How to do mouse hovering for group of buttons in python - python-2.7

Question

I am just trying a mouse hovering function for a group of 5 buttons, mouse hovering color
setting is running fine with one button sample but it is not giving proper result
when I tried for multiple buttons, it is affecting only last binded button, can anyone
help me quick on what is wrong with my below code, I tried with bind and bind_all methods!
both are failing to give desired output.
import os
from Tkinter import *
import Tkinter as tk
global btnPlaceY
btnPlaceY=10
class App():
def __init__ (self):
root['width']=400
root['height']=600
root['bg']='blue'
self.btnGroupCreate()
def btnDisplaymessage1(self):
print "Function testing1"
def btnDisplaymessage2(self):
print "Function testing2"
def btnGroupCreate(self):
btnNameNameGroup="btnSample"
self.btnWidget={}
lstBtnTitle=['A','B','C','D','E']
lstBtnCommands=[ lambda:self.btnDisplaymessage1(),
lambda:self.btnDisplaymessage2() ]
for B in range(5):
btnName=btnNameNameGroup+lstBtnTitle[B]
global btnPlaceY
btnPlaceY=btnPlaceY+70
# Button commands
if B==0:
btnMenuCommand=lstBtnCommands[0]
if B==1:
btnMenuCommand=lstBtnCommands[1]
if B==2:
btnMenuCommand=lstBtnCommands[0]
if B==3:
btnMenuCommand=lstBtnCommands[1]
if B==4:
btnMenuCommand=lstBtnCommands[1]
if B==5:
btnMenuCommand=lstBtnCommands[0]
self.btnWidget[btnName]= tk.Button(root, width=4, height=2,text=lstBtnTitle[B],bg="gray",relief=FLAT,command=btnMenuCommand) #activebackground="red"
widget = self.btnWidget[btnName]
widget.bind_all("<Enter>", lambda event: event.widget.configure(bg="red"))
widget.bind_all("<Leave>", lambda event: event.widget.configure(bg="gray"))
self.btnWidget[btnName].pack()
self.btnWidget[btnName].place(x=40, y = btnPlaceY)
def make_callback(self,btnName):
print "make_callback"
root['bg']='green'
widget = self.btnWidget[btnName]
def callback(event):
print "callback"
widget.configure(bg="red")
root['bg']='green'
return callback
root=tk.Tk()
app = App()
root.mainloop()

Answer 1

The problem is that in Python the closures such as your lambda function, close over the names in the surrounding block, not the values. As such, the value of btnName as observed in the lambda depends on when it is executed - as the events can occur only after the loop is exited, the btnName is the last one seen.
There are many ways to avoid this problem, one being the Kevin's default value trick. However, the default value approach is prone to errors - now the function accepts 2 arguments instead of 1 and can be called erroneously in some context.
Thus a better approach is to use a callback maker:
def make_callback(btnName):
def callback(event):
self.btnWidget[btnName].configure(bg="red"))
return callback
self.btnWidget[btnName].bind_all("<Enter>", make_callback(btnName))
However, one notices that the btnWidget is repeated, so it is possible to do this into
widget = self.btnWidget[btnName]
def make_callback(btnName):
def callback(event):
widget.configure(bg="red")
return callback
widget.bind_all("<Enter>", make_callback(widget))
Which could be written with lambdas as:
widget = self.btnWidget[btnName]
widget.bind_all("<Enter>",
(lambda widget: lambda event: widget.configure(bg="red"))(widget))
However, in this case none of this trickery is necessary as the only thing needed in callback is the widget on which the event occurs. And that is readily available in the TkInter events in the attribute widget, so really all you need to do is
widget = self.btnWidget[btnName]
widget.bind_all("<Enter>", lambda event: event.widget.configure(bg="red"))
widget.bind_all("<Leave>", lambda event: event.widget.configure(bg="yellow"))

Answer 2

self.btnWidget[btnName].bind_all("<Enter>", lambda event: self.btnWidget[btnName].configure(bg="red"))
self.btnWidget[btnName].bind_all("<Leave>", lambda event: self.btnWidget[btnName].configure(bg="yellow"))
Inside the lambda expressions on each of these lines, btnName will have the last value that it held during the loop, rather than the value it had when you bound the event. It will be "btnSampleE", even for buttons A through D. Additionally, you should be using bind instead of bind_all for widget-specific events.
The typical way to force "early binding" behavior, so that btnName keeps its current value, is to supply a default argument to the lambda, containing the value you need.
self.btnWidget[btnName].bind("<Enter>", lambda event, x = btnName: self.btnWidget[x].configure(bg="red"))
self.btnWidget[btnName].bind("<Leave>", lambda event, x = btnName: self.btnWidget[x].configure(bg="yellow"))
You can use whatever variable name you want for this argument; you could even call it btnName, as the local value will overshadow the nonlocal one.
self.btnWidget[btnName].bind("<Enter>", lambda event, btnName=btnName: self.btnWidget[btnName].configure(bg="red"))
self.btnWidget[btnName].bind("<Leave>", lambda event, btnName=btnName: self.btnWidget[btnName].configure(bg="yellow"))