Entry Validation

Central to our task is determining whether the user input is valid or not. Tkinter provides validatation tools, generally both tkinter and ttk entry widgets react in the same way. We shall use ttk as it continues to validate without having to be reset. By the way the same system can be used for both the spinbox and combobox.

There are 3 options used for entry validation, validate together with validatecommand and invalidcommand. Before a validatecommand or invalidcommand can be used it first of all must be registered, creating a TCL wrapper. These options point to a callback function that checks the input and returns a True or False, if True then the input is made otherwise the input is blocked.

Important

Every check in the callback function can only return True or False otherwise the validation stops working.

Validate Options

Validate has the following options.

Note

The last option is none, not to be confused with the reserved word None.

Validatecommand and Invalidcommand Options

The commands can take the following substitution codes, as enhancements to the callback function.

Methods Used with Validation

The three most important types of input, string, integer and float, will be dealt within this section. Colour Picker shows how to deal with a hexadecimal input. The colour picker uses check limits with different widgets using the same validation function. As with any function only call the actual validation parameters being being used.

As we are validating simple options the validate function method together with key is used and the validatecommand. Within these scripts the following aliases are used :-

text %P

The value of the entry if the edit is allowed

input %S

The text string being inserted/deleted

index %i

Index of char string to be inserted/deleted

action %d

Type of action: 1 for insert, 0 for delete, or -1 for focus, forced or textvariable validation