Problems with Focus - python-3.x

Question

I'm trying to return the focus to the first entry. If you move the focus to the next entry or the button and the you click on the button, the focus returns fine to first entry. When I try doing the same thing by using the tab key, the focus_set method fails. I've tried many different ways, but the result is always the same. Anyone knows why? And might be so kind as to showing me how to do it right? Thanks in advance.
This is what I got so far:
from tkinter import *
w = Tk()
def focus():
box1.focus_set()
def check(event):
if str(event.widget) == '.!entry2':
print('focus back to box1')
focus()
box1 = Entry(w, width=15)
box2 = Entry(w, width=15)
box1.focus_set()
box2.bind('<Tab>', check)
box1.pack()
box2.pack()
btn = Button(w, text='Box 1 Focus', command=focus)
btn.pack()
w.mainloop()

Answer 1

If I run your code, str(event.widget) is something like ".36580648", not ".!entry2". You can give your widget a custom name like
box2 = Entry(w, width=15, name='second')
You can then check if str(event.widget) == '.second'.
Alternatively, you can just check if event.widget == box2: which is easier and less prone to error.
If you do one of these things, you will see that 'focus back to box1' is printed, but the focus is still transferred to the button instead of the label. This is because your custom event is triggered before the default event for <Tab>, which is to move focus to the next widget. You can stop the default event handling by returning 'break' in your function.
The complete example would become:
from tkinter import *
w = Tk()
def focus():
box1.focus_set()
def check(event):
if event.widget == box2:
print('focus back to box1')
focus()
return 'break'
box1 = Entry(w, width=15)
box2 = Entry(w, width=15)
box1.focus_set()
box2.bind('<Tab>', check)
box1.pack()
box2.pack()
btn = Button(w, text='Box 1 Focus', command=focus)
btn.pack()
w.mainloop()