Problem with IF statements

26 Feb, 2015

Art S writes (excerpted):

First, let me say that I am having a great time learning Python from Python for Kids, even though I am just a kid at heart. I'm an old, old guy (I'll be 52 in April), but my students keep me young - I've been teaching high school mathematics for 25 years, and I am finally getting around to checking "Learn Python" off of my to-do list.

Here's the problem. All of my code has been running fine through IDLE until I got to the chapter on IF statements. When I typed into IDLE the first example from Chapter 5, page 54, nothing happened - the string 'You are too old!' (coincidence?) was not returned. All of the other examples involving IF's lead to the same result - nothing.

if-statement

I thought I would give it one more try, just to be sure. When I did, I accidentally hit enter twice, and the desired output appeared. I feel good that it worked, but why should I have to hit enter twice?

So there's nothing you're actually doing wrong here. When you hit enter after typing the second line (the print part), Python is just waiting for the next statement in that block of code. Python uses white-space for the program structure, so the only way it knows you've actually finished that particular block of code, is when you hit enter again on the next line. So Python only runs the code once you've actually said you've finished... and the way you say you're finished, in this case, is to hit enter on an empty line.

.....

Miquel C writes:

Do you know where to learn Python? I've been searching and I've only found Codecademy. I'd be very happy if you know where to go from Python For Kids. All kind of learning would be great.

I also downloaded an IRC bot, and I've translated it to my language. I've been looking the code and learning a bit.

Once you've mastered the basics of programming, my suggestion would be to find a small project to work on. Rather than looking at other people's code, or reading another, more advanced book, try creating your own version of an application you're interested in. It should be challenging, but not impossible. For example, when I was younger, I learned a new programming language by creating my own version of Tetris. You'll hit problems you don't know how to solve, so you'll end up reading the Python documentation, or googling to find the answers, or asking questions on sites like StackOverflow. But that's the one of the best ways to carry on learning (in my opinion).


Error opening IDLE on Mac OS X 10.6

10 Jan, 2015

Lucas writes (excerpted):

When I try to open IDLE as instructed on page 11 I get an error message.

Error dialog

Any suggestions?

It depends on which version of Python you're running, but if you've installed the latest (3.4.2), then instead of putting the following in Automator:

open -a "/Applications/Python 3.4/IDLE.app" --args -n

Try putting this instead:

/Library/Frameworks/Python.framework/Versions/3.4/bin/idle3.4 -n

The reason the text in the book doesn't work is a difference between Automator in versions of OS X prior to 10.7 (I believe 10.7, anyway) and later versions of the operating system.


Incrementing photo filenames

28 Dec, 2014

Emmeline writes:

I'm twelve and I recently got a raspberry Pi camera module. I really wanted to do stop animation with it but, when I used Tkinter to make a button it would take a picture when I pressed it and save it as I specified. The problem is if I wanted to use the button again it deleted the old image ex image.jpg and replaced it with a new image (image.jpg). Do you have any suggestions of how to change the code so it will name the photos numerically. Ex image1.jpg then image2.jpg. I'm doing all this in idle3. Thanks a lot for the book. "Python for kids" is great.

Glad you like the book.

There's a few ways you can change your code to have an incrementing filename. You could write a class, which uses a variable to store the current number. The class would have a function which increments the variable and returns the value. Something like this would work:

>>> class Counter:
...     def __init__(self):
...         self.counter = 0
...     def next(self):
...         self.counter += 1
...         return self.counter
...

You could use the class by creating an object like this:

>>> c = Counter()
>>> c.next()
1
>>> c.next()
2

You could then create a function to return a unique image name each time it's called:

c = Counter()

def get_image_name():
    return 'image%s.jpg' % c.next()

You could also just adapt the code above to create a class which returns unique image names:

>>> class ImageNames:
...     def __init__(self):
...         self.counter = 0
...     def next_image_name(self):
...         self.counter += 1
...         return 'image%s.jpg' % self.counter
... 
>>> im = ImageNames()
>>> im.next_image_name()
'image1.jpg'
>>> im.next_image_name()
'image2.jpg'

By the way, you can find more about embedding values in strings in Chapter 3, and classes and objects in Chapter in Chapter 8.

Hope that helps.


Well done Amazon - truly awful UX

22 Oct, 2014

How to ensure that your customers never, ever... ever want to sign up for Prime... ever:

Step 1.
Prompt user to sign up for Amazon Prime while processing their order.

Step 2.
Customer clicks "No Thanks" (because customer was previously signed up for Prime and decided they didn't want to pay for it - so of course you should ask again and again... and again)

Step 3.
Display final checkout page to customer and automatically select the paid shipping option (okay fine, that's fairly normal)

Step 4.
Display popup window to customer asking them to sign up for Amazon Prime again (did I mention again?), only this time position the Sign up link in the exact same position that that the radio button for "Free Super Saver Delivery" is displayed (because you know that's the button they're probably going to select). Better yet, you should time the display of this popup so that by the time they've moved the mouse cursor into place, customer is clicking on said radio button at the very moment the popup appears.

Step 5.
By no means should you ask for confirmation before signing the customer up, when they do mistakenly click.

Frankly, don't know whether it was just exceptionally poor, or whether it was nefarious UX design to get people on Prime, but it's a really, really good way to annoy your customers. Or at least this customer.


Multiple statements found

08 Sep, 2014

Arijjan V writes (excerpted):

When I enter code into the compiler I continue to get multiple statement error messages. Even if I copy the code from the book.

I use Idle 34 on windows professional 7. This is what I typed into the Idle Shell.

found_coins = 20
magic_coins = 70
stolen_coins = 3
coins = found_coins
for week in range(1, 53):
    coins = coins + magic_coins - stolen_coins
    print('Week %s = %s' % (week, coins))
**Click Enter**
SyntaxError: multiple statements found while compiling a single statement

I searched online but it's not an indent error. I'd appreciate any help.

To be honest, I'm not quite sure how you managed to get that error when typing the code into IDLE - the only way I can get it to happen, is if I copy-and-paste directly into the Shell:

Multiple statements showing error

The reason being is that you can only copy-and-paste line-by-line into the Shell window (a quirk of the way it works). What you should actually be entering looks like this:

Multiple statements without error

If you want to paste in a large chunk of code, then click File, then New File, then paste the code, and save the file before trying to run it:

Multiple statements in new file

If you can reproduce the problem when typing in the code, send a screenshot (take it after entering a few lines) -- I'd be interested to see if it's obvious from the screenshot what the cause is...