Python Tkinter - Can only unbind and rebind Button-1 once

Question

I'm trying to deactivate button 1 linked to a parent function. I need to reassign button 1 to move an image on the canvas. All works well the first time the code is executed. The second time I'm getting this error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1540, in __call__
    return self.func(*args)
  File "./wman2", line 244, in move_window
    MoveWindow(mycanvas, _item_id, _canvas_xy, _b1_bind, popup)
  File "./wman2", line 256, in __init__
    self.canvas.unbind("<ButtonPress-1>", self.b1_bind)
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1102, in unbind
    self.deletecommand(funcid)
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 450, in deletecommand
    self.tk.deletecommand(name)
TclError: can't delete Tcl command

The code:

I've commented out the code that generates the error below:

def move_window ():
    global _b1_bind, popup
    ''' Can only move a window, not a monitor or entire canvas '''
    if _item_type == "Window" :
        MoveWindow(mycanvas, _item_id, _canvas_xy, _b1_bind, popup)

    else :
        display_info()

class MoveWindow:
    ''' Drag image on canvas '''

    def __init__(self, canvas, item, coords, b1_bind, func_bind):
        self._drag_data = {"x": 0, "y": 0, "item": None}
        self.item = item
        self.coords = coords
        self.canvas = canvas
        self.b1_bind = b1_bind
        self.func_bind = func_bind
#        self.canvas.unbind("<ButtonPress-1>", self.b1_bind)
        self.p_bind = self.canvas.bind("<ButtonPress-1>", \
                                       self.on_image_press, "+")
        self.r_bind = self.canvas.bind("<ButtonRelease-1>", \
                                       self.on_image_release, "+")
        self.m_bind = self.canvas.bind("<B1-Motion>", \
                                       self.on_image_motion, "+")

    def on_image_press(self, event):
        '''Begining drag of an object'''
        root.config(cursor='hand1')
        # record the item and its location
        self._drag_data["item"] = self.item
        self._drag_data["x"] = self.coords[0]
        self._drag_data["y"] = self.coords[1]
        ''' Initial call in case mouse off image '''

    def on_image_release(self, event):
        '''End drag of an object'''
        root.config(cursor='')
        self.canvas.unbind("<ButtonPress-1>", self.p_bind)
        self.canvas.unbind("<ButtonRelease-1>", self.r_bind)
        self.canvas.unbind("<B1-Motion>", self.m_bind)
#        self.b1_bind = self.canvas.bind("<Button-1>", self.func_bind, "+")

    def on_image_motion(self, event):
        '''Handle dragging of an object'''
        # compute how much the mouse has moved
        delta_x = event.x - self._drag_data["x"]
        delta_y = event.y - self._drag_data["y"]
        # move the object the appropriate amount
        self.canvas.move(self._drag_data["item"], delta_x, delta_y)
        # record the new position
        self._drag_data["x"] = event.x
        self._drag_data["y"] = event.y

Further down in the code:

# attach popup to canvas
menu.bind("<FocusOut>", popup_focus_out)
_b3_bind = mycanvas.bind("<Button-3>", popup, "+")
# Button 1 conflicts with MoveWindow()
_b1_bind=None
#_b1_bind = mycanvas.bind("<Button-1>", popup, "+")

Summary

The three lines I had to comment out are:

_b1_bind = mycanvas.bind("<Button-1>", popup, "+")
self.canvas.unbind("<ButtonPress-1>", self.b1_bind)
self.b1_bind = self.canvas.bind("<Button-1>", self.func_bind, "+")

Currently my only option is to not let the parent function use button 1 at all. I think the problem is the first time button 1 is reassigned to the parent the button ID changes and needs to be reassigned to the global variable _b1_bind but I'm not sure how to do that?

Credit: Code is from Brian Oakley.

@stovfl Yes I have that in my history: First successful unbind: `Starting value: 140585244928592popup` First successful rebind: `Ending value: 140585244928592popup` **Second unbind which crashes program:** `Starting value: 140585244928592popup` — WinEunuuchs2Unix, Dec 25 '19 at 20:12
***"Second unbind `140585244928592popup`***: You can't `.unbind(...` twice with the same `iid`. For monitoring do `self.br_bind = None` after `.unbind(...` — stovfl, Dec 25 '19 at 21:06
@stovfl I think you mean `self.b1_bind = None`? I thought I could unbind it a second time because it was recreated a second time with: `self.b1_bind = self.canvas.bind("", self.func_bind, "+")` — WinEunuuchs2Unix, Dec 25 '19 at 21:15
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/204850/discussion-between-stovfl-and-wineunuuchs2unix). — stovfl, Dec 25 '19 at 21:29

score 1 · Answer 1 · answered Dec 26 '19 at 00:12

Thanks to the help of stovfl it's working now.

The solution is to change _b1_bind from a global variable to a variable encapsulated within mycanvas.

First step is to change this line:

_b1_bind = mycanvas.bind("<Button-1>", popup, "+")

To:

mycanvas.b1_bind = mycanvas.bind("<ButtonPress-1>", popup, "+")

Now the ID for the bound button is no longer a global variable but rather part of mycanvas scoped variables.

The next step is to change the unbind from:

self.canvas.unbind("<ButtonPress-1>", self.b1_bind)

To:

self.canvas.unbind("<ButtonPress-1>", self.canvas.b1_bind)

Remember canvas is a parameter to MoveWindow class which is now called with:

MoveWindow(mycanvas, _item_id, _canvas_xy, popup)

And is received with:

def __init__(self, canvas, item, coords, func_bind):

The last line to change was:

self.b1_bind = self.canvas.bind("<Button-1>", self.func_bind, "+")

Which was changed to:

self.canvas.b1_bind = self.canvas.bind("<ButtonPress-1>", \
                                       self.func_bind, "+")

Python Tkinter - Can only unbind and rebind Button-1 once

The code:

Further down in the code:

Summary

1 Answers1