Callbacks

Formation allows you to connect callbacks that will be invoked when a certain action occurs. These can be set on the following widget attributes:

• command: Invoked when the widget like a button is clicked.

• validatecommand: Invoked when the widget like an entry is validated.

• invalidcommand: Invoked when the widget like a entry is invalid.

• postcommand: Invoked after a menu is posted.

• tearoffcommand: Invoked when the a menu is torn off.

• xscrollcommand: Invoked when the widget like a scrollbar is horizontally scrolled.

• yscrollcommand: Invoked when the widget like a scrollbar is vertically scrolled.

Note

Some of these command attributes may require a callabck that accepts a number of arguments. Before connecting a callback, make sure you know the arguments that the callback will receive.

Callback Format

To set a simple callback without any arguments you can just use the callback name as defined in your code

callback

Or set it as a call with no arguments

callback()

To pass arguments to a callabck you simply have to use the call format

callback(arg1, arg2, arg3)

The arguments (arg1, arg2, and arg3) will be passed to the callback You can also pass kwargs to the callback using the format

callback(arg1, arg2, arg3, kwarg1=value1, kwarg2=value2)

Any number of arguments and kwargs can be passed to the callback this way. The arguments can also be expressions that will be evaluated before being passed to the callback.

callback("SomeText".lower())
callback(1 + 2, 3 * 4, 5 ** 6)

The function will get the result of the expression as the argument.

You can also pass the widget that triggered the callback as the first argument by prefixing the callback name with ::

::callback
::callback(arg1, arg2, arg3)
::callback(arg1, arg2, arg3, kwarg1=value1, kwarg2=value2)

If you have additional arguments, they will be passed after the widget.

::callback(20)

def callback(widget, value):
    print(widget, value) # <tkinter object> 20

Below is an example where we connect a callabck to each key of a keypad. The callback is a function named key_pressed which takes the widget and the value of the key as arguments.

Connecting a callback to a keypad

The code for the callback is as follows

def key_pressed(widget, value):
    print(widget, value)
    # do something with the value and widget

Note

The :: prefix is not part of the callback name. It is only used to indicate that the widget should be passed as the first argument to the callback.

Connecting callbacks to your code

When loading a design from a file, you can connect the callbacks to your code by using the connect_callbacks() method of Builder. You can either pass a dictionary with the callback names as keys and the functions as values or pass an instance of a class that has the callback names as methods.

...

def on_click(event):
    print("button clicked")

def on_keypress(event):
    print("key pressed")

# method #1: manual dictionary

build = Builder(parent, path="my_design.xml")
build.connect_callbacks({
    "on_click": on_click,
    "on_keypress: on_keypress,
})

# method #2: globals() dictionary

build = Builder(path="my_design.xml")
build.connect_callbacks(globals())

...

# method #3: class instance

class App(tkinter.Tk):

    def __init__(self):
        self.build = Builder(self, path="my_design.xml")
        self.build.connect_callbacks(self)

    def on_click(self, event):
        print("button clicked")

    def on_keypress(self, event):
        print("key pressed")

app = App()
app.mainloop()
...