Add English exercises for part 3 of the course

23 files changed, 1086 insertions, 4 deletions

diff --git a/Makefile b/Makefile
index 2653337..8467d7f 100644
--- a/Makefile
+++ b/Makefile
@@ -19,6 +19,7 @@ EN_EXERCISES = \
 	13_extend-class \
 	14_property \
 	15_private-methods \
+	17_button-and-label \
 	$(NULL)
 ES_EXERCISES = \
 	1_imprimir-nombres \
@@ -53,6 +54,7 @@ ZIPS = \
 	python-ejercicios.zip \
 	python-exercises.zip \
 	$(NULL)
+
 EN_PYTHON_FILES = \
 	$(addprefix exercises/en/,$(addsuffix .py,$(EN_EXERCISES))) \
 	$(addprefix exercises/en/,$(addsuffix _solution.py,$(EN_EXERCISES))) \
@@ -63,6 +65,25 @@ ES_PYTHON_FILES = \
 	$(addprefix exercises/es/,$(addsuffix _solucion.py,$(ES_EXERCISES))) \
 	$(NULL)
 
+EN_EXERCISE_FILES = \
+	$(EN_PYTHON_FILES) \
+    exercises/en/16_gnome-devel-demos.txt \
+	exercises/en/17_button-and-label.py \
+	exercises/en/17_button-and-label_solution.py \
+	exercises/en/Exercise18.activity/ \
+	exercises/en/18_gtk-to-activity.txt \
+    exercises/en/Exercise19.activity/ \
+	exercises/en/19_translation.txt \
+	exercises/en/Exercise20.activity/ \
+	exercises/en/20_packaging.txt \
+	exercises/en/21_complete-activity.txt \
+	$(NULL)
+
+ES_EXERCISE_FILES = \
+	$(ES_PYTHON_FILES) \
+	exercises/es/16_gnome-devel-demos.txt \
+	$(NULL)
+
 all: $(PDFS) $(ZIPS)
 
 presentation.pdf: presentation.tex $(TEX_FILES) $(IMAGE_FILES)
@@ -74,15 +95,15 @@ handout.pdf: handout.tex $(TEX_FILES) $(IMAGE_FILES)
 	pdflatex "\providecommand\locale{$(LANG)}\input{$<}"
 
 # Zip up the exercises.
-python-exercises.zip: $(EN_PYTHON_FILES)
+python-exercises.zip: $(EN_EXERCISE_FILES)
 	zip --quiet --no-dir-entries $@ $^
 
-python-ejercicios.zip: $(ES_PYTHON_FILES)
+python-ejercicios.zip: $(ES_EXERCISE_FILES)
 	zip --quiet --no-dir-entries $@ $^
 
 check:
-	$(foreach ex,$(EN_EXERCISES),pep8 exercises/en/${ex}.py exercises/en/${ex}_solution.py;)
-	$(foreach ex,$(ES_EXERCISES),pep8 exercises/es/${ex}.py exercises/es/${ex}_solucion.py;)
+	pep8 $(EN_PYTHON_FILES)
+	pep8 $(ES_PYTHON_FILES)
 
 clean:
 	rm -f $(PDFS:.pdf=.aux)
diff --git a/exercises/en/16_gnome-devel-demos.txt b/exercises/en/16_gnome-devel-demos.txt
new file mode 100644
index 0000000..a1c013e
--- /dev/null
+++ b/exercises/en/16_gnome-devel-demos.txt
@@ -0,0 +1,28 @@
+Exercise 16 is not assessed. Instead, you should go to:
+	https://developer.gnome.org/gnome-devel-demos/unstable/tutorial.py.html.en
+and work through the following examples. For each example, copy the code into
+a file and run it using Python. Read the documentation on the web pages and
+ensure you understand how the example works.
+
+You should look at the following tutorial pages, at a minimum:
+ • 1.1
+ • 2.1
+ • 2.2
+ • 2.3
+ • 3.1
+ • 4.1
+ • 4.2
+ • 4.3
+ • 4.4
+ • 5.1
+ • 5.2
+ • 5.4
+ • 5.5
+ • 5.7
+ • 7.3
+ • 8.2
+ • 8.3
+ • 9.1
+ • 11.3
+
+Note that tutorial page 14.1 is covered separately in a later exercise.
diff --git a/exercises/en/17_button-and-label.py b/exercises/en/17_button-and-label.py
new file mode 100644
index 0000000..2eee908
--- /dev/null
+++ b/exercises/en/17_button-and-label.py
@@ -0,0 +1,64 @@
+
+#!/usr/bin/python
+# coding=utf-8
+
+"""This program should display a GTK+ window containing a button and label.
+
+You must add two widgets to the box in the window: a Gtk.Button and a
+Gtk.Label.
+
+The Gtk.Button must show the text 'Change Label'. The Gtk.Label must show the
+text 'Initial label.'.
+
+After you have added the two widgets, you must connect a callback to the
+button's "clicked" signal. In this callback, you must change the Gtk.Label's
+text to 'Final label.'. This means that when the program is run and the button
+is clicked, the label should change its text.
+
+You can find documentation for the Gtk.Button "clicked" signal here:
+    https://developer.gnome.org/gtk3/stable/GtkButton.html#GtkButton-clicked
+
+Don't forget to document any new methods you add.
+
+Hint: Don't forget to call the show() method on the button and label to make
+them visible after adding them to the box.
+"""
+
+from gi.repository import Gtk
+
+
+class SimpleWindow(Gtk.Window):
+    """A simple window which displays a label and a button."""
+    def __init__(self):
+        super(SimpleWindow, self).__init__(title='Simple Window')
+
+        # Set a default size for the window.
+        self.set_default_size(200, 200)
+
+        # Set up a close handler for the window. Do not modify this.
+        self.connect('destroy', self.__window_destroy_cb)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.add(box)
+
+        # You must add code here to create and set up the button and label.
+        # Don't forget to make them visible and add them to the box.
+
+    # You must add a signal callback here to handle the button's "clicked"
+    # signal and change the label's text.
+
+    def __window_destroy_cb(self, window):
+        """Callback to quit the program when the window is closed.
+
+        Do not modify this."""
+        Gtk.main_quit()
+
+
+# Run the program. Do not modify this.
+SimpleWindow().show()
+Gtk.main()
diff --git a/exercises/en/17_button-and-label_solution.py b/exercises/en/17_button-and-label_solution.py
new file mode 100644
index 0000000..30d644b
--- /dev/null
+++ b/exercises/en/17_button-and-label_solution.py
@@ -0,0 +1,77 @@
+#!/usr/bin/python
+# coding=utf-8
+
+"""This program should display a GTK+ window containing a button and label.
+
+You must add two widgets to the box in the window: a Gtk.Button and a
+Gtk.Label.
+
+The Gtk.Button must show the text 'Change Label'. The Gtk.Label must show the
+text 'Initial label.'.
+
+After you have added the two widgets, you must connect a callback to the
+button's "clicked" signal. In this callback, you must change the Gtk.Label's
+text to 'Final label.'. This means that when the program is run and the button
+is clicked, the label should change its text.
+
+You can find documentation for the Gtk.Button "clicked" signal here:
+    https://developer.gnome.org/gtk3/stable/GtkButton.html#GtkButton-clicked
+
+Don't forget to document any new methods you add.
+
+Hint: Don't forget to call the show() method on the button and label to make
+them visible after adding them to the box.
+"""
+
+from gi.repository import Gtk
+
+
+class SimpleWindow(Gtk.Window):
+    """A simple window which displays a label and a button."""
+    def __init__(self):
+        super(SimpleWindow, self).__init__(title='Simple Window')
+
+        # Set a default size for the window.
+        self.set_default_size(200, 200)
+
+        # Set up a close handler for the window. Do not modify this.
+        self.connect('destroy', self.__window_destroy_cb)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.add(box)
+
+        # Create and set up the label and button.
+        label = Gtk.Label('Initial label.')
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button('Change Label')
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback.
+        self._label = label
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self._label.set_text('Final label.')
+
+    def __window_destroy_cb(self, window):
+        """Callback to quit the program when the window is closed.
+
+        Do not modify this."""
+        Gtk.main_quit()
+
+
+# Run the program. Do not modify this.
+SimpleWindow().show()
+Gtk.main()
diff --git a/exercises/en/18_gtk-to-activity.txt b/exercises/en/18_gtk-to-activity.txt
new file mode 100644
index 0000000..8836099
--- /dev/null
+++ b/exercises/en/18_gtk-to-activity.txt
@@ -0,0 +1,3 @@
+The instructions for exercise 18 are in Exercise18.activity/exercise18.py,
+which is the only file you need to modify.
+The example solution is in Exercise18.activity/exercise18_solution.py.
diff --git a/exercises/en/19_translation.txt b/exercises/en/19_translation.txt
new file mode 100644
index 0000000..520f31a
--- /dev/null
+++ b/exercises/en/19_translation.txt
@@ -0,0 +1,3 @@
+The instructions for exercise 19 are in Exercise19.activity/exercise19.py.
+The example solution is in Exercise19.activity/exercise19_solution.py. No
+example solution files are provided for the POT and PO files.
diff --git a/exercises/en/20_packaging.txt b/exercises/en/20_packaging.txt
new file mode 100644
index 0000000..c43ffeb
--- /dev/null
+++ b/exercises/en/20_packaging.txt
@@ -0,0 +1,2 @@
+The instructions for exercise 20 are in Exercise20.activity/exercise20.py. There
+is no example solution, but there are detailed instructions to follow.
diff --git a/exercises/en/21_complete-activity.txt b/exercises/en/21_complete-activity.txt
new file mode 100644
index 0000000..325275a
--- /dev/null
+++ b/exercises/en/21_complete-activity.txt
@@ -0,0 +1,27 @@
+Exercise 20
+
+For this final exercise, you must create a new Sugar activity from scratch. You
+have two options of activities to create:
+ A. An ‘odd one out’ game. It should display four buttons on the screen, each
+    with a conjugation of a verb as its text. The four conjugations should be
+    different, and one should be incorrect. The user must click on the button
+    with the incorrect conjugation. If they do, their score is increased and
+    another set of four conjugations is displayed. If they click on the wrong
+    button, their score is not incremented and the conjugations do not change.
+    The user must click the odd one out to proceed.
+    The user’s score should be displayed in a label in the toolbar (ask if you
+    need help with this, or look at the code in PascalTriangle which does a
+    similar thing by putting a GtkScale in its toolbar).
+ B. Similar to the above game, but using mathematical equalities instead of
+    verb conjugations. The game should present the user with four equalities
+    (e.g. “5 × 6 + 3 = 33”, “(1 + 4) × 2 = 10”, etc.), one of which should be
+    incorrect. The user must pick the incorrect one to proceed.
+
+Once you’ve created the activity, tested it in emulation and tested it on a
+physical XO, you should get someone else to review all of your code and see if
+they can find any problems with it. Code review like this is always helpful, as
+every programmer makes mistakes (no matter how experienced they are) and a
+fresh pair of eyes can spot them. Ideally, your code should be reviewed by
+someone who wrote the other type of activity to you, so they have to rely on
+comments in your code to work out how the code functions. Remember to comment
+your code well!
diff --git a/exercises/en/Exercise18.activity/activity/activity.info b/exercises/en/Exercise18.activity/activity/activity.info
new file mode 100644
index 0000000..e0ffeda
--- /dev/null
+++ b/exercises/en/Exercise18.activity/activity/activity.info
@@ -0,0 +1,9 @@
+[Activity]
+name = Exercise 18
+summary = Python exercise 18. Not a real activity.
+activity_version = 1
+host_version = 1
+bundle_id = hn.educatrachos.exercise18
+icon = activity
+exec = sugar-activity exercise18.Exercise18Activity
+license = GPLv2+
diff --git a/exercises/en/Exercise18.activity/activity/activity.svg b/exercises/en/Exercise18.activity/activity/activity.svg
new file mode 100644
index 0000000..580b0e6
--- /dev/null
+++ b/exercises/en/Exercise18.activity/activity/activity.svg
@@ -0,0 +1,163 @@
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+
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diff --git a/exercises/en/Exercise18.activity/exercise18.py b/exercises/en/Exercise18.activity/exercise18.py
new file mode 100644
index 0000000..753eb4a
--- /dev/null
+++ b/exercises/en/Exercise18.activity/exercise18.py
@@ -0,0 +1,99 @@
+#!/usr/bin/python
+# coding=utf-8
+
+"""Convert a simple GTK+ program to be a Sugar activity.
+
+You must convert the simple GTK+ program from exercise 17 to be a Sugar
+activity.
+
+All the other files for the activity (activity.info, setup.py, etc.) have been
+provided already; you must simply modify the code in this file to derive the
+class from sugar3.activity.activity.Activity instead of from Gtk.Window, and
+make a few other changes.
+
+ 1. Remove the “#!/usr/bin/python” line from the top of the file.
+    The first line of the file should now be “# coding=utf-8”.
+ 2. Rename the class to “Exercise18Activity”.
+ 3. Add a ‘handle’ parameter to __init__ and propagate it to the parent
+    constructor in the super() call.
+ 4. Remove the window title from the constructor super() call.
+ 5. Remove the call to set_default_size() and the window destroy callback.
+ 6. Add an activity toolbar (ToolbarBox), and add an activity button
+    (ActivityToolbarButton) and a stop button (StopButton) to it.
+ 7. Change the self.add() call to self.set_canvas().
+ 8. In a terminal, change to this directory and run ‘./setup.py dev’ to add the
+    activity to Sugar on your computer.
+ 9. In a terminal, run ‘sugar-emulator’ and test the activity.
+
+Note that the necessary ‘import’ statements have already been added to the
+file.
+
+Don't forget to document any new methods you add.
+
+Hint: The code necessary for step 6 is:
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+from sugar3.graphics.toolbutton import ToolButton
+
+
+class SimpleWindow(Gtk.Window):
+    """A simple window which displays a label and a button."""
+    def __init__(self):
+        super(SimpleWindow, self).__init__(title='Simple Window')
+
+        # Set a default size for the window.
+        self.set_default_size(200, 200)
+
+        # Set up a close handler for the window. Do not modify this.
+        self.connect('destroy', self.__window_destroy_cb)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.add(box)
+
+        # Create and set up the label and button.
+        label = Gtk.Label('Initial label.')
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button('Change Label')
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback.
+        self._label = label
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self._label.set_text('Final label.')
+
+    def __window_destroy_cb(self, window):
+        """Callback to quit the program when the window is closed.
+
+        Do not modify this."""
+        Gtk.main_quit()
diff --git a/exercises/en/Exercise18.activity/exercise18_solution.py b/exercises/en/Exercise18.activity/exercise18_solution.py
new file mode 100644
index 0000000..da880e8
--- /dev/null
+++ b/exercises/en/Exercise18.activity/exercise18_solution.py
@@ -0,0 +1,101 @@
+# coding=utf-8
+
+"""Convert a simple GTK+ program to be a Sugar activity.
+
+You must convert the simple GTK+ program from exercise 17 to be a Sugar
+activity.
+
+All the other files for the activity (activity.info, setup.py, etc.) have been
+provided already; you must simply modify the code in this file to derive the
+class from sugar3.activity.activity.Activity instead of from Gtk.Window, and
+make a few other changes.
+
+ 1. Remove the “#!/usr/bin/python” line from the top of the file.
+    The first line of the file should now be “# coding=utf-8”.
+ 2. Rename the class to “Exercise18Activity”.
+ 3. Add a ‘handle’ parameter to __init__ and propagate it to the parent
+    constructor in the super() call.
+ 4. Remove the window title from the constructor super() call.
+ 5. Remove the call to set_default_size() and the window destroy callback.
+ 6. Add an activity toolbar (ToolbarBox), and add an activity button
+    (ActivityToolbarButton) and a stop button (StopButton) to it.
+ 7. Change the self.add() call to self.set_canvas().
+ 8. In a terminal, change to this directory and run ‘./setup.py dev’ to add the
+    activity to Sugar on your computer.
+ 9. In a terminal, run ‘sugar-emulator’ and test the activity.
+
+Note that the necessary ‘import’ statements have already been added to the
+file.
+
+Don't forget to document any new methods you add.
+
+Hint: The code necessary for step 6 is:
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+from sugar3.graphics.toolbutton import ToolButton
+
+
+class Exercise18Activity(activity.Activity):
+    """A simple window which displays a label and a button."""
+    def __init__(self, handle):
+        super(Exercise18Activity, self).__init__(handle)
+
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.set_canvas(box)
+
+        # Create and set up the label and button.
+        label = Gtk.Label('Initial label.')
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button('Change Label')
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback.
+        self._label = label
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self._label.set_text('Final label.')
diff --git a/exercises/en/Exercise18.activity/setup.py b/exercises/en/Exercise18.activity/setup.py
new file mode 100755
index 0000000..77fda74
--- /dev/null
+++ b/exercises/en/Exercise18.activity/setup.py
@@ -0,0 +1,4 @@
+#!/usr/bin/env python
+from sugar.activity import bundlebuilder
+if __name__ == "__main__":
+    bundlebuilder.start()
diff --git a/exercises/en/Exercise19.activity/activity/activity.info b/exercises/en/Exercise19.activity/activity/activity.info
new file mode 100644
index 0000000..3a0739c
--- /dev/null
+++ b/exercises/en/Exercise19.activity/activity/activity.info
@@ -0,0 +1,9 @@
+[Activity]
+name = Exercise 19
+summary = Python exercise 19. Not a real activity.
+activity_version = 1
+host_version = 1
+bundle_id = hn.educatrachos.exercise19
+icon = activity
+exec = sugar-activity exercise19.Exercise19Activity
+license = GPLv2+
diff --git a/exercises/en/Exercise19.activity/activity/activity.svg b/exercises/en/Exercise19.activity/activity/activity.svg
new file mode 100644
index 0000000..580b0e6
--- /dev/null
+++ b/exercises/en/Exercise19.activity/activity/activity.svg
@@ -0,0 +1,163 @@
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+
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+   xmlns:svg="http://www.w3.org/2000/svg"
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diff --git a/exercises/en/Exercise19.activity/exercise19.py b/exercises/en/Exercise19.activity/exercise19.py
new file mode 100644
index 0000000..d912d00
--- /dev/null
+++ b/exercises/en/Exercise19.activity/exercise19.py
@@ -0,0 +1,106 @@
+# coding=utf-8
+
+"""Add internationalisation support to a Sugar activity.
+
+Below is a simple Sugar activity which displays a different string every time
+the button is pressed. You must modify it to add internationalisation support
+so that its UI can be viewed in a different language.
+
+All the other files for the activity (activity.info, setup.py, etc.) have been
+provided already; you must modify the code in this file to add
+internationalisation support, generate a POT file, and then translate it to
+Spanish.
+
+Note that the original strings in the program are always in English, by
+convention.
+
+ 1. Add an ‘import’ statement for the gettext module:
+        from gettext import gettext as _
+ 2. Mark all the relevant strings in the program as translatable using the _()
+    function.
+ 3. Generate a POT file by executing ‘./setup.py genpot’ in a terminal (after
+    changing to this directory).
+ 4. Copy the generated po/Exercise19.pot file to po/es.po.
+ 5. Either open es.po in a text editor, or use a translation tool like
+    gtranslator, and translate the strings to Spanish.
+ 6. Run ‘./setup.py dev’ to add the activity to Sugar on your computer.
+ 7. Run ‘./setup.py build’ to compile the .po files into machine-readable .mo
+    files in the ‘locale’ directory.
+ 7. Run ‘LANG=es sugar-emulator’ to run the Sugar emulator in Spanish and test
+    the new translation. The whole activity should appear in Spanish.
+
+Note: It is not possible to use Pootle for this exercise because it’s an online
+service and would require the exercise’s code to be uploaded to the internet.
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+
+
+class Exercise19Activity(activity.Activity):
+    """A simple window which displays a label and a button."""
+    def __init__(self, handle):
+        super(Exercise19Activity, self).__init__(handle)
+
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.set_canvas(box)
+
+        # Set up a list of text strings to display in the label.
+        self._strings = (
+            'Initial string.',
+            'Second string.',
+            'Handle object is ‘%s’.' % handle,
+            'Another string.',
+        )
+        self._next_string = 0
+
+        # Create and set up the label and button.
+        label = Gtk.Label()
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button('Change Label')
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback, and then set the initial text for the label.
+        self._label = label
+        self.__rotate_label_text()
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self.__rotate_label_text()
+
+    def __rotate_label_text(self):
+        """Change the label to display the next string in the rotation.
+
+        The modulus operation is used to wrap self._next_string around when
+        it reaches the final index in the self._strings array.
+        """
+        self._label.set_text(self._strings[self._next_string])
+        self._next_string = (self._next_string + 1) % len(self._strings)
diff --git a/exercises/en/Exercise19.activity/exercise19_solution.py b/exercises/en/Exercise19.activity/exercise19_solution.py
new file mode 100644
index 0000000..4818446
--- /dev/null
+++ b/exercises/en/Exercise19.activity/exercise19_solution.py
@@ -0,0 +1,107 @@
+# coding=utf-8
+
+"""Add internationalisation support to a Sugar activity.
+
+Below is a simple Sugar activity which displays a different string every time
+the button is pressed. You must modify it to add internationalisation support
+so that its UI can be viewed in a different language.
+
+All the other files for the activity (activity.info, setup.py, etc.) have been
+provided already; you must modify the code in this file to add
+internationalisation support, generate a POT file, and then translate it to
+Spanish.
+
+Note that the original strings in the program are always in English, by
+convention.
+
+ 1. Add an ‘import’ statement for the gettext module:
+        from gettext import gettext as _
+ 2. Mark all the relevant strings in the program as translatable using the _()
+    function.
+ 3. Generate a POT file by executing ‘./setup.py genpot’ in a terminal (after
+    changing to this directory).
+ 4. Copy the generated po/Exercise19.pot file to po/es.po.
+ 5. Either open es.po in a text editor, or use a translation tool like
+    gtranslator, and translate the strings to Spanish.
+ 6. Run ‘./setup.py dev’ to add the activity to Sugar on your computer.
+ 7. Run ‘./setup.py build’ to compile the .po files into machine-readable .mo
+    files in the ‘locale’ directory.
+ 7. Run ‘LANG=es sugar-emulator’ to run the Sugar emulator in Spanish and test
+    the new translation. The whole activity should appear in Spanish.
+
+Note: It is not possible to use Pootle for this exercise because it’s an online
+service and would require the exercise’s code to be uploaded to the internet.
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+from gettext import gettext as _
+
+
+class Exercise19Activity(activity.Activity):
+    """A simple window which displays a label and a button."""
+    def __init__(self, handle):
+        super(Exercise19Activity, self).__init__(handle)
+
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.set_canvas(box)
+
+        # Set up a list of text strings to display in the label.
+        self._strings = (
+            _('Initial string.'),
+            _('Second string.'),
+            _('Handle object is ‘%s’.') % handle,
+            _('Another string.'),
+        )
+        self._next_string = 0
+
+        # Create and set up the label and button.
+        label = Gtk.Label()
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button(_('Change Label'))
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback, and then set the initial text for the label.
+        self._label = label
+        self.__rotate_label_text()
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self.__rotate_label_text()
+
+    def __rotate_label_text(self):
+        """Change the label to display the next string in the rotation.
+
+        The modulus operation is used to wrap self._next_string around when
+        it reaches the final index in the self._strings array.
+        """
+        self._label.set_text(self._strings[self._next_string])
+        self._next_string = (self._next_string + 1) % len(self._strings)
diff --git a/exercises/en/Exercise19.activity/setup.py b/exercises/en/Exercise19.activity/setup.py
new file mode 100755
index 0000000..77fda74
--- /dev/null
+++ b/exercises/en/Exercise19.activity/setup.py
@@ -0,0 +1,4 @@
+#!/usr/bin/env python
+from sugar.activity import bundlebuilder
+if __name__ == "__main__":
+    bundlebuilder.start()
diff --git a/exercises/en/Exercise20.activity/.gitignore b/exercises/en/Exercise20.activity/.gitignore
new file mode 100644
index 0000000..6a592c9
--- /dev/null
+++ b/exercises/en/Exercise20.activity/.gitignore
@@ -0,0 +1,3 @@
+*.pyc
+dist/
+locale/
diff --git a/exercises/en/Exercise20.activity/activity/activity.info b/exercises/en/Exercise20.activity/activity/activity.info
new file mode 100644
index 0000000..d258ee6
--- /dev/null
+++ b/exercises/en/Exercise20.activity/activity/activity.info
@@ -0,0 +1,9 @@
+[Activity]
+name = Exercise 20
+summary = Python exercise 20. Not a real activity.
+activity_version = 1
+host_version = 1
+bundle_id = hn.educatrachos.exercise20
+icon = activity
+exec = sugar-activity exercise20.Exercise20Activity
+license = GPLv2+
diff --git a/exercises/en/Exercise20.activity/exercise20.py b/exercises/en/Exercise20.activity/exercise20.py
new file mode 100644
index 0000000..766f453
--- /dev/null
+++ b/exercises/en/Exercise20.activity/exercise20.py
@@ -0,0 +1,76 @@
+# coding=utf-8
+
+"""Package a Sugar activity to produce a bundle.
+
+Below is a simple Sugar activity which displays a static image. It is very
+boring, but does not need changing. In this exercise, you must add the relevant
+files to git and then build a bundle for the activity and install it on a
+physical XO laptop.
+
+Before packaging the activity, an icon needs to be created.
+
+ 1. Open Inkscape and create a new 48×48 pixel icon (‘icon_48x48’ template).
+ 2. Draw the icon, using only a single stroke colour and a single background
+    colour. Save it as activity/activity.svg.
+ 3. Open the created SVG file with a text editor and replace the stroke colours
+    as described in the ‘Create An Icon’ here:
+        http://en.flossmanuals.net/make-your-own-sugar-activities/package-the-activity/
+ 4. Execute the following commands in a terminal open in the
+    Exercise20.activity/ directory. They will add the relevant files to git:
+        git init
+        git status
+        git add .gitignore exercise20.py setup.py images/logonew2.png
+        git add activity/activity.info
+        git add activity/activity.svg
+        git status
+        git commit
+    They create a new git repository in the current directory, check its
+    status, add the required files to the repository, check its status again
+    to ensure we don’t commit anything we don’t mean to, then commit the new
+    files and changes to the repository.
+
+    The .gitignore file is a special file which lists files that git shouldn’t
+    allow you to add to the repository, and which will be hidden from the
+    output of `git status`.
+ 5. Run ‘./setup.py dist_xo’ to build an XO bundle.
+ 6. Copy the dist/Exercise20-1.xo file to a USB stick and install it on a
+    physical XO laptop.
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+import os
+
+
+class Exercise20Activity(activity.Activity):
+    """A simple window which displays a static image."""
+    def __init__(self, handle):
+        super(Exercise20Activity, self).__init__(handle)
+
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+
+        # Add an image to the window.
+        box = Gtk.Box()
+        box.show()
+        self.set_canvas(box)
+
+        image = Gtk.Image()
+        filename = os.path.join(os.path.dirname(__file__),
+                                'images', 'logonew2.png')
+        image.set_from_file(filename)
+        image.show()
+        box.pack_start(image, True, True, 8)
diff --git a/exercises/en/Exercise20.activity/images/logonew2.png b/exercises/en/Exercise20.activity/images/logonew2.png
new file mode 100644
index 0000000..d523b0b
--- /dev/null
+++ b/exercises/en/Exercise20.activity/images/logonew2.png
diff --git a/exercises/en/Exercise20.activity/setup.py b/exercises/en/Exercise20.activity/setup.py
new file mode 100755
index 0000000..77fda74
--- /dev/null
+++ b/exercises/en/Exercise20.activity/setup.py
@@ -0,0 +1,4 @@
+#!/usr/bin/env python
+from sugar.activity import bundlebuilder
+if __name__ == "__main__":
+    bundlebuilder.start()