how to make tkinter event work when <button-1> is pressed?

Question

I want to fill rectangles just like in paint. When mouse button is pressed I want every rectangle I enter to be filled and otherwise I want no event to take place.

Answer 1

There's no need to use <Enter> at all. Try:

from tkinter import Canvas
import tkinter

_width = 50
_height = 50
_size = 8

root = tkinter.Tk()
root.title("draw me a lovely matrix")
canv = Canvas(root, width=_width * _size, height=_height * _size)
rects = []

# B1-Motion activates every time the mouse is moved with button 1 held down
# See https://effbot.org/tkinterbook/tkinter-events-and-bindings.htm
# We fill every item that the mouse comes in contact with while button 1 is held
# The tag tied to the item the event was called on, "current", points to the
#   item under the pointer when the button was pressed, i.e. the first box.
# Only the x and y properties change, so we configure the item closest to those.
def motion(event):
  canv.itemconfig(canv.find_closest(event.x, event.y), fill="#aaa")
canv.bind("<B1-Motion>", motion)

for i in range(_size):
  for j in range(_size):
    rects.append(canv.create_rectangle(_width * j, _height * i, _width * (j + 1), _height * (i + 1), fill="#fff", width=0))
    # don't need to bind anything to <Enter>

canv.pack()
root.mainloop()

Unfortunately, there's no possible solution using <Enter>. The following non-working code would theoretically be correct if <Enter> worked the way you wanted it to.

import tkinter

_width = 50
_height = 50
_size = 8

root = tkinter.Tk()
root.title("draw me a lovely matrix")
canv = tkinter.Canvas(root, width=_width * _size, height=_height * _size)
rects = []

# This javascript-like approach doesn't work in tkinter because
# <Enter> and <Leave> events aren't called while any mouse buttons
# are held.
def fill(tag):
  canv.itemconfig(tag, fill="#aaa")
def enter(event):
  fill("current")
def mousedown(event):
  fill("current")
  canv.tag_bind("all", "<Enter>", enter, add="+")
def mouseup(event):
  canv.tag_unbind("all", "<Enter>")
canv.bind("<Button-1>", mousedown)
canv.bind("<ButtonRelease-1>", mouseup)

for i in range(_size):
  for j in range(_size):
    rects.append(canv.create_rectangle(_width * j, _height * i, _width * (j + 1), _height * (i + 1), fill="#fff", width=0))

canv.pack()
root.mainloop()

Tested in python 3.8.2, tkinter 8.6