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Events in wxPython

Events in wxPython

Events are integral part of every GUI application. All GUI applications are event-driven. An application reacts to different event types which are generated during its life. Events are generated mainly by the user of an application. But they can be generated by other means as well. e.g. internet connection, window manager, timer. So when we call MainLoop() method, our application waits for events to be generated. The MainLoop() method ends when we exit the application.

Definitions

Event is a piece of application-level information from the underlying framework, typically the GUI toolkit. Event loop is a programming construct that waits for and dispatches events or messages in a program. The event loop repeatedly looks for events to process. A dispatcher is a process which maps events to event handlers. Event handlers are methods that react to events.
Event object is an object associated with the event. It is usually a window. Event type is a unique event, that has been generated. Event binder is an object, that binds an event type with an event handler.

A simple event example

In the following section we will describe a simple event. We will talk about a move event.
A move event is generated, when we move a window to a new position. The event type is wx.MoveEvent. The event binder for this event is wx.EVT_MOVE
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
        
        
    def InitUI(self):

        wx.StaticText(self, label='x:', pos=(10,10))
        wx.StaticText(self, label='y:', pos=(10,30))
        
        self.st1 = wx.StaticText(self, label='', pos=(30, 10))
        self.st2 = wx.StaticText(self, label='', pos=(30, 30))

        self.Bind(wx.EVT_MOVE, self.OnMove)

        self.SetSize((250, 180))
        self.SetTitle('Move event')
        self.Centre()
        self.Show(True)  

    def OnMove(self, e):
        
        x, y = e.GetPosition()
        self.st1.SetLabel(str(x))
        self.st2.SetLabel(str(y))


def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
The example displays the current position of the window. 
self.Bind(wx.EVT_MOVE, self.OnMove)
Here we bind the wx.EVT_MOVE event binder to the OnMove() method. 
def OnMove(self, e):
    
    x, y = e.GetPosition()
    self.st1.SetLabel(str(x))
    self.st2.SetLabel(str(y))
The event parameter in the OnMove() method is an object specific to a particular event type. In our case it is the instance of a wx.MoveEvent class. This object holds information about the event. For example the Event object or the position of the window. In our case the Event object is the wx.Frame widget. We can find out the current position by calling the GetPosition() method of the event.
Move event
Figure: Move event

Event binding

Working with events is straightforward in wxPython. There are three steps:
  • Identify the event binder name: wx.EVT_SIZE, wx.EVT_CLOSE etc
  • Create an event handler. It is a method, that is called, when an event is generated
  • Bind an event to an event handler.
In wxPython we say to bind a method to an event. Sometimes a word hook is used. You bind an event by calling the Bind() method. The method has the following parameters: 
Bind(event, handler, source=None, id=wx.ID_ANY, id2=wx.ID_ANY)
  • event is one of EVT_* objects. It specifies the type of the event.
  • handler is an object to be called. In other words, it is a method, that a programmer binds to an event.
  • source parameter is used when we want to differentiate between the same event type from different widgets.
  • id parameter is used, when we have multiple buttons, menu items etc. The id is used to differentiate among them.
  • id2 is used when it is desirable to bind a handler to a range of ids, such as with EVT_MENU_RANGE.
Note that method Bind() is defined in class EvtHandler. It is the class, from which wx.Window inherits. wx.Window is a base class for most widgets in wxPython. There is also a reverse process. If we want to unbind a method from an event, we call the Unbind() method. It has the same paremeters as the above one.

Vetoing events

Sometimes we need to stop processing an event. To do this, we call the method Veto(). 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):

        self.Bind(wx.EVT_CLOSE, self.OnCloseWindow)

        self.SetTitle('Event veto')
        self.Centre()
        self.Show(True)

    def OnCloseWindow(self, e):

        dial = wx.MessageDialog(None, 'Are you sure to quit?', 'Question',
            wx.YES_NO | wx.NO_DEFAULT | wx.ICON_QUESTION)
            
        ret = dial.ShowModal()
        
        if ret == wx.ID_YES:
            self.Destroy()
        else:
            e.Veto()

def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In our example, we process a wx.CloseEvent. This event is called, when we click the X button on the titlebar, press Alt + F4 or select close from the system menu. In many applications, we want to prevent from accidentally closing the window, if we made some changes. To do this, we must bind the wx.EVT_CLOSE event binder. 
dial = wx.MessageDialog(None, 'Are you sure to quit?', 'Question',
    wx.YES_NO | wx.NO_DEFAULT | wx.ICON_QUESTION)
    
ret = dial.ShowModal()
During the processing of the close event, we show a message dialog. 
if ret == wx.ID_YES:
    self.Destroy()
else:
    event.Veto()
Depending on the return value from the dialog, we destroy the window, or veto the event. Notice that to close the window, we must call the Destroy() method. By calling the Close() method, we would end up in an endless cycle.

Event propagation

There are two types of events. Basic events and command events. They differ in propagation. Event propagation is travelling of events from child widgets to parent widgets and grand parent widgets etc. Basic events do not propagate. Command events do propagate. For example wx.CloseEvent is a basic event. It does not make sense for this event to propagate to parent widgets.
By default, the event that is catched in a event handler stops propagating. To continue propagation, we must call the Skip() method. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class MyPanel(wx.Panel):
    
    def __init__(self, *args, **kw):
        super(MyPanel, self).__init__(*args, **kw) 

        self.Bind(wx.EVT_BUTTON, self.OnButtonClicked)

    def OnButtonClicked(self, e):
        
        print 'event reached panel class'
        e.Skip()


class MyButton(wx.Button):
    
    def __init__(self, *args, **kw):
        super(MyButton, self).__init__(*args, **kw) 

        self.Bind(wx.EVT_BUTTON, self.OnButtonClicked)

    def OnButtonClicked(self, e):
        
        print 'event reached button class'
        e.Skip()
        

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
        
        
    def InitUI(self):

        mpnl = MyPanel(self)

        MyButton(mpnl, label='Ok', pos=(15, 15))

        self.Bind(wx.EVT_BUTTON, self.OnButtonClicked)

        self.SetTitle('Propagate event')
        self.Centre()
        self.Show(True)  

    def OnButtonClicked(self, e):
        
        print 'event reached frame class'
        e.Skip()


def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In our example, we have a button on a panel. The panel is placed in a frame widget. We define a handler for all widgets. 
def OnButtonClicked(self, e):
    
    print 'event reached button class'
    e.Skip()
We process the button click event in our custom button class. The Skip() method propagates the event further to to panel class. 
event reached button class
event reached panel class
event reached frame class
We get this, when we click on the button. The event travels from the button to the panel and to the frame.
Try to omit some Skip() methods and see, what hapens.

Window identifiers

Window identifiers are integers that uniquely determine the window identity in the event system. There are three ways to create window id's.
  • let the system automatically create an id
  • use standard identifiers
  • create your own id
Each widget has an id parameter. This is a unique number in the event system. If we work with multiple widgets, we must differantiate among them. 
wx.Button(parent, -1)
wx.Button(parent, wx.ID_ANY)
If we provide -1 or wx.ID_ANY for the id parameter, we let the wxPython automatically create an id for us. The automatically created id's are always negative, whereas user specified id's must always be positive. We usually use this option when we do not need to change the widget state. For example a static text, that will never be changed during the life of the application. We can still get the id, if we want. There is a method GetId(), which will determine the id for us. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):

        pnl = wx.Panel(self)
        exitButton = wx.Button(pnl, wx.ID_ANY, 'Exit', (10, 10))

        self.Bind(wx.EVT_BUTTON,  self.OnExit, id=exitButton.GetId())

        self.SetTitle("Automatic id")
        self.Centre()
        self.Show(True)

    def OnExit(self, event):

        self.Close()

def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In this example, we do not care about the actual id value. 
self.Bind(wx.EVT_BUTTON,  self.OnExit, id=exitButton.GetId())
We get the automatically generated id by calling the GetId() method.
It is recommended to use standard identifiers. The identifiers can provide some standard graphics or behaviour on some platforms. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):

        pnl = wx.Panel(self)
        grid = wx.GridSizer(3, 2)

        grid.AddMany([(wx.Button(pnl, wx.ID_CANCEL), 0, wx.TOP | wx.LEFT, 9),
            (wx.Button(pnl, wx.ID_DELETE), 0, wx.TOP, 9),
            (wx.Button(pnl, wx.ID_SAVE), 0, wx.LEFT, 9),
            (wx.Button(pnl, wx.ID_EXIT)),
            (wx.Button(pnl, wx.ID_STOP), 0, wx.LEFT, 9),
            (wx.Button(pnl, wx.ID_NEW))])

        self.Bind(wx.EVT_BUTTON, self.OnQuitApp, id=wx.ID_EXIT)

        pnl.SetSizer(grid)

        self.SetSize((220, 180))
        self.SetTitle("Standard ids")
        self.Centre()
        self.Show(True)

    def OnQuitApp(self, event):
        
        self.Close()

def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In our example we use standard identifiers on buttons. On Linux, the buttons have icons. 
grid.AddMany([(wx.Button(pnl, wx.ID_CANCEL), 0, wx.TOP | wx.LEFT, 9),
    (wx.Button(pnl, wx.ID_DELETE), 0, wx.TOP, 9),
    (wx.Button(pnl, wx.ID_SAVE), 0, wx.LEFT, 9),
    (wx.Button(pnl, wx.ID_EXIT)),
    (wx.Button(pnl, wx.ID_STOP), 0, wx.LEFT, 9),
    (wx.Button(pnl, wx.ID_NEW))])
We add six buttons to a grid sizer. The wx.ID_CANCEL, wx.ID_DELETE, wx.ID_SAVE, wx.ID_EXIT, wx.ID_STOP and wx.ID_NEW are standard identifiers. 
self.Bind(wx.EVT_BUTTON, self.OnQuitApp, id=wx.ID_EXIT)
We bind the button click event to the OnQuitApp() event handler. The id parameter is used to differantiate among the buttons. We uniquely identify the source of the event.
Standard identifiers
Figure: Standard identifiers
The last option is to use own identifiers. We define our own global ids. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

ID_MENU_NEW = wx.NewId()
ID_MENU_OPEN = wx.NewId()
ID_MENU_SAVE = wx.NewId()

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):
        
        self.CreateMenuBar()
        self.CreateStatusBar()
        
        self.SetSize((250, 180))
        self.SetTitle('Global ids')
        self.Centre()
        self.Show(True)  
        
    def CreateMenuBar(self):
        
        mb = wx.MenuBar()
        
        fMenu = wx.Menu()
        fMenu.Append(ID_MENU_NEW, 'New')
        fMenu.Append(ID_MENU_OPEN, 'Open')
        fMenu.Append(ID_MENU_SAVE, 'Save')
        
        mb.Append(fMenu, '&File')
        self.SetMenuBar(mb)
        
        self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_NEW)
        self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_OPEN)
        self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_SAVE)        
        
    def DisplayMessage(self, e):
        
        sb = self.GetStatusBar()
                
        eid = e.GetId()
        
        if eid == ID_MENU_NEW:
            msg = 'New menu item selected'
        elif eid == ID_MENU_OPEN:
            msg = 'Open menu item selected'
        elif eid == ID_MENU_SAVE:
            msg = 'Save menu item selected'
        
        sb.SetStatusText(msg)

def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In the code example, we create a menu with three menu items. The ids for this menu items are created globally. 
ID_MENU_NEW = wx.NewId()
ID_MENU_OPEN = wx.NewId()
ID_MENU_SAVE = wx.NewId()
The wx.NewId() method creates a new unique id. 
self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_NEW)
self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_OPEN)
self.Bind(wx.EVT_MENU, self.DisplayMessage, id=ID_MENU_SAVE)
All three menu items are identified by their unique id. 
eid = e.GetId()

if eid == ID_MENU_NEW:
    msg = 'New menu item selected'
elif eid == ID_MENU_OPEN:
    msg = 'Open menu item selected'
elif eid == ID_MENU_SAVE:
    msg = 'Save menu item selected'
From the event object we retrieve the id. Depending on the id value, we prepare the message, which is displayed in the statusbar of the application.

PaintEvent

A paint event is generated when a window is redrawn. This happens when we resize a window or when we maximize it. A paint event can be generated programatically as well. For example, when we call SetLabel() method to change a wx.StaticText widget. Note that when we minimize a window, no paint event is generated. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):

        self.count = 0
        self.Bind(wx.EVT_PAINT, self.OnPaint)

        self.SetSize((250, 180))
        self.Centre()
        self.Show(True)  

    def OnPaint(self, e):
        
        self.count += 1
        self.SetTitle(str(self.count))
        
def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In our example we count the number of paint events and set the current number of generated events to the title of the frame window. 
self.Bind(wx.EVT_PAINT, self.OnPaint)
We bind the EVT_PAINT event to the OnPaint() method. 
def OnPaint(self, e):
    
    self.count += 1
    self.SetTitle(str(self.count))
Inside the OnPaint() event, we increase the counter and set it to the title of the frame window.

Focus event

The focus indicates the currently selected widget in application. The text entered from the keyboard or pasted from the clipboard is sent to the widget, which has the focus. There are two event types concerning focus. The wx.EVT_SET_FOCUSevent, which is generated when a widget receives focus. The wx.EVT_KILL_FOCUS is generated, when the widget looses focus. The focus is changed by clicking or by a keybord key. Usually Tab/Shift+Tab. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class MyWindow(wx.Panel):
    
    def __init__(self, parent):
        super(MyWindow, self).__init__(parent)

        self.color = '#b3b3b3'

        self.Bind(wx.EVT_PAINT, self.OnPaint)
        self.Bind(wx.EVT_SIZE, self.OnSize)
        self.Bind(wx.EVT_SET_FOCUS, self.OnSetFocus)
        self.Bind(wx.EVT_KILL_FOCUS, self.OnKillFocus)

    def OnPaint(self, e):
        
        dc = wx.PaintDC(self)

        dc.SetPen(wx.Pen(self.color))
        x, y = self.GetSize()
        dc.DrawRectangle(0, 0, x, y)

    def OnSize(self, e):
        
        self.Refresh()

    def OnSetFocus(self, e):
        
        self.color = '#0099f7'
        self.Refresh()

    def OnKillFocus(self, e):
        
        self.color = '#b3b3b3'
        self.Refresh()

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
        
        
    def InitUI(self):

        grid = wx.GridSizer(2, 2, 10, 10)
        grid.AddMany([(MyWindow(self), 0, wx.EXPAND|wx.TOP|wx.LEFT, 9),
            (MyWindow(self), 0, wx.EXPAND|wx.TOP|wx.RIGHT, 9), 
            (MyWindow(self), 0, wx.EXPAND|wx.BOTTOM|wx.LEFT, 9), 
            (MyWindow(self), 0, wx.EXPAND|wx.BOTTOM|wx.RIGHT, 9)])


        self.SetSizer(grid)

        self.SetSize((350, 250))
        self.SetTitle('Focus event')
        self.Centre()
        self.Show(True)  

    def OnMove(self, e):
        
        print e.GetEventObject()
        x, y = e.GetPosition()
        self.st1.SetLabel(str(x))
        self.st2.SetLabel(str(y))


def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In our example, we have four panels. The panel with focus is highlighted. 
self.Bind(wx.EVT_SET_FOCUS, self.OnSetFocus)
self.Bind(wx.EVT_KILL_FOCUS, self.OnKillFocus)
We bind two focus events to event handlers. 
def OnPaint(self, e):
    
    dc = wx.PaintDC(self)

    dc.SetPen(wx.Pen(self.color))
    x, y = self.GetSize()
    dc.DrawRectangle(0, 0, x, y)
In the OnPaint() method, we draw on the windows. The color of the outline depends on whether the window has focus or not. The outline of the focused window is drawn in blue color. 
def OnSetFocus(self, e):
    
    self.color = '#0099f7'
    self.Refresh()
In the OnSetFocus() method, we set the self.color variable to some blue color. We refresh the frame window which will generate a paint event for all its child widgets. The windows are redrawn and the one with the focus has a new color for its outline.
Focus event
Figure: Focus event

KeyEvent

When we press a key on our keyboard, wx.KeyEvent is generated. This event is sent to the widget that has currently focus. There are three different key handlers:
  • wx.EVT_KEY_DOWN
  • wx.EVT_KEY_UP
  • wx.EVT_CHAR
A common request is to close application, when the Esc key is pressed. 
#!/usr/bin/python
# -*- coding: utf-8 -*-


import wx

class Example(wx.Frame):
           
    def __init__(self, *args, **kw):
        super(Example, self).__init__(*args, **kw) 
        
        self.InitUI()
                
    def InitUI(self):

        pnl = wx.Panel(self)
        pnl.Bind(wx.EVT_KEY_DOWN, self.OnKeyDown)
        pnl.SetFocus()

        self.SetSize((250, 180))
        self.SetTitle('Key event')
        self.Centre()
        self.Show(True)  

    def OnKeyDown(self, e):
        
        key = e.GetKeyCode()
        
        if key == wx.WXK_ESCAPE:
            
            ret  = wx.MessageBox('Are you sure to quit?', 'Question', 
                wx.YES_NO | wx.NO_DEFAULT, self)
                
            if ret == wx.YES:
                self.Close()               
        
def main():
    
    ex = wx.App()
    Example(None)
    ex.MainLoop()    


if __name__ == '__main__':
    main()
In this example, we process the Esc key press. A message box is shown to confirm, whether we really want to close the application. 
pnl.Bind(wx.EVT_KEY_DOWN, self.OnKeyDown)
We bind an event handler to the EVT_KEY_DOWN event. 
key = e.GetKeyCode()
Here we get the key code of the pressed key. 
if key == wx.WXK_ESCAPE:
We check the key code. The Esc key has wx.WXK_ESCAPE code.
In this chapter, we talked about events in wxPython.
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