tkinter app adding a right click context menu?

You would create a Menu instance and write a function that calls
its post() or tk_popup() method.

The tkinter documentation doesn’t currently have any information about tk_popup().
Read the Tk documentation for a description, or the source:

library/menu.tcl in the Tcl/Tk source:

::tk_popup --
This procedure pops up a menu and sets things up for traversing
the menu and its submenus.

Arguments:
menu  - Name of the menu to be popped up.
x, y  - Root coordinates at which to pop up the menu.  
entry - Index of a menu entry to center over (x,y).  
        If omitted or specified as {}, then menu's  
        upper-left corner goes at (x,y).  

tkinter/__init__.py in the Python source:

def tk_popup(self, x, y, entry=""):
    """Post the menu at position X,Y with entry ENTRY."""
    self.tk.call('tk_popup', self._w, x, y, entry)

You associate your context menu invoking function with right-click via:
the_widget_clicked_on.bind("<Button-3>", your_function).

However, the number associated with right-click is not the same on every platform.

library/tk.tcl in the Tcl/Tk source:

On Darwin/Aqua, buttons from left to right are 1,3,2.  
On Darwin/X11 with recent XQuartz as the X server, they are 1,2,3; 
other X servers may differ.

Here is an example I wrote that adds a context menu to a Listbox:

import tkinter # Tkinter -> tkinter in Python 3

class FancyListbox(tkinter.Listbox):

    def __init__(self, parent, *args, **kwargs):
        tkinter.Listbox.__init__(self, parent, *args, **kwargs)

        self.popup_menu = tkinter.Menu(self, tearoff=0)
        self.popup_menu.add_command(label="Delete",
                                    command=self.delete_selected)
        self.popup_menu.add_command(label="Select All",
                                    command=self.select_all)

        self.bind("<Button-3>", self.popup) # Button-2 on Aqua

    def popup(self, event):
        try:
            self.popup_menu.tk_popup(event.x_root, event.y_root, 0)
        finally:
            self.popup_menu.grab_release()

    def delete_selected(self):
        for i in self.curselection()[::-1]:
            self.delete(i)

    def select_all(self):
        self.selection_set(0, 'end')


root = tkinter.Tk()
flb = FancyListbox(root, selectmode="multiple")
for n in range(10):
    flb.insert('end', n)
flb.pack()
root.mainloop()

The use of grab_release() was observed in an example on effbot.
Its effect might not be the same on all systems.