Validation

Validation Modes

There are two main validation modes: prevalidation, in which the -validatecommand is evaluated prior to each edit and the return value is used to determine whether to accept or reject the change; and revalidation, in which the -validatecommand is evaluated to determine whether the current value is valid.

The -validate option determines when validation occurs; it may be set to any of the following values:

none: Default. This means validation will only occur when specifically requested by the validate widget command.
key: The entry will be prevalidated prior to each edit (specifically, whenever the insert or delete widget commands are called). If prevalidation fails, the edit is rejected.
focus: The entry is revalidated when the entry receives or loses focus.
focusin: The entry is revalidated when the entry receives focus.
focusout: The entry is revalidated when the entry loses focus.
all: Validation is performed for all above conditions.

The -invalidcommand is evaluated whenever the -validatecommand returns a false value.

The -validatecommand and -invalidcommand may modify the entry widget’s value via the widget insert or delete commands, or by setting the linked -textvariable. If either does so during prevalidation, then the edit is rejected regardless of the value returned by the -validatecommand.

If -validatecommand is empty (the default), validation always succeeds.

Validation Script Substitutions

It is possible to perform percent substitutions on the -validatecommand and -invalidcommand, just as in a bind script. The following substitutions are recognized:

%d: Type of action: 1 for insert prevalidation, 0 for delete prevalidation, or -1 for revalidation.
%i: Index of character string to be inserted/deleted, if any, otherwise -1.
%P: In prevalidation, the new value of the entry if the edit is accepted. In revalidation, the current value of the entry.
%s: The current value of entry prior to editing.
%S: The text string being inserted/deleted, if any, {} otherwise.
%v: The current value of the -validate option.
%V: The validation condition that triggered the callback (key, focusin, focusout, or forced).
%W: The name of the entry widget.

Differences from Tk Entry Widget Validation

The standard Tk entry widget automatically disables validation (by setting -validate to none) if the -validatecommand or -invalidcommand modifies the entry’s value. The Tk themed entry widget only disables validation if one of the validation scripts raises an error, or if -validatecommand does not return a valid boolean value. (Thus, it is not necessary to re-enable validation after modifying the entry value in a validation script).

In addition, the standard entry widget invokes validation whenever the linked -textvariable is modified; the Tk themed entry widget does not.