import tkinter as tki


class MyCalculatorApp:
    NUM_DIGITS = 10

    def __init__(self,parent):
        self._parent = parent
        self._display_label = tki.Label(parent)
        self._display_label.pack(side=tki.TOP)

        lower_frame = tki.Frame(parent)
        lower_frame.pack()

        self._create_digit_buttons(lower_frame)
        self._create_op_buttons(lower_frame)

        self._reset()

    def _reset(self):
        self._display_label.configure(text="0")
        self._current_num = ""
        self._prev_num = 0
        self._prev_op = lambda x, y: x+y

    def _create_digit_buttons(self,parent):
        digit_frame = tki.Frame(parent)
        digit_frame.pack(side=tki.LEFT)

        button = [None]*MyCalculatorApp.NUM_DIGITS
        for digit in range(MyCalculatorApp.NUM_DIGITS):
            button[digit] = tki.Button(digit_frame,
                                text=str(digit),
                                command=self._digit_event_h(digit))
            button[digit].grid(row = 3-(digit+2)//3, column=(digit-1)%3)

    def _create_op_buttons(self,parent):
        separator = tki.Frame(parent, width=10)
        separator.pack(side = tki.LEFT)
        op_frame = tki.Frame(parent)
        op_frame.pack(side=tki.LEFT)

        plus_button = tki.Button(op_frame,text="+",
                                 command=self._op_event_h(lambda x, y: x+y))
        times_button = tki.Button(op_frame,text="*",
                                  command=self._op_event_h(lambda x, y: x*y))
        op_equals = lambda x, y: x if self._current_num == "" else y
        eq_button = tki.Button(op_frame, text="=",
                               command=self._op_event_h(op_equals))
        clear_button = tki.Button(op_frame, text="C",
                                  command=self._reset)

        plus_button.pack(side=tki.TOP)
        times_button.pack(side=tki.TOP)
        eq_button.pack(side=tki.TOP)
        clear_button.pack(side=tki.TOP)

    def _digit_event_h(self,digit):
        def digit_press():
            self._current_num += str(digit)
            self._display_label.configure(text = self._current_num)
        return digit_press

    def _op_event_h(self,op_func):
        def op_event():
            if self._current_num == "":
                cur_num = 0
            else:
                cur_num = int(self._current_num)
            self._prev_num = self._prev_op(self._prev_num,int(cur_num))
            self._prev_op = op_func
            self._current_num = ""
            self._display_label.configure(text=str(self._prev_num))
        return op_event

root = tki.Tk()
root.wm_title("My Simple Calculator")
MyCalculatorApp(root)
root.mainloop()