Buttons, Labels, and Entry Widgets

The label widget displays static text or an image and accepts no direct user input—no clicks, no typing—making it the simplest Tk widget for captions, instructions, or read-only status text. Its content is set either with -text "Ready" for a fixed string or, more usefully for text that changes over time, with -textvariable statusVar, which links the label to a Tcl variable so that setting set statusVar "Saving..." anywhere in the script automatically updates the label on screen without calling configure again. A label can also display an image with -image myPhoto (after loading a Tk photo image with the image create photo command), and -compound controls how text and image combine when both are set, accepting values like left, right, top, bottom, or center to position the text relative to the image.

A button widget's core behavior comes from -command scriptOrProc, which is invoked whenever the button is clicked (or activated via keyboard when it has focus), and the same script can be triggered programmatically at any time with widgetPath invoke, which is useful for testing or for wiring a keyboard shortcut to the same action a click would trigger. Setting -state disabled grays the button out and makes it unclickable until you set -state normal again, which is the standard way to prevent double-submission of a form while a save operation is in progress. Tk offers both the classic button widget and the themed ttk::button; both share the -text, -command, and -state options, but ttk::button follows the active theme's appearance and is generally preferred for new applications, while classic button supports a few extra options like -default active that give it a distinct highlighted border to mark it as the form's default action.

The entry widget provides a single-line text field, most commonly linked to a Tcl variable via -textvariable, which lets code read the current contents at any time with set value $usernameVar rather than issuing an explicit .username get call, though get still works and is the only option for entries not linked to a variable. Programmatic edits use .username delete 0 end followed by .username insert 0 "default text" to clear and replace the contents, with 0 and end acting as character-index positions the same way string indices do elsewhere in Tcl. For input validation, -validate key combined with -validatecommand {checkInput %P} runs a validation procedure on every keystroke before the change is accepted, where %P substitutes the proposed new value of the entry (what it would become if the edit is allowed), letting the procedure return 0 to reject the keystroke or 1 to allow it—for example, rejecting any character that isn't a digit to build a numeric-only field.

The -textvariable pattern used by label and entry extends to other widgets through similarly named options: checkbutton and radiobutton use -variable to link their checked state to a Tcl variable (often paired with -onvalue/-offvalue for checkbuttons or a shared -variable with distinct -value per button for a radiobutton group), and scale uses -variable to link a slider's numeric position. Because these are ordinary Tcl variables, you can attach a trace to them with trace add variable usernameVar write onUsernameChanged, which calls onUsernameChanged automatically every time the entry's linked variable changes for any reason—whether the user typed a character or another part of the program set the value programmatically—making traces a clean way to react to input changes without polling the widget or duplicating logic across every code path that might modify the field.