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 |
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