from gi.repository import Gtk

import sys

class MyWindow(Gtk.ApplicationWindow):

    def __init__(self, app):

        Gtk.Window.__init__(self, title="Calculator", application=app)

        self.set_default_size(350, 200)

        self.set_border_width(10)

        # an entry

        self.entry = Gtk.Entry()

        # with an initial text

        self.entry.set_text('0')

        # text aligned on the right

        self.entry.set_alignment(1)

        # the text in the entry cannot be modified writing in it

        self.entry.set_can_focus(False)

        # a grid

        grid = Gtk.Grid()

        grid.set_row_spacing(5)

        # to attach the entry

        grid.attach(self.entry, 0, 0, 1, 1)

        # the labels for the buttons

        buttons = [7, 8, 9, '/',

                   4, 5, 6, '*',

                   1, 2, 3, '-',

                   'C', 0, '=', '+']

        # each row is a ButtonBox, attached to the grid

        for i in range(4):

            hbox = Gtk.ButtonBox.new(Gtk.Orientation.HORIZONTAL)

            hbox.set_spacing(5)

            grid.attach(hbox, 0, i + 1, 1, 1)

            # each ButtonBox has 4 buttons, connected to the callback function

            for j in range(4):

                button = Gtk.Button(label=buttons[i * 4 + j])

                button.set_can_focus(False)

                button.connect("clicked", self.button_clicked)

                hbox.add(button)

        # some variables for the calculations

        self.first_number = 0

        self.second_number = 0

        self.counter = 0

        self.operation = ""

        # add the grid to the window

        self.add(grid)

    # callback function for all the buttons

    def button_clicked(self, button):

        # for the operations

        if button.get_label() == '+':

            self.counter += 1

            if self.counter > 1:

                self.do_operation()

            self.entry.set_text('0')

            self.operation = "plus"

        elif button.get_label() == '-':

            self.counter += 1

            if self.counter > 1:

                self.do_operation()

            self.entry.set_text('0')

            self.operation = "minus"

        elif button.get_label() == '*':

            self.counter += 1

            if self.counter > 1:

                self.do_operation()

            self.entry.set_text('0')

            self.operation = "multiplication"

        elif button.get_label() == '/':

            self.counter += 1

            if self.counter > 1:

                self.do_operation()

            self.entry.set_text('0')

            self.operation = "division"

        # for =

        elif button.get_label() == '=':

            self.do_operation()

            self.entry.set_text(str(self.first_number))

            self.counter = 1

        # for Cancel

        elif button.get_label() == 'C':

            self.first_number = 0

            self.second_number = 0

            self.counter = 0

            self.entry.set_text('')

            self.operation = ""

        # for a digit button

        else:

            new_digit = int(button.get_label())

            if self.entry.get_text() == 'error':

                number = 0

            else:

                number = int(self.entry.get_text())

            number = number * 10 + new_digit

            if self.counter == 0:

                self.first_number = number

            else:

                self.second_number = number

            self.entry.set_text(str(number))

    def do_operation(self):

        if self.operation == "plus":

            self.first_number += self.second_number

        elif self.operation == "minus":

            self.first_number -= self.second_number

        elif self.operation == "multiplication":

            self.first_number *= self.second_number

        elif self.operation == "division":

            try:

                self.first_number /= self.second_number

            except ZeroDivisionError:

                self.first_number = 0

                self.second_number = 0

                self.counter = 0

                self.entry.set_text('error')

                self.operation = ""

                return

        else:

            self.first_number = 0

            self.second_number = 0

            self.counter = 0

            self.entry.set_text('error')

class MyApplication(Gtk.Application):

    def __init__(self):

        Gtk.Application.__init__(self)

    def do_activate(self):

        win = MyWindow(self)

        win.show_all()

    def do_startup(self):

        Gtk.Application.do_startup(self)

app = MyApplication()

exit_status = app.run(sys.argv)

sys.exit(exit_status)