How can I add cut/copy/paste functionality to my application?

Question

I'm developing an Electoral Calculator (https://launchpad.net/electoralcalculator) using Python 2.7, Gtk+3 and Glade and I want to add cut/copy/paste functionality to my application.

I want to give the user the opportunity to cut/copy/paste text using menu items (Edit > Cut, Edit > Copy, and Edit > Paste), toolbar buttons or keyboard shortcuts (Ctrl+X, Ctrl+C, and Ctrl+V).

How can I get the selected text to cut/copy it? There are many text entry widgets and the selected text can be in any of them.

How can I know where the cursor is, so I can paste the contents of the clipboard there?

I found about this example:
http://python-gtk-3-tutorial.readthedocs.org/en/latest/clipboard.html

But while in this example there is only one text entry widget, in my application there are plenty of them.
How can I know where the selected text is (in which text entry widget) to cut/copy it?
How can I know where the cursor is for the paste functionality?

English is not my first language, please forgive my errors.

Thanks for your help

EDIT:
I wrote an example with working cut, copy, and paste buttons based on Ian's/Timo's code.
Thank you both, Timo and Ian B., for your help. I really appreciate it.

Let me know if there's something wrong in the example.

The keyboard shortcuts (Ctrl+X, Ctrl+C, and Ctrl+V) work automagically without adding any code.

from gi.repository import Gtk, Gdk

class TwotextWindow(Gtk.Window):
    __gtype_name__ = "TwotextWindow"

    def __init__(self):
        super(TwotextWindow, self).__init__()
        self.connect('delete-event', Gtk.main_quit)

        self.vbox = Gtk.VBox(False, 8)
        for x in range(4):
            self._build_entry()

        button_cut = Gtk.Button(label='Cut')
        button_cut.connect('clicked', self.on_cut_clicked)
        self.vbox.pack_start(button_cut, False, False, 0)

        button_copy = Gtk.Button(label='Copy')
        button_copy.connect('clicked', self.on_copy_clicked)
        self.vbox.pack_start(button_copy, False, False, 0)

        button_paste = Gtk.Button(label='Paste')
        button_paste.connect('clicked', self.on_paste_clicked)
        self.vbox.pack_start(button_paste, False, False, 0)

        self.add(self.vbox)
        self.show_all()

        # Code for other initialization actions should be added here.
        self.clipboard = Gtk.Clipboard.get(Gdk.SELECTION_CLIPBOARD)

    def _build_entry(self):
        entry = Gtk.Entry(text='Hello, World!')
        entry.connect('focus-in-event', self.on_entry_focus)
        self.vbox.pack_start(entry, False, False, 0)

    def on_cut_clicked(self, widget):
        # Get the bounds of the selected text
        bounds = self.focus.get_selection_bounds()

        # if the bounds of the selection are not an empty tuple,
        # put the selection in the variable chars
        # and copy it to the clipboard
        # (get_selection_bounds returns an empty tuple if there is no selection)
        # then delete the selection
        if bounds:
            chars = self.focus.get_chars(*bounds)
            print "Copying '%s' from: %s" % (chars, self.focus)
            self.clipboard.set_text(chars, -1)
            print "Deleting text selection: characters from position %s to %s" % (bounds[0], bounds[1])
            self.focus.delete_text(bounds[0], bounds[1])
        else:
            print "Can't cut if you don't select text"

    def on_copy_clicked(self, widget):
        # Get the bounds of the selected text
        bounds = self.focus.get_selection_bounds()

        # if the bounds of the selection are not an empty tuple,
        # put the selection in the variable chars
        # and copy it to the clipboard
        # (get_selection_bounds returns an empty tuple if there is no selection)
        if bounds:
            chars = self.focus.get_chars(*bounds)
            print "Copying '%s' from: %s" % (chars, self.focus)
            self.clipboard.set_text(chars, -1)
        else:
            print "Can't copy if you don't select text"

    def on_paste_clicked(self, widget):
        # Get the text from the clipboard
        text = self.clipboard.wait_for_text()

        if text != None:
            # If there's text selected in the target
            # delete it and paste the contents of the clipboard
            bounds = self.focus.get_selection_bounds()
            if bounds:
                print "Deleting text selection: characters from position %s to %s" % (bounds[0], bounds[1])
                self.focus.delete_text(bounds[0], bounds[1])
                print "Pasting '%s' into: '%s' at the position %s" % (text, self.focus, bounds[0])
                self.focus.insert_text(text, bounds[0])

            # else insert the text in the current position of the cursor in the target
            else:
                pos = self.focus.get_position()
                #print "Cursor position in the target: %s" % pos
                print "Pasting '%s' into: '%s' at the position %s" % (text, self.focus, pos)
                self.focus.insert_text(text, pos)
        else:
            print "No text on the clipboard."

    def on_entry_focus(self, widget, event):
        print "Focused:", widget
        self.focus = widget


if __name__ == '__main__':
    win = TwotextWindow()
    Gtk.main()

Answer 1

How about using an internal variable to store the last active widget? Use the entry's focus-in-event signal (when keyboard takes focus) to modify that variable with its name (can use a common callback for all text entries). Then when you need to copy or paste something, you can use that variable to know where to put it (via a getattr). Here's a little example I cooked up.

Original code edited to work standalone and solve questions

from gi.repository import Gtk, Gdk

class TwotextWindow(Gtk.Window):
    __gtype_name__ = "TwotextWindow"

    def __init__(self):
        super(TwotextWindow, self).__init__()
        self.connect('delete-event', Gtk.main_quit)

        self.vbox = Gtk.VBox(False, 8)
        for x in range(4):
            self._build_entry()

        button = Gtk.Button(label='Copy')
        button.connect('clicked', self.on_copy_clicked)
        self.vbox.pack_start(button, False, False, 0)

        self.add(self.vbox)
        self.show_all()

        # Code for other initialization actions should be added here.
        self.clipboard = Gtk.Clipboard.get(Gdk.SELECTION_CLIPBOARD)

    def _build_entry(self):
        entry = Gtk.Entry(text='Hello, World!')
        entry.connect('focus-in-event', self.on_entry_focus)
        self.vbox.pack_start(entry, False, False, 0)

    def on_copy_clicked(self, widget):
        bounds = self.focus.get_selection_bounds()
        chars = self.focus.get_chars(*bounds)
        print "Copying '%s' from: %s" % (chars, self.focus)
        #TODO: do the actual copying

    def on_entry_focus(self, widget, event):
        print "Focused:", widget
        self.focus = widget


if __name__ == '__main__':
    win = TwotextWindow()
    Gtk.main()

Don't know if there's a better way to do this. I'm pretty new to this too.