Validation in Tkinter

Tkinter's validation system is one of those parts of the toolkit that is less than intuitive. It relies on the following three configuration options that we can pass into any input widget:

• validate: This option determines which type of event will trigger the validation callback
• validatecommand: This option takes the command that will determine if the data is valid
• invalidcommand: This option takes a command that will run if validatecommand returns False

This seems pretty straightforward, but there are some unexpected curves.

The values we can pass to validate are as follows:

Validates string	Triggers when
none	It is none that turns off validation
focusin	The user enters or selects the widget
unfocus	The user leaves the widget
focus	Either focusin or focusout
key	The user enters text in the widget
all	focusin, focusout, and key

 

The validatecommand argument is where things get tricky. You might think this takes the name of a Python function or method, but that's not quite it. Instead, we...