Restarting the game

29 Aug, 2014

Lyna M writes (excerpted):

I bought your book Python for Kids with the intention to teach my son Python later on but have now been using it myself to learn Python as I find your book to be quite comprehensive even if it was 'for kids'.

I've been doing the Bounce tutorial and would like to find a way to add a restart button so that each time the game ends, the player can restart the game if they want to.

I've applied the codes for Game Over but would like to add a restart button after that.

if ball.hit_bottom == True:
    canvas.itemconfig(game_over_text, state='normal')
    btn = Button(tk, text="Click to Restart Game")

tk = Tk()
btn = Button(tk, text="Click to Restart Game")

How to tie in the button so that it restarts the game when clicked? I've been trying out codes from forums and blogs but they don't seem to work or are too hard to understand.

There's a few ways you can handle the restart. If you look at the programming puzzles in Chapter 14, the first challenge is to delay the game start using the <Button-1> binding; to tie a mouse button click to a function which then starts the game. You could adapt that function to also restart the game once it's over. That's probably the most seamless approach.

If you really want to use a button, you need a way to add it just once -- because where you've added it (inside the loop), you'll get one button added to the window for every iteration of the loop. After the loop finishes isn't really the right place either.

First of all you'll need a Game class which will tie all the game objects together. Basically the new class would have object variables to 'store' the paddle, the ball, the score, the game-over-text, and also your restart button (initially the restart button variable would be set to None). So rather than adding the button inside the if-statement, you might do something like this:

    game = Game(canvas, paddle, ball, score, game_over_text)

    ---  ---

    while 1:

        ---  ---

        if ball.hit_bottom == True:
            canvas.itemconfig(game_over_text, state='normal')
            if game.restart_button is None:

In the above code, we check if the value of the game variable restart_button is None (in other words, the button is not currently displayed), we call a function add_restart - so that fixes the problem of adding the button more than once. Here's the code for adding the button:

class Game:

    ---  ---

    def add_restart(self):
        self.restart_button = Button(tk, text="Click to Restart Game", command=self.restart)

The command associated with the button is the function on our Game class called restart - this is the bit of code that takes care of hiding the "Game Over" text, putting the ball and paddle back in their starting positions, and so on.

Hope that helps somewhat.


28 Aug, 2014

Marta C writes:

I'm new in programming. I just want to know if it would be too hard or complicated to use Python 2.7 with the Python for Kids book? I am still practicing for a class I took with v 2.7. Can I have both IDLE 2.7 and 3 in the system and use them separately and not run into problems? Thank you.

Most of the examples in the book will work with Python 2.7 -- but some won't without modification (particularly anything using the tkinter module). And you may hit subtle problems which will be harder to figure out as a consequence. However, you can install both Python 2.7 and 3 on the same system (you didn't say, but I'm guessing you're using Windows -- after installation, you'll find you've got both c:\Python27 and c:\Python34 directories, assuming you install the latest Python version, on your hard drive) -- just make sure you select the correct directory when setting up the IDLE shortcut (described in Chapter 1).

Is The Animation Loop Necessary?

17 Aug, 2014

Christopher D writes:

I'm currently working through your book and enjoying it immensely. The following is baffling me though, so hoping you can help.

In Chapter 13, page 197 (after creating the ball object), you wrote: "If you run this program now using Run>Module, the canvas will appear for a split second and then vanish". You then go on to write about how its necessary to add the animation loop to stop this happening.

My window appears and draws the ball correctly, but does not vanish. Reading through the code I don't understand why it's expected to disappear either. This seems to be a key point so would be grateful if you could shed light! I'm running Python 3.4.1 by the way.

Thank you in advance for your help!

You've found an error in the book, I'm afraid. If you run the code using the Python console (try it by opening a command prompt, or terminal, and running python, the window will indeed appear and vanish. The basic idea is, there's nothing left for it to do, so no reason for the window to remain open. That's the reason for adding an animation loop of some kind (in the book I use a very basic while-loop, but tkinter also has a mainloop function for doing this). However, when you run the same code with Idle (the Python Shell), which is written using tkinter, the window remains open because there's already a main loop running (started by the Shell itself). So the book is both right and wrong, I guess, depending on how you want to run the code. The errata has been updated accordingly. Thanks for the "bug report".

Trouble with Automator

27 Jul, 2014

Jean-Jacques B writes:

I am trying to set up my iMac for the book, having trouble with the automator script:

open -a "Applications/Python 3.2/" --args -n

Run Shell Script failed open: unrecognized option '--args'

I can get python to launch the other ways, but then turtle doesn't work.

This had me stumped for a moment (couldn't find any similar errors when I googled).

It actually looks like a poorly handled error in the Automator. If you copy+paste what you entered into the Terminal, you get an entirely different error message:

$ open -a "Applications/Python 3.2/" --args -n
Unable to find application named 'Applications/Python 3.2/'

Which hopefully helps you figure out where you went wrong... try adding a / (slash) in front of the path to IDLE. eg.:

open -a "/Applications/Python 3.2/" --args -n

Python 3.4 Shell

15 Jun, 2014

Kirk K writes:

On page 12 it says File>New Window but I cannot find it on our Windows version 3.4.1. Please advise.

It looks like, as of Python 3.4, they've changed the menu, so File->New Window has become File->New File (which makes the Shell consistent with other text editors, at least). It still has the same behaviour though - opening a new window for typing in your Python code.

Restart when running tkinter

13 Jun, 2014

Craig S writes:

My son is having a problem running the random rectangle program on page 174 of Python for Kids. He is using version 2.7.7, so the Tkinter module must be capitalized (just so you don't suggest to capitalize it). Every time he tries to run the module, it comes up with a new window saying, "RESTART". He tried to copy and paste your code, but came up with the same results. This is the code he's written. from Tkinter import * import random tk = Tk() canvas = Canvas(tk, width=400,height=400) def random_rectangle(width, height):     ... random_rectangle(400, 400)

This is what comes up when he runs it. Python 2.7.7 (default, Jun 1 2014, 14:17:13) on win32 Type "copyright", "credits" or "license()" for more information. >>> =============== RESTART ===============

Please share some advice. Thanks.

A couple of thoughts:

  1. He's missing the pack function after the 4th line of that code (canvas.pack())
  2. You're probably not running Idle in "No Subprocess" mode (see Chapter 1 of the book for setup instructions). If that doesn't work for you, for some reason, then best thing to do is use a text editor, save the code and then run it from the command prompt by typing python (see for instructions on running the command prompt if you're not aware how to do that)
  3. You may hit other weird issues if you're not using Python3, since all the code in the book was written and tested with 3.x. Much of it is probably fine, but no guarantees there...

Which format?

12 Jun, 2014

Koki Y writes:

I have a question. You use % for format instead .format(). In the python tutorial, it is written that % format is an older version of format. What's the point of using % format?

The book uses the % operator (also known as the interpolation operator) basically through omission more than anything else. The first electronic version was written for Python2 (back in 2006/2007), then updated to be compatible with both 2 and 3. While it was almost totally rewritten, to be published as "Python for Kids", some of the existing text was repurposed -- however, the string formatting section wasn't specifically updated for Python 3. I suspect that's somehing we should probably fix when/if it's reprinted again.

By all means, use the format function as detailed in the Python documentation.

Questions and remarks on Python for Kids

19 May, 2014

Marcin S writes:

I am currently testing your book on my girlfriend who is not a kid anymore but a person that has not written any code in her entire life. To assist her better I have read the whole book. I think I spent my money well when I bought it.

Still, I have some questions and remarks.

Did you consider to emphasize more we are using Python 3 in the entire book? I know that the reader is instructed to download that version, but if someone uses Python 2 for some reason (e.g. computer science classroom with outdated software), one will get different results. In the context of your book I have found two important cases: 1) division always produces a float in Python3 because we no longer have a strict difference between integers and floats, 2) input() works like raw_input() from Python2, while input() in Python2 made implicit eval(). [1]

The errata mentions input() behaves differently than readline() depending on the context -- console or shell. I am unable to replicate it. BTW, the parameter to read()/readline() specifies the maximum number of characters to read, not the exact number. And there is one more thing -- if one reads less characters than available on stdin, remaining characters will be read on the next call to read()/readline(). [2]

Are strings enclosed in triple double-quotes omitted as reserved for documentation? I think only triple single-quotes are mentioned in the book. [3]

Why did you choose to name classes with plural names (class Giraffes) instead of singular (class Giraffe)? [4]

The "is" keyword is mentioned in the keywords section as a complicated concept and is omitted in the examples. I have one remark on that: PEP8 suggests using "is" when checking if something is None (not with "=="). [5]

I have seen range() with 3 arguments mentioned later in the book. Did you consider mentioning slices ([a:b:c]) as well? [6]

Did you consider mentioning "for" and "while" with the optional "else" block? [6]

The book mentions random.choice() but the bounce game uses shuffle() and picking by index instead. [7]

Mr. Stick Man checks for collisions using comparisons glued with "and". Did you consider constructs like "x1 < x < x2" instead of "x1 < x and x < x2"? [8]

Why did you choose a Tk construct based on sequence: update_idletasks(), update(), sleep() instead of mainloop() with rescheduling with Tk.after()? There would not be any active wait then and it would be event-driven. Is not the update_idletasks() included in update()? [9]

I've annotated your comments so I can address each in turn, but the answer to many of your points is somewhat similar:

[1] No, at the time of writing, I didn't consider more emphasis was needed, other than what is detailed in the first chapter. But as time has gone on, the differences between 2 and 3 account for a significantly large percentage of the questions I get about the book, so perhaps something additional makes sense. [2] The problem is reproducible in Python3.2 (and older versions). In 3.3 onwards, the max characters parameter works as expected. [3] No, the triple double-quotes were omitted essentially for simplification reasons. To make it less confusing for younger readers. [4] I chose plural class names mainly to try to indicate class as a kind of grouping - trying to distinguish between that and singular object instances. Yes, not particularly standard, but I had a (possibly vain) hope it would help to highlight the difference between class and object. [5] "is" was omitted again for simplification reasons -- to avoid a complicated description of how object identity works. [6] Again, slices and else blocks in for/while loops were omitted for simplification. [7] You'll find a description of the shuffle function on page 136. [8] Another one omitted to simplify things. Combining conditions with and and or is, I believe, more straightforward for kids to understand (even if x < y < z is a bit cleaner) [9] This one came up in the review from one of the technical proof readers. mainloop is definitely the better (read as correct) approach, but I wanted to stick with a loop and update/update_idletasks because it made it obvious, to the reader, what is happening. Each loop tells one of the objects to do something (move and draw), and then triggers Tk to redraw the screen (the reason for using both update and update_idletasks is that when writing the book -- with an earlier version of Python3 -- I saw some animation jittering when using only the one call. Using both seemed to resolve that issue. Perhaps I should've checked why at the time, and maybe it's no longer an problem with Python3.4)

Hope that helps clarify some of your questions...

Intermediate Python Resources

18 May, 2014

Jim G writes:

I'm currently going through your book python for kids. I was wondering what books or other sources you recommend for learning more about python

Well, there are a heap of online tutorials, and dead-tree, printed books to choose from. However, I have an alternate suggestion to further reading.

Once you've learned the basics, why not pick a project? Find something you want to create - whether that's a game, a desktop application, a website or something else - make sure you have the Python documentation bookmarked, and a search engine to hand, and start working on it. You can't beat actually coding for improving your knowledge and ability.

Let's say you decide to write a program to send a message from one computer to another, on your home network. That might lead you to the socket module in the Python modules documentation. Assuming you find that too opaque, you might search for a "python networking tutorial" or perhaps a "simple python socket example" - but either way, the investigation, and putting that information to work in a program of your own will be a much more valuable learning experience than just reading a dry chapter about networking in some resource.

Python on the iPad?

10 Mar, 2014

Beth S writes:

I teach gifted elementary-aged students in Albuquerque, NM. A few of my students and I have been working through your book. We have limited desktops in our classroom, but we also have access to several iPads. In researching the various apps available, I’ve realized that most (if not all?) do not include the _tkinter module. To be honest, I’m rather new to programming myself!

Do you have a recommendation for an iPad app that would be appropriate for students using your book?

Thank you for your time and your ability to clearly explain the basics of Python!

I'm afraid I don't know of any way to use an iPad for Python, other than by jail-breaking the device (and even then I have no idea whether tkinter would work anyway - I think unlikely).

The only thought I had was whether you could use the desktop PCs in your class to run multiple VMs (using something like VirtualBox) and then connect to those using a Remote Desktop App for the iPad. Not straightforward to set up, but it might accomplish what you want (assuming the desktops are reasonably powerful of course).

Sorry I can't be much help there.

Update: since sending my initial reply, I came across this discussion, which explains why you're not likely to see tkinter on the iPad anytime soon.