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()

• kttii: here is some similar code that might help: stackoverflow.com/questions/25295302/…
• gene: Look at What does if name == "main" do?

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.

• GreyEyedPallas: Well that's embarrassing, thanks! Your explanation is much clearer than the others I've come across.