Nuts and Volts - October 2013

“ backwards-compatible.” In other words, some of the changes in Python 3 caused programs written in earlier versions of Python to produce syntax errors. As a simple example, if we wanted to display “Hello World!” on the RPi monitor (which we will shortly do), in Python 3 we would write print (“Hello World!”); in all earlier versions of Python, we would write print “Hello World!” Understandably, many Python programmers were somewhat upset by this situation. However, others welcomed the changes because they make Python more consistent and more readable. (Search for “Python 2 vs. Python 3” if you’re interested in the details of the debate.)

Version 2

October 2013 13

SHARPENING YOUR TOOLS OF CREATIVITY

for index = 1 to 20
 if index // 4 = 0 then
 high LED
 else
 low LED
 endif
 wait 1
next index

Both code snippets implement the same task — which could also be accomplished with simpler code — but that’s not the point. Rather, the question is which snippet is easier to read and understand? (If you chose Version 2, you’re absolutely correct!)

If you’re a regular Primer reader, you know that I often encourage readers to email me with any PICAXE-related questions or problems they may be having. I frequently receive software questions accompanied by a program listing.

Of course, we have to make a choice at this point: Are we going to use Python 2 or Python 3 for our Sometimes (fortunately not often) the listings look a lot like Version 1 above. Before I can even begin to try to figure out what’s going on, I have to edit the program so that it’s properly formatted and more easily readable.

PICAXE-Pi experiments? I have to admit, I’m having a little trouble deciding!

The point of all this is that I’ve switched back to using Python 3. 2. Naturally, I can’t rule out the possibility that I won’t run into additional problems with Python 3. Fortunately, however, Python 2. 7 and Python 3. 2 are both pre-installed in the latest version of the RPi’s operating system, so switching between the two versions won’t be a problem.

Experimenting With Python 3. 2

With Python programs, I won’t need to do that because Python forces us to correctly format all our programs. That’s why I love this feature! (There’s another major benefit to formatting a Python program; we’ll discuss that one when we get to our “Hello World!” stuff.)

When I first started experimenting with Python, I chose to use version 3. 2 because I knew the Python community was already moving in that direction, and I didn’t want to end up trying to learn both versions, probably getting confused in the process. At that point, I intended to use I2C to communicate between the PICAXE and RPi, but I

quickly discovered that an I2C library was not yet available in Python 3. 2, so I switched to using Python 2. 7.

After experimenting with I2C for a few days, I realized that it’s fairly complicated to implement reliable two-way communications between the two processors. It turns out that good old-fashioned serial communication looks like a better choice. (We’ll get into those details in a future column.)

Python 2 vs.

Python 3

Python has been around for almost 20 years, and it has evolved into a very popular and easy to use language. However, when Python 3.0 was introduced in 2008, it created quite a stir in the Python community because it was the first version that wasn’t

■ FIGURE 1. The IDLE3 Python shell.

When we program in PICAXE BASIC, we use either the

Programming Editor or AXEpad software — both of which are PICAXE IDEs (Integrated Development Environments). Similarly, there are several IDEs for Python programming. In the Python world, an IDE is usually referred to as a Python shell, and the shell that’s included in the RPi operating system is called IDLE. Actually, when you look at the RPi desktop, you will see two different IDLE icons. IDLE is used for Python 2.x programming, and IDLE3 is used for Python 3 programming.