This is my first ever *complete*, simple, amateurish, kinda-useless, desktop application built in Gtk toolkit

calculator.py

from gi.repository import Gtk


class MainWindow(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self)

        self.connect("delete-event", Gtk.main_quit)
        self.set_size_request(240, 240)
        self.set_resizable(False)

        header = Gtk.HeaderBar()
        header.props.title = "Calculator"
        header.props.show_close_button = True
        self.set_titlebar(header)

        main = Gtk.Grid()
        self.add(main)

        top_box = Gtk.Box()
        main.attach(top_box, 1, 0, 1, 1)

        self.entry = Gtk.Entry()
        self.entry.set_icon_from_stock(Gtk.EntryIconPosition.SECONDARY,
                                       Gtk.STOCK_CLOSE)
        self.entry.set_icon_activatable(Gtk.EntryIconPosition.SECONDARY, True)
        self.entry.set_size_request(240, 50)
        self.entry.connect("icon-press", self.on_clear_icon_click)
        self.entry.set_icon_tooltip_text(Gtk.EntryIconPosition.SECONDARY,
                                         "Click to clear the text box")
        self.entry.set_icon_sensitive(Gtk.EntryIconPosition.SECONDARY,
                                      True)
        top_box.add(self.entry)

        number_grid = Gtk.Grid()
        number_grid.set_column_spacing(2)
        number_grid.set_row_spacing(2)
        main.attach(number_grid, 1, 1, 1, 1)
        self.add_numbers(number_grid)

        tools_grid = Gtk.Grid()
        tools_grid.set_column_spacing(2)
        tools_grid.set_row_spacing(2)
        main.attach(tools_grid, 1, 3, 1, 1)
        self.add_tools(tools_grid)

    def add_numbers(self, container):
        row_cnt = 1
        col_cnt = 0
        for i in xrange(10):
            button = Gtk.Button(str(i))
            button.set_size_request(47, 30)
            button.connect("clicked", self.on_button_click)
            if(i > 4):
                container.attach(button, col_cnt, row_cnt+1, 1, 1)
                col_cnt += 1
            else:
                container.attach(button, i, row_cnt, 1, 1)

    def add_tools(self, container):
        plus = Gtk.Button("+")
        plus.connect("clicked", self.on_button_click)
        plus.set_size_request(47, 30)
        container.attach(plus, 0, 0, 1, 1)

        sub = Gtk.Button("-")
        sub.connect("clicked", self.on_button_click)
        sub.set_size_request(47, 30)
        container.attach(sub, 1, 0, 1, 1)

        div = Gtk.Button("/")
        div.connect("clicked", self.on_button_click)
        div.set_size_request(47, 30)
        container.attach(div, 2, 0, 1, 1)

        mul = Gtk.Button("*")
        mul.connect("clicked", self.on_button_click)
        mul.set_size_request(47, 30)
        container.attach(mul, 3, 0, 1, 1)

        evaluate = Gtk.Button("=")
        evaluate.connect("clicked", self.on_eval_click)
        evaluate.set_size_request(47, 30)
        container.attach(evaluate, 4, 0, 1, 1)

        open_bracket = Gtk.Button("(")
        open_bracket.connect("clicked", self.on_button_click)
        open_bracket.set_size_request(47, 30)
        container.attach(open_bracket, 0, 1, 1, 1)

        close_bracket = Gtk.Button(")")
        close_bracket.connect("clicked", self.on_button_click)
        close_bracket.set_size_request(47, 30)
        container.attach(close_bracket, 1, 1, 1, 1)

        point = Gtk.Button(".")
        point.connect("clicked", self.on_button_click)
        point.set_size_request(47, 30)
        container.attach(point, 2, 1, 1, 1)

        image = Gtk.Image()
        image.set_from_stock(Gtk.STOCK_UNDO, True)
        back_space = Gtk.Button()
        back_space.connect("clicked", self.on_clear_click)
        back_space.set_size_request(47, 30)
        back_space.set_image(image)
        container.attach(back_space, 3, 1, 1, 1)

    def on_clear_click(self, button):
        entry_text = self.entry.get_text()
        self.entry.set_text(entry_text[0:len(entry_text)-1])

    def on_button_click(self, button):
        entry_text = self.entry.get_text()
        entry_text += button.get_label()
        self.entry.set_text(entry_text)

    def on_eval_click(self, button):
        try:
            self.entry.set_text(str(eval(self.entry.get_text())))
        except:
            self.entry.set_text('')

    def on_clear_icon_click(self, widget, icon_pos, event):
        self.entry.set_text('')

main = MainWindow()
main.show_all()
Gtk.main()