from Tkinter import *

root = Tk()

class MyOwnEntry:

    def __init__(self):
        self.variable = StringVar()
        self.variable.trace("w", self.Validation)
        self.MyOwnEntry = Entry(root, textvariable = self.variable)
        self.MyOwnEntry.pack()

    def Validation(self,*args):
        self.newvar = self.variable.get()
        try:
            if isinstance(float(self.newvar),float):
                return self.newvar
            elif float(self.newvar)<1000:
                self.newvar = self.newvar[:-1]
                self.variable.set(self.newvar)
            else:
                self.newvar = self.newvar[:-1]
                self.variable.set(self.newvar)
        except ValueError:
            self.newvar = self.newvar[:-1]
            self.variable.set(self.newvar)




Input1 = MyOwnEntry()


root.mainloop()