Retrieve/get back command callback function from TKinter widget

Question

I am (for some elaborate setup reasons) trying to retrieve the actual command callback function from tkinter widgets, for example setting up a callback for a button b

import tkinter as tk
root = tk.Tk()
b = tk.Button(root, text='btn', command=lambda:print('foo'))

both

b['command']
b.cget('command')

which I think both are equivalent to

b.tk.call(b._w, 'cget', '-command')

will only return a string like "2277504761920<lambda\>" and not the actual command function. Is there a way to get the actual callback function?

Answer 1

I cannot imagine any case and Im not sure at all if this answers your question but it maybe equivalent for what you are looking for:

---

The invoke method of the button seems pretty equivalent to me. So solution-1 would be:

import tkinter as tk

def hi():
    print('hello')

root = tk.Tk()
b = tk.Button(root, text='test', command=hi)
b.pack()

cmd = b.invoke
#cmd = lambda :b._do('invoke')
root.mainloop()

---

If this isnt what you looking for you could call the function in tcl level. Solution-2:

import tkinter as tk

def hi():
    print('hello')

root = tk.Tk()
b = tk.Button(root, text='test', command=hi)
b.pack()
cmd = lambda :root.tk.call(b['command'])
#cmd= lambda :root.tk.eval(b['command'])
cmd()
root.mainloop()

---

Solution 3, would be to return your function by invoke:

import tkinter as tk

def hi():
    print('hello')
    return hi

root = tk.Tk()
b = tk.Button(root, text='test', command=hi)
b.pack()
cmd = b.invoke()
print(cmd) #still a string but comparable
root.mainloop()

Answer 2

This is a more complex solution. It patches Misc._register, Misc.deletecommand and Misc.destroy to delete values from dict tkinterfuncs. In this example there are many print to check that values are added and removed from the dict.

import tkinter as tk

tk.tkinterfuncs = {} # name: func

def registertkinterfunc(name, func):
    """Register name in tkinterfuncs."""
    # print('registered', name, func)
    tk.tkinterfuncs[name] = func
    return name

def deletetkinterfunc(name):
    """Delete a registered func from tkinterfuncs."""
    # some funcs ('tkerror', 'exit') are registered outside Misc._register
    if name in tk.tkinterfuncs:
        del tk.tkinterfuncs[name]
        # print('delete', name, 'tkinterfuncs len:', len(tkinterfuncs))

def _register(self, func, subst=None, needcleanup=1):
    """Return a newly created Tcl function. If this
    function is called, the Python function FUNC will
    be executed. An optional function SUBST can
    be given which will be executed before FUNC."""
    name = original_register(self, func, subst, needcleanup)
    return registertkinterfunc(name, func)

def deletecommand(self, name):
    """Internal function.
    Delete the Tcl command provided in NAME."""
    original_deletecommand(self, name)
    deletetkinterfunc(name)

def destroy(self):
    """
    Delete all Tcl commands created for
    this widget in the Tcl interpreter.
    """
    if self._tclCommands is not None:
        for name in self._tclCommands:
            # print('- Tkinter: deleted command', name)
            self.tk.deletecommand(name)
            deletetkinterfunc(name)
        self._tclCommands = None

def getcommand(self, name):
    """
    Gets the command from the name.
    """
    return tk.tkinterfuncs[name]


original_register = tk.Misc.register
tk.Misc._register = tk.Misc.register = _register 
original_deletecommand = tk.Misc.deletecommand
tk.Misc.deletecommand = deletecommand
tk.Misc.destroy = destroy
tk.Misc.getcommand = getcommand

if __name__ == '__main__':
    def f():
        root.after(500, f)

    root = tk.Tk()
    root.after(500, f)
    but1 = tk.Button(root, text='button1', command=f)
    but1.pack()
    but2 = tk.Button(root, text='button2', command=f)
    but2.pack()
    but3 = tk.Button(root, text='button3', command=lambda: print(3))
    but3.pack()
    print(root.getcommand(but1['command']))
    print(root.getcommand(but2['command']))
    print(root.getcommand(but3['command']))
    but3['command'] = f
    print(root.getcommand(but3['command']))
    root.mainloop()