问如何在Tkinter上只允许输入字段中的某些参数

EN

import Tkinter as tk

class LimitedFloatEntry(tk.Entry):
    '''A new type of Entry widget that allows you to set limits on the entry'''
    def __init__(self, master=None, **kwargs):
        self.var = tk.StringVar(master, 0)
        self.var.trace('w', self.validate)
        self.get = self.var.get
        self.from_  = kwargs.pop('from_', 0)
        self.to = kwargs.pop('to', 1)
        self.old_value = 0
        tk.Entry.__init__(self, master, textvariable=self.var, **kwargs)

    def validate(self, *args):
        try:
            value = self.get()
            # special case allows for an empty entry box
            if value not in ('', '-') and not self.from_ <= float(value) <= self.to:
                raise ValueError
            self.old_value = value
        except ValueError:
            self.set(self.old_value)

    def set(self, value):
        self.delete(0, tk.END)
        self.insert(0, str(value))

root = tk.Tk()
e1 = LimitedFloatEntry(root, from_=-2, to=5)
e1.pack()
root.mainloop()