How to add a child window (or non-modal dialog)

by David Primmer

This is the third walkthrough in a series of tutorial-style walkthroughs to help newcomers get started using PythonCard.

Overview, Scope and Purpose

This walkthrough covers PythonCard Version 0.8.

The first two PythonCard walkthrough tutorials (which can be found, like this one, in the docs/html directory of your PythonCard installation) have created single-window applications. It is, of course, often necessary to create applications with multiple windows. Adding another background as a child window will allow you to modularize your application. You can allow users to hide and show windows (by using the visible attribute), and you can clean up your user interface and split widgets off into logical groups.

In this walkthrough, we will extend our simple Counter application (the subject of walkthrough2) to add a child window which launches when the application opens, interacts with it, and exhibits some demonstrative behavior when the child window is closed. This walkthrough will make more sense if you've worked through walkthrough2.

Modal vs. Non-modal

There are two types of child windows: those that require the user to stop any other activity and pay attention only to the current window (modal) and windows that allow simultaneous interaction with the main window (non-modal or modeless). The first thing to decide when creating a child window is whether you require the window to be dismissed before the user can continue to work with the main application window. With a modal window, users can choose to cancel, they can respond to a message, or they can set parameters and assign values to content in the active application. An example of a simple modal window is a message box or alert.

You may want your window to stick around while the user is working in another window. The radioclient sample, for example, has a child window that displays a view of the current document as it would be rendered in a Web browser. It's important to decide between modal and non-modal before you begin creating your child window because the standard window type in PythonCard, the background, cannot be modal.


This is the standard class definition for a PythonCard app:

class Minimal(model.Background):

One of the most important concepts when dealing with child windows in PythonCard is the background. The background is unique to PythonCard and it basically encapsulates a wxFrame and a wxPanel. Each PythonCard application is a class that derives from model.Background. You can't make a class derived from model.Background modal. That is a wxWidgets limitation, not PythonCard. If you want a modal dialog then your class has to be derived from model.CustomDialog. They are similar, but different and the resource format is slightly different as well. dbBrowser, resourceEditor, textEditor, and textRouter all use custom modal dialogs.

Overview of Designing a Child Window

Making a non-modal child window using resourceEditor is simple, following these steps:

  1. Create a window with resourceEditor. In this walkthrough, we'll do so by starting with the minimal sample and adding features.
  2. In your main application, import the module you created at step 1 and add code to the main application's startup routine to create an instance of your imported child window class.
  3. Modify the attributes of the new object, calling its methods and using its components. It is now part of the main application.

Create a stand-alone PythonCard application

Each PythonCard .py file contains a class definition derived from PythonCard's model.Background as well as a stub to instantiate that class. A child window is simply an instantiation of a class from within another application. Tutorials in walkthrough1 and walkthrough2 cover how to create a stand-alone PythonCard application so we won't cover that here.

It is possible that your child window will be interacting with the data in your main application, so there may be limits to how much design can be done independently of your main application. But it is still a good idea to try to separate their functions as much as possible to allow code re-use and to simplify debugging. In the current example, we will use the minimal sample as the basis of our child window while using the counter sample as our main application. has one control, a text field called 'field1'. We will add a button to to reset the value of the field in the main counter application to 0 when it is pressed. We will also connect the buttons in the main counter application to update field1's contents in the child window ( to match those of the field in counter's window.

Even though these are relatively trivial interactions, they serve to demonstrate the techniques involved in getting two (or more) windows communicating with one another in a PythonCard application.

Start by creating a new folder to hold your two resource files and your two PythonCard script files. Copy,,, and from their respective folders in the samples directory into this new folder. You can leave their names the same, though in practice you will generally change the names of files to match the application you are constructing.

Open in the PythonCard resourceEditor and add a button to it as shown in Figure 1.

button added to minimal application
Figure 1. Button Added to minimal Sample Window

Label the button "Clear Counter" and name it btnReset. Save the resource file.

We'll get back to scripting this button shortly.

Launching the Child Window From the Parent Application

Opening the child window in our main application's code is simply a matter of importing the class and creating an object of that class, attached to the current background.

In addition to the standard imports for Counter, we'll import minimal:

from PythonCard import model
import minimal

Next we'll add an event handler to be executed when the Counter application is started. This handler acts something like autoexec.bat on a PC or .login in a Unix shell. Place an on_initialize handler right below the class definition. (Placement isn't important but following this convention will make it easier for you to work through and maintain multi-window applications.) Here is the class declaration of our Counter application with on_initialize added:

class Counter(model.Background):
    def on_initialize(self, event):

and here is the code that we will add to the on_initialize method:

        self.minimalWindow = model.childWindow(self, minimal.Minimal)

We create a minimal window object uisng the childWindow function and give it the name minimalWindow by passing two parameters to the function: the parent window (self), and the background class (minimal.Minimal) we want to use.

That is all is needed to create a minimal window object, but at this point, it is still hidden and not much good to us.

Communicating With Your Child Window

Continuing in the on_initialize handler, we make calls to set the position and visibility of the new window:

        # override resource position
        self.minimalWindow.position = (200, 5)
        self.minimalWindow.visible = True

We now have a window that is an attribute of our main background, just like any of the menus or buttons that are already a part of Counter.

As constructed before we began this project, the increment and decrement buttons in Counter modify the value of the text field in Counter. To cause the Counter application's buttons to update the text value in the minimal child window minimalWindow, we simply add one more call to update the control in that window as well (the new lines are in bold type):

    def on_incrBtn_mouseClick(self, event):
        startValue = int(self.components.field1.text)
        endValue = startValue + 1
        self.components.field1.text = str(endValue)
        self.minimalWindow.components.field1.text = str(endValue)

    def on_decrBtn_mouseClick(self, event):
        startValue = int(self.components.field1.text)
        endValue = startValue - 1
        self.components.field1.text = str(endValue)
        self.minimalWindow.components.field1.text = str(endValue)

Notice that we reference components in the child window by a collection of objects starting with the main application (self) and then pointing first to the child window, then to its components property, then to the specific component, then to the property of that component we wish to change. If we wanted to execute a method of that component or the background, we would use a similar construct.

This is obviously very simplistic, (not to mention somewhat redundant coding). Many times, you will be using a child window to modify components or data associated with the parent window. PythonCard is not passing events between windows in this release, but in many cases you can simply call the event handler directly. If you are interested in passing an arbitrary event such as a mouseClick, that will require creating the event and then posting it using wx.PostEvent. Custom events are beyond the scope of this tutorial. However, the child window is able to access the parent window directly by traversing up the stack of windows.

For example we can place a control on our child window that updates a control on our main background. Let's connect the Reset Counter button we added to the minimal application above. Add the following code to

    def on_initialize(self, event):
        self.parent = self.getParent()

    def on_btnReset_mouseClick(self, event):
        self.parent.components.field1.text = "0"

When our child window it initialized, it calls getParent() to get a reference to its parent window, and then stores that reference. We place the code that handles this task in the on_initialize handler so that the reference is available to all handlers in the application.

You'll notice that the text field on the Counter background is reset to zero but the text field on the Minimal background is not. (This might be a little confusing because both fields are called 'field1'. In a real application, the fields should be named something more descriptive.) At this point, our minimal sample is no longer a stand-alone. If you run Minimal by itself, self.parent is set to None. If you were using the child window in other roles or wanted to make it multi-purpose, you could place the self.getParent() call in a try...except block.

Closing the Child Window

Your main application window will clean up the child window when your app is closed. If the user has the ability to close the child window before the main window closes (by using the the child's File->Exit menu or by clicking the close box on the window) it's a good idea to just hide the window instead of destroying it. This will allow you to unhide the window without re-initializing it and also permits you to communicate with the window while it is hidden. In the process, you avoid runtime errors that could result from the child window being non-existent as far as the application is concerned.

We do this by overriding the on_close event handler. on_close would normally destroy the window but we change it so it hides the window by setting the visible attribute to False, hiding the window. Just to make things a little more interesting, we've also added a custom doExit function that sets the counter's field1 to an arbitrary value just to confirm the connection between the two windows visibly:

    def doExit(self):
        self.parent.components.field1.text = "99"

    def on_close(self, event):
        self.visible = False

    def on_exit_command(self, event):

The resource file ( defines an exit command for the File->Exit menu so we use a command handler to override the default behavior. As the above code shows, the File->Exit menu item just calls the close() method to close the window. That is the same as clicking the close box on the window and triggers the close window event, so that on_close is called. We placed the work to be done when the document is closing in the doExit method. In this case it just sets the counter field in the parent to "99".

In addition, in doExit() you could modify some properties of the parent window to keep track of the state of your child window. For example, assuming you have a View menu with an item that hides or unhides your child window, you could use doExit() to check or uncheck the 'View Minimal Window' menu item on the parent. The code would look something like this:

        self.parent.menuBar.setChecked('menuViewMinimalWindow', False)

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$Revision: 1.11 $ : $Author: kasplat $ : Last update $Date: 2005/12/30 06:29:36 $