import tkinter as tk
import tkinter.ttk as ttk

class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)

        # give invalid entries a red background
        style = ttk.Style()
        style.map("TEntry",  foreground=[('invalid', "red")])
        # style.map("TEntry",  selectforeground=[('invalid', "green")])     # More weirdness when uncommented

        self.entryVar = tk.StringVar()
        self.entry = ttk.Entry(self, textvariable=self.entryVar)
        self.button = ttk.Button(self, text="Button")

        # this label will show the current state, updated
        # every second.
        self.label = tk.Label(self, anchor="w")
        self.after_idle(self.updateLabel)

        # layout the widgets
        self.entry.pack(side="top", fill="x")
        self.button.pack(side="top", fill="x")
        self.label.pack(side="bottom", fill="x")

        # add trace on the variable to do custom validation
        self.entryVar.trace("w", self.validate)

        # set up bindings to also do the validation when we gain
        # or lose focus
        self.entry.bind("<FocusIn>", self.validate)
        self.entry.bind("<FocusOut>", self.validate)

    def updateLabel(self):
        '''Display the current entry widget state'''
        state = str(self.entry.state())
        self.label.configure(text=state)
        self.after(1000, self.updateLabel)

    def validate(self, *args):
        '''Validate the widget contents'''
        value = self.entryVar.get()
        if "invalid" in value:
            self.entry.state(["invalid"])
        else:
            self.entry.state(["!invalid"])

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).pack(fill="both", expand=True)
    root.mainloop()