Trying to get a panel on wxPython to change a TextCtrl on another panel

Question

I am in the process of writing a chat program that integrates with skype. I have most of the legwork done, but I am having issues with the Notebook control in wxPython. I want to create a new tab on the notebook when a user sends a message, which I have working, but the issue I have is how do I reference the TextCtrl on the panel on the tab? The following is code from a different project I pulled from online:

import wx

class Page(wx.Panel):
  def __init__(self, parent):
     wx.Panel.__init__(self, parent)
     t = wx.StaticText(self, -1, "THIS IS A PAGE OBJECT", (20,20))

class MainFrame(wx.Frame):
  def __init__(self):
    wx.Frame.__init__(self, None, title="Notebook Remove Pages Example")

    pannel  = wx.Panel(self)
    vbox    = wx.BoxSizer(wx.VERTICAL)
    hbox    = wx.BoxSizer(wx.HORIZONTAL)

    self.buttonRemove = wx.Button(pannel, id=wx.ID_ANY, label="DELETE", size=(80, 25))
    self.buttonRemove.Bind(wx.EVT_BUTTON, self.onButtonRemove)
    hbox.Add(self.buttonRemove)

    self.buttonInsert = wx.Button(pannel, id=wx.ID_ANY, label="CREATE", size=(80, 25))
    self.buttonInsert.Bind(wx.EVT_BUTTON, self.onButtonInsert)
    hbox.Add(self.buttonInsert)

    self.buttonMessage = wx.Button(pannel, id=wx.ID_ANY, label="Message", size=(80, 25))
    self.buttonMessage.Bind(wx.EVT_BUTTON, self.onButtonMessage)
    hbox.Add(self.buttonList)

    vbox.Add(hbox)

    self.Notebook3 = wx.Notebook(pannel)
    vbox.Add(self.Notebook3, 2, flag=wx.EXPAND)

    pannel.SetSizer(vbox)

    self.pageCounter = 0
    self.addPage()

def addPage(self):
    self.pageCounter += 1
    page      = Page(self.Notebook3)
    pageTitle = "Page: {0}".format(str(self.pageCounter))
    self.Notebook3.AddPage(page, pageTitle)

def onButtonRemove(self, event):   
    page_to_delete = self.Notebook3.GetSelection()
    self.Notebook3.DeletePage(page_to_delete)

def onButtonInsert(self, event):   
    self.addPage()

def onButtonMessage(self, event):

    self.Notebook3.StaticText(0).AppendText("Yeah right. Like this works")

if __name__ == "__main__":
   app = wx.App()
   MainFrame().Show()
   app.MainLoop()

I just can't seem to get it together quite right. Any help is appreciated.

acattle · Accepted Answer

Yoriz's answer is good but you need to be very careful in these situations to not violate encapsulation and abstraction.

When it comes to encapsulation, my personal rule-of-thumb is that any of my frames/panels/controls/dialogs/etc. are only allowed to call functions one level away. That is, a panel can call functions from its parent and from its immediate children. In their solution, Yoriz calls a function two levels away; AppendText() is called on page.textCtrl:

def onButtonMessage(self, event):
    page = self.notebook3.GetCurrentPage()
    page.textCtrl.AppendText("Yeah this works ")

The problem is that MainFrame is calling functions on objects it does not have immediate control of. This can be easily solved by creating some wrapper function in Page like this:

class Page(wx.Panel):
    ...

    def AppendText(text):
        self.textCtrl.AppendText(text)

And calling it in MainFrame like this:

class MainFrame(wx.Frame):
    def onButtonMessage(self, event):
        page = self.notebook3.GetCurrentPage()
        page.AppendText("Yeah this works ")

This too has a problem. In order to promote code-reuse we want our design to be as abstracted as possible with the fewest possible dependencies. MainFrame is dependent on notebook3 existing and having a textCtrl object (in Yoriz's answer) or an AppendText() function (in my suggestion above) and will create an error if this isn't the case. This creates difficulties if you attempt to reuse MainFrame in a different project.

There are several ways to reduce these types of dependencies but my personal favourite is wxPython's pubsub library (you can find a more detailed tutorial here). When the button is pressed in one panel you can send out a message using pub.sendMessage() which can then be received by the other panel which can deal with it appropriately. You can even include arguments (in your case, like what text to set textCtrl to). This is conceptually very similar to how wxPython already handles button presses, just substitute "message" for "event".

Using pubsub, MainFrame doesn't need to know anything about workbook3. Heck, pannel andworkbook3 can communicate without even knowing each other exist; they only need to know what messages to send/subscribe to (I suggest declaring your messages as constants). This makes all of your components more flexible and reusable.

In code, it looks like this:

import wx
from wx.lib.pubsub import setupkwargs #this line not required in wxPython2.9.
                                      #See documentation for more detail
from wx.lib.pubsub import pub

#This message requires the argument "text"
MSG_CHANGE_TEXT = "change.text"

class Page(wx.Panel):
    def __init__(self, parent):
        self.textCtrl = wx.TextCtrl(self, -1, "THIS IS A PAGE OBJECT ",
                                style=wx.TE_MULTILINE | wx.BORDER_NONE)
        ...

        pub.subscribe(self.onChangeText, MSG_CHANGE_TEXT)

    def onChangeText(self, text):
        self.textCtrl.AppendText(text)

class MainFrame(wx.Frame):
    ...

    def onButtonMessage(self, event):
        pub.sendMessage(MSG_CHANGE_TEXT, text="Yeah this works ")

Obviously, the sample code you gave us is pretty simple so in this particular case you could easily say that the advantages gained by using pubsub are not worth the effort. However, what if pannel and workbook3 didn't share the same parent? In that case it would be very difficult to pass an instance of workbook3 to pannel and would require a lot of dependencies over a large number of classes. In situations like this pubsub offers a simple, easy, and clean solution.

Trying to get a panel on wxPython to change a TextCtrl on another panel

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