Key-Press Widget Placement

Demonstrates the key_down event handler API: press a key while the plot is focused to add an overlay widget centred on the current cursor position, or press Backspace / Delete to remove the last widget you clicked.

Key bindings

Key	Action
q	Add a rectangle
w	Add a circle
e	Add an annulus
Backspace (macOS ⌫) Delete (Windows / Linux)	Remove last-clicked

Built-in 2-D shortcuts (not overridden in this example):

Key	Action
r	Reset zoom / pan
c	Toggle colorbar
l	Toggle log scale
s	Toggle symlog scale

The cursor coordinates are available as event.xdata and event.ydata in image-pixel space (column, row), so widgets are centred exactly where the cursor was when the key was pressed.

Note

Move the mouse over the image first so the plot panel receives focus, then press a key. On macOS the backspace key (⌫) is used for deletion; on Windows / Linux use the Delete key.

⚡

import numpy as np
import anyplotlib as apl

# ── Synthetic test image ──────────────────────────────────────────────────────
rng = np.random.default_rng(0)
N = 256
x = np.linspace(0, 4 * np.pi, N)
XX, YY = np.meshgrid(x, x)
data = np.sin(XX) * np.cos(YY) + 0.15 * rng.standard_normal((N, N))

# ── Figure ────────────────────────────────────────────────────────────────────
fig, ax = apl.subplots(figsize=(520, 520))
plot = ax.imshow(data)

# ── Key handlers ─────────────────────────────────────────────────────────────

@plot.add_event_handler("key_down")
def add_rectangle(event):
    """Press 'q' — add a rectangle centred on the cursor."""
    if event.key != 'q':
        return
    cx, cy = event.xdata, event.ydata
    half_w, half_h = N * 0.08, N * 0.08
    plot.add_widget(
        "rectangle",
        x=cx - half_w, y=cy - half_h,
        w=half_w * 2,  h=half_h * 2,
        color="#ffd54f",
    )


@plot.add_event_handler("key_down")
def add_circle(event):
    """Press 'w' — add a circle centred on the cursor."""
    if event.key != 'w':
        return
    plot.add_widget(
        "circle",
        cx=event.xdata, cy=event.ydata,
        r=N * 0.07,
        color="#80cbc4",
    )


@plot.add_event_handler("key_down")
def add_annulus(event):
    """Press 'e' — add an annulus centred on the cursor."""
    if event.key != 'e':
        return
    plot.add_widget(
        "annular",
        cx=event.xdata, cy=event.ydata,
        r_outer=N * 0.12,
        r_inner=N * 0.06,
        color="#ce93d8",
    )


# macOS sends 'Backspace' for the ⌫ key; Windows/Linux send 'Delete'.
# Register both so the example works cross-platform.
@plot.add_event_handler("key_down")
def delete_last(event):
    """Press Backspace/Delete — remove the last widget that was clicked."""
    if event.key not in ('Backspace', 'Delete'):
        return
    wid = event.last_widget_id
    if wid and wid in {w.id for w in plot.list_widgets()}:
        plot.remove_widget(wid)


# ── Catch-all handler (optional) — log every registered key press ─────────────

@plot.add_event_handler("key_down")
def log_key(event):
    xdata = event.xdata
    ydata = event.ydata
    pos = f"({xdata:.1f}, {ydata:.1f})" if xdata is not None else "n/a"
    print(f"[key_down] key={event.key!r}  img={pos}"
          f"  last_widget={event.last_widget_id!r}")

fig  # Interactive