from Tkinter import *
root = Tk()
L1 = Label(root, text="Login page", bg = "blue", fg = "white")
L1.pack(fill = X, ipadx = 5, ipady = 5)
V = StringVar(root, value='Enter username here')
E1 = Entry(root, textvariable=V)
E1.pack(side = LEFT, padx = 5, pady = 5)
username = V.get()
print "Username is '" + username + "'"
B1 = Button(root, text ="Login" , command = Login)
B1.pack(side = RIGHT, fill = X, pady=5)
Think about scope for a moment. When your program ends, all memory (meaning variables, objects, etc.) are released. The only 2 ways I can think of to pass something from one program to another is:
1) Write the username value to a file which the next program can read as part of its startup.
2) Have a third "controller" or "launcher" program that runs the program above, takes a return value from that program, then passes that value as a parameter to the next program.
But in any case, you will have to save that value past the scope of the program above.