWidth(1)
        self.point = None
        self.points = []

        def canvasPressEvent(self, e):
            self.point = self.toMapCoordinates(e.pos())
            m = QgsVertexMarker(self.canvas)
            m.setCenter(self.point)
            m.setColor(QColor(0, 255, 0))
            m.setIconSize(5)
            m.setIconType(QgsVertexMarker.ICON_BOX)
            m.setPenWidth(3)
            self.points.append(self.point)
            self.isEmittingPoint = True
            self.showPoly()

        def showPoly(self):
            self.rubberband.reset(QGis.Polygon)
            for point in self.points[:-1]:
                self.rubberband.addPoint(point, False)
                self.rubberband.addPoint(self.points[-1], True)
                self.rubberband.show()


class PointMapTool(QgsMapToolEmitPoint):
    def __init__(self, canvas):
        self.canvas = canvas

        QgsMapToolEmitPoint.__init__(self, self.canvas)

        self.point = None

        def canvasPressEvent(self, e):
            self.point = self.toMapCoordinates(e.pos())
            print self.point.x(), self.point.y()
            m = QgsVertexMarker(self.canvas)
            m.setCenter(self.point)
            m.setColor(QColor(0, 255, 0))
            m.setIconSize(5)
            m.setIconType(QgsVertexMarker.ICON_BOX, ICON_X)
            m.setPenWidth(3)


class MainApp(QApplication):
    def __init__(self):
        QApplication.__init__(self, [], True)
        wdg = MyWnd()
        wdg.show()
        self.exec_()

if __name__=="main":
    import sys
    app = MainApp()

Answer 1

It looks like your issue might be caused by as simple syntax error.

The correct syntax is if __name__ == '__main__': (with double underscores around main)

That if statement is essentially used to check whether the file is being executed as a standalone script or if it has been imported by some other python script. Anything inside the if statement will only be executed if the file is being run as a standalone script.