Variables & reactivity#

Widgets that hold a value — entries, checkbuttons, radiobuttons, scales — can be bound to a variable object instead of you reading and writing the widget directly. A variable is the single source of truth for that value: set the variable and the widget updates; the user edits the widget and the variable updates. That two-way link — plus running code when a value changes — is reactivity: your interface follows your data instead of you wiring every update by hand.

Note

This page covers the essentials. For the variable types in depth, traces (read / write / unset) and their cleanup, and patterns like computed fields, see the Variables guide.

Binding a widget to a variable#

Create a variable, then hand it to the widget through the option that fits: textvariable for text-bearing widgets (Entry, Label), variable for selection controls (Checkbutton, Radiobutton, Scale). Read and write with .get() and .set():

name = ttk.StringVar(value="Ada")

ttk.Entry(app, textvariable=name).pack()
ttk.Label(app, textvariable=name).pack()   # mirrors the entry live

name.set("Grace")                          # entry and label both update

ttkbootstrap provides four variable classes — ttk.StringVar, ttk.IntVar, ttk.BooleanVar, ttk.DoubleVar — and the class you pick determines what .get() returns:

StringVar — text (entries, labels).
IntVar — whole numbers; a Checkbutton’s on/off value; a Radiobutton choice.
BooleanVar — True/False (a Checkbutton’s checked state).
DoubleVar — floating-point numbers (a Scale’s position).

Reacting to changes#

To run code whenever a variable changes, attach a trace. The callback fires on every write — handy for live updates like enabling a button or refreshing a summary. Swallow the three callback arguments with *_:

name.trace_add("write", lambda *_: print("value is now", name.get()))

Traces come in read/write/unset modes and can be removed again for cleanup — see the Variables guide.

A worked example#

Reactivity in one screen: an entry feeds a variable, a trace recomputes a second variable from it, and a label bound to that one updates live as you type — no widget is read or refreshed by hand.

import ttkbootstrap as ttk

app = ttk.App(title="Reactivity")

name = ttk.StringVar()
greeting = ttk.StringVar(value="Hello there!")

def greet(*_):
    who = name.get().strip()
    greeting.set(f"Hello, {who}!" if who else "Hello there!")

name.trace_add("write", greet)            # recompute whenever name changes

ttk.Entry(app, textvariable=name).pack(padx=20, pady=(20, 8))
ttk.Label(app, textvariable=greeting, bootstyle="primary").pack(padx=20, pady=(0, 20))

app.mainloop()