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Scale

Scale is a direct-manipulation input control for selecting a numeric value from a continuous or stepped range.

Unlike NumericEntry, which is optimized for precise typing, Scale is designed for gesture-based adjustment — ideal for volume, zoom, thresholds, and any setting where users benefit from immediate visual feedback.

Image placeholder: ![Scale overview](../_img/widgets/scale/overview.png) Suggested shot: horizontal scale + value label + live update.

Quick start

import bootstack as bs

app = bs.App()

scale = bs.Scale(app, from_=0, to=100, value=50)
scale.pack(fill="x", padx=20, pady=10)

app.mainloop()

When to use

Use Scale when:

  • users benefit from visual, continuous adjustment
  • relative changes matter more than precision
  • live feedback improves usability

Consider a different control when...

Appearance

orient

Controls orientation.

bs.Scale(app, from_=0, to=100, orient="horizontal")  # or "vertical"

Orientation

Horizontal scales are preferred for most desktop layouts. Vertical scales work well for audio levels, side panels, or compact tool areas.

Design System

Scale styling follows the theme's color palette. See Design System for customization options.

Examples and patterns

Value model

A Scale always produces a numeric value within its configured range.

  • The displayed value updates while the user drags the thumb.
  • A committed value is produced when the user releases the thumb.

If your app treats "live preview" differently from "final commit", use the appropriate event hook (see Events).

from_ and to

Defines the numeric range.

bs.Scale(app, from_=10, to=200)

value

Sets the initial value.

bs.Scale(app, from_=0, to=100, value=25)

step

If supported by your implementation, constrains values to discrete increments.

bs.Scale(app, from_=0, to=10, step=1)

Continuous vs stepped

  • Continuous scales are best for perception-based adjustment

  • Stepped scales are best when values must align to discrete states

Events

Scale distinguishes between live input and committed change.

Event When it fires
on_input while the thumb is moving
on_changed when the user releases the thumb 
def handle_changed(event):
    print("final value:", event.data)

scale.on_changed(handle_changed)

Live feedback

Use on_input(...) to update previews or labels while dragging. Use on_changed(...) when the value is committed.

Validation and constraints

Because a Scale inherently constrains values to a known range, explicit validation is usually minimal.

Validation is most useful when:

  • the scale is conditionally enabled/required
  • the valid range changes dynamically
  • the value must satisfy cross-field rules

Behavior

Scales are commonly paired with a label or preview to reflect the current value.

value_label = bs.Label(app, text="50")

def update_value(event):
    value_label.config(text=str(int(event.data)))

scale.on_input(update_value)

Additional resources

API reference