How to use Tkinter Canvas find_closest to select points while ignoring grid lines (with tags)

Learn how to make Tkinter Canvas find_closest pick the nearest oval, not grid lines. Understand halo and start, and use tags to hide items for hit testing.

Picking the right object on a Tkinter Canvas can be trickier than it looks. When you want clicks to snap to the nearest point while ignoring other shapes like grid lines, a naive call to find_closest(x, y) quickly runs into edge cases. The function happily returns whatever is closest in display space, which may be a line segment rather than a tiny oval you actually care about. Attempts to constrain the search with start and halo often don’t help in the way you might expect.

Reproducing the issue

The example below draws two 3×3 sets of small ovals and a simple grid. Clicking on the canvas should mark the nearest oval, but the call with start can still return a grid line.

import tkinter as tk
from tkinter import ttk
class PlotPad:
    def __init__(self, master):
        self.edge = 600
        self.master = master
        self.shell = ttk.Frame(self.master)
        self.board = tk.Canvas(self.shell, width=self.edge, height=self.edge)
        self.board.bind("<Motion>", self.on_move)
        self.board.bind("<Button-1>", self.on_click)
        self.footer = ttk.Frame(self.shell)
        self.txt = ttk.Label(self.footer, relief=tk.GROOVE, text='Coordinates go here')
        self.paint_dots(-150, 'blue', 'first_batch')
        self.paint_dots(-20, 'black', 'second_batch')
        self.paint_grid()
        self.shell.pack()
        self.board.pack()
        self.footer.pack(fill='both', expand=True)
        self.txt.pack(side='left', fill='x', expand=True)
    def on_move(self, event):
        px, py = event.x, event.y
        self.txt.config(text=f"Mouse coordinates: ({px}, {py})")
    def on_click(self, event):
        px, py = event.x, event.y
        # Attempt to prefer ovals by constraining with start
        nearest = self.board.find_closest(px, py, halo=0, start=9)
        bbox = self.board.coords(nearest[0])
        cx, cy = (bbox[0] + bbox[2]) / 2, (bbox[1] + bbox[3]) / 2
        span = 5
        self.board.create_line(cx - span, cy - span, cx + span, cy + span, fill='red')
        self.board.create_line(cx - span, cy + span, cx + span, cy - span, fill='red')
    def paint_dots(self, offset, color, tagname):
        for ix in range(1, 4):
            for iy in range(1, 4):
                self.board.create_oval(
                    ix * 200 + offset - 1,
                    iy * 200 + offset - 1,
                    ix * 200 + offset + 1,
                    iy * 200 + offset + 1,
                    fill=color,
                    tags=tagname
                )
    def paint_grid(self):
        for ix in range(1, 4):
            self.board.create_line(
                ix * 200 + 10,
                0,
                ix * 200 + 10,
                600,
                fill='green',
                tags='grid_lines')
        for iy in range(1, 4):
            self.board.create_line(
                0,
                iy * 200 + 10,
                600,
                iy * 200 + 10,
                fill='green',
                tags='grid_lines')
if __name__ == '__main__':
    root = tk.Tk()
    app = PlotPad(root)
    root.mainloop()

What’s actually going on

There are two Canvas.find_closest arguments that look promising but don’t solve this scenario. The halo parameter expands the click point into a circle of the given radius, effectively increasing the hit area around the mouse location. It does not filter object types. The start parameter does not limit the search to a range of IDs or a tag; instead, if multiple objects are at the same closest location, it picks the one that is below start in the display list. That makes it useful for breaking ties in z-order, not for excluding entire classes of shapes like grid lines.

Because of this, find_closest may still return a line if it is closer to the click point than any small oval.

Practical fix: temporarily hide what you don’t want to hit

A straightforward workaround is to toggle the state of the shapes you want to ignore, run the pick, and restore them. Since the grid lines all share the same tag, this becomes a simple, targeted change. Hide the 'grid_lines' before calling find_closest and restore them afterward.

def on_click(self, event):
    px, py = event.x, event.y
    # Temporarily remove grid lines from hit testing
    self.board.itemconfig('grid_lines', state="hidden")
    nearest = self.board.find_closest(px, py)
    self.board.itemconfig('grid_lines', state="normal")
    bbox = self.board.coords(nearest[0])
    cx, cy = (bbox[0] + bbox[2]) / 2, (bbox[1] + bbox[3]) / 2
    span = 5
    self.board.create_line(cx - span, cy - span, cx + span, cy + span, fill='red')
    self.board.create_line(cx - span, cy + span, cx + span, cy - span, fill='red')

This avoids geometric shuffling or moving items off-canvas. It relies on a consistent tag to isolate the objects you want to suppress during the hit test, which is often how interactive layers are managed on a Canvas anyway.

Why this matters

Pointer picking is at the heart of many UI behaviors, from map editors and diagramming tools to lightweight CAD-like interactions. Understanding that halo enlarges the click area and start influences tie-breaking in display order helps prevent dead ends when you need to restrict selection to a subset of items. When filtering is required, temporarily hiding irrelevant elements by tag is a dependable pattern.

Takeaways

If you need find_closest to operate on a subset of Canvas items, don’t expect start or halo to filter classes of objects. Use clear tagging and momentarily hide the layers you want to exclude, then restore them immediately after selection. If you must constrain the search to specific items without hiding, another approach is iterating over items with a target tag and computing the distance manually, but where tags are well-structured, state toggling is concise and effective.