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.

from tkinter import *
root=Tk()
root.geometry("500x400+200+100")

class Menu_Entry(Entry):
    def __init__(self,perant,*args,**kwargs):
        Entry.__init__(self,perant,*args,**kwargs)
        self.popup_menu=Menu(self,tearoff=0,background='#1c1b1a',fg='white',
                                     activebackground='#534c5c',
                             activeforeground='Yellow')
        self.popup_menu.add_command(label="Cut                     ",command=self.Cut,
                                    accelerator='Ctrl+V')
        self.popup_menu.add_command(label="Copy                    ",command=self.Copy,compound=LEFT,
                                    accelerator='Ctrl+C')
    
        self.popup_menu.add_command(label="Paste                   ",command=self.Paste,accelerator='Ctrl+V')
        self.popup_menu.add_separator()
        self.popup_menu.add_command(label="Select all",command=self.select_all,accelerator="Ctrl+A")
        self.popup_menu.add_command(label="Delete",command=self.delete_only,accelerator=" Delete")
        self.popup_menu.add_command(label="Delete all",command=self.delete_selected,accelerator="Ctrl+D")
        self.bind('<Button-3>',self.popup)
        self.bind("<Control-d>",self.delete_selected_with_e1)
        self.bind('<App>',self.popup)
        self.context_menu = Menu(self, tearoff=0)
        self.context_menu.add_command(label="Cut")
        self.context_menu.add_command(label="Copy")
        self.context_menu.add_command(label="Paste")
         
    def popup(self, event):
      try:
        self.popup_menu.tk_popup(event.x_root, event.y_root, 0)
      finally:
        self.popup_menu.grab_release()

    def Copy(self):
      self.event_generate('<<Copy>>')

    def Paste(self):
      self.event_generate('<<Paste>>')

    def Cut(self):
      self.event_generate('<<Cut>>')

    def delete_selected_with_e1(self,event):
      self.select_range(0, END)
      self.focus()
      self.event_generate("<Delete>")

    def delete_selected(self):
      self.select_range(0, END)
      self.focus()
      self.event_generate("<Delete>")

    def delete_only(self):
      self.event_generate("<BackSpace>")

    def select_all(self):
      self.select_range(0, END)
      self.focus()



ent=Menu_Entry(root)
ent.pack()


root.mainloop()

I made some changes to the conext menu code above in order to adjust my demand and I think it would be useful to share:

Version 1:

import tkinter as tk
from tkinter import ttk

class Main(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master)
        master.geometry('500x350')
        self.master = master
        self.tree = ttk.Treeview(self.master, height=15)
        self.tree.pack(fill='x')
        self.btn = tk.Button(master, text='click', command=self.clickbtn)
        self.btn.pack()
        self.aMenu = tk.Menu(master, tearoff=0)
        self.aMenu.add_command(label='Delete', command=self.delete)
        self.aMenu.add_command(label='Say Hello', command=self.hello)
        self.num = 0

        # attach popup to treeview widget
        self.tree.bind("<Button-3>", self.popup)

    def clickbtn(self):
        text = 'Hello ' + str(self.num)
        self.tree.insert('', 'end', text=text)
        self.num += 1

    def delete(self):
        print(self.tree.focus())
        if self.iid:
            self.tree.delete(self.iid)

    def hello(self):
        print ('hello!')

    def popup(self, event):
        self.iid = self.tree.identify_row(event.y)
        if self.iid:
            # mouse pointer over item
            self.tree.selection_set(self.iid)
            self.aMenu.post(event.x_root, event.y_root)            
        else:
            pass

root = tk.Tk()
app=Main(root)
root.mainloop()

Version 2:

import tkinter as tk
from tkinter import ttk

class Main(tk.Frame):
    def __init__(self, master):
        master.geometry('500x350')
        self.master = master
        tk.Frame.__init__(self, master)
        self.tree = ttk.Treeview(self.master, height=15)
        self.tree.pack(fill='x')
        self.btn = tk.Button(master, text='click', command=self.clickbtn)
        self.btn.pack()
        self.rclick = RightClick(self.master)
        self.num = 0

        # attach popup to treeview widget
        self.tree.bind('<Button-3>', self.rclick.popup)
    def clickbtn(self):
        text = 'Hello ' + str(self.num)
        self.tree.insert('', 'end', text=text)
        self.num += 1

class RightClick:
    def __init__(self, master):
       
        # create a popup menu
        self.aMenu = tk.Menu(master, tearoff=0)
        self.aMenu.add_command(label='Delete', command=self.delete)
        self.aMenu.add_command(label='Say Hello', command=self.hello)

        self.tree_item = ''

    def delete(self):
        if self.tree_item:
            app.tree.delete(self.tree_item)

    def hello(self):
        print ('hello!')

    def popup(self, event):
        self.aMenu.post(event.x_root, event.y_root)
        self.tree_item = app.tree.focus()

root = tk.Tk()
app=Main(root)
root.mainloop()