Episode: 2933
Title: HPR2933: A walk through my PifaceCAD Python code – Part 1
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2933/hpr2933.mp3
Transcribed: 2025-10-24 13:29:59

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It's Wednesday the 30th of October 2019 and this is HPR episode 2933 entitled, I walk
through my PiFaceCAD Python code part 1. It's part of a series a little bit of Python
and it's the 50th anniversary show for MrX. The show itself is about 50 minutes long and
carries an explicit fact. The summary is, in this series I do a whirlwind tour of the
Python code I developed for my PiFaceCAD board. This episode of HPR is brought to you by
an honesthost.com. Get 15% discount on all shared hosting with the offer code HPR15. That's
HPR15. Better web hosting that's honest and fair at An Honesthost.com.
Hello and welcome hacker public radio audience. My name is MrX and welcome to this podcast.
I'd like to start as usual by thanking the people HPR for making this service available
to us all. It really is a truly invaluable service to bring to all of us. HPR is a community
led podcast provided by the community for the community. That means you can contribute to
just pick up a microphone, MP3 player, mobile phone, computer if you've got one, hit record
enough you go and then send it in simple as that. Well it's been a while since I've recorded
anything and I'm feeling decidedly rusty. I wasn't sure if I was going to record this episode.
I'm hummed and hooded about it for quite a while. So in this episode I thought I'd do a quick run
through try to keep it quick of my Python program which I use on my PiFace command and display
board which sits on top of my Raspberry Pi which I use to control a mock P on another Raspberry Pi
upstairs. It allows me to play and pause a track playing on mock P music and command line player
and allows me to go to the next track, previous track and so on and so forth. I explained all that
in a previous episode hopefully. I've just turned it on and it says there are only 23 days to a free
HPR slot and the blinks on the side is glowing green which means this is an adequate, I want to say
plenty, it's never plenty, adequate number of shows in the queue. Ken would never want to admit
that. There's never adequate, there's always space for more. So why don't you contribute something
and give us some shows? I've got contributed to some shows. More than we know what to do with.
So yes, one with the shoe. So this is going to be a bit off the cuff.
So I've got, I'll just move screens here and I'll just quit the program with a queue and I'm
logged into my Raspberry Pi with the PiFace board installed. So I've got a copy here open and
of caddas. So where am I? What was I saying? So this script has been in development for quite a
long time and I mean, nowadays I hardly ever tweak it. It can go months without change. I did do
some changes recently, which I'll talk about later on as I come to them. My most recent changes.
But I'm just looking back at the date stamps on the script. So I've got a backup script which
backs up my scripts directory as it's up. Well, tj, tj, tjz it. And let me just see. Yeah, so
so the very earliest version of my scripts folder goes back to 2014. So I should, does that mean
I've been working with it? So it might be in 2014 when I started working on it. I'm not quite
sure, but it goes back a long way anyway. And the most recent one was that most recent change was
on the 5th of December 2018, just for him to sick. So the program's called caddas menu.py.
It's a Python program. Basically, so I'll just do I have that open? Let's just see. Yeah,
yes, here we go. So I had a few, but some, a few problems with an update. And I had to go back
from Python 3 to Python 2. But I'll explain that in another podcast. Can I ignore that for now?
Hopefully, I won't tweak it too much. I did wonder, as I said, I wondered whether to release this or not
because it's always nerve-wracking when you think about releasing code into the wild. You know,
it's going to show how rubbish programmers are and all the mistakes and terrible things. And
I'm actually just spilling. And I always have been so, Dave will have a Canadian when he sees the
a devil spelling in this script, in English. But then I suppose, well, I'm not a professional
programmer, I'm a hobby programmer. It's a good thing, it's a curse, but Python's so easy
to play with, you can pull code together really quickly and really easily. Probably you can
write quite nasty code quite easily as well, of course. But I was kind of impatient just
to build this thing up, and it kind of morphed from very simple thing. It wasn't well planned
out like a good software, but software should be, you know, was, oh, what they want to do
well. Of course, the first thing I wanted to do, too, was to able to pause and play, pause
and pause and play audio playback from my Raspberry Pi upstairs. It was running mock P, mock
P being the command line player, which plays audio. And I wanted to run, I wanted to be able
to, at the very least, play and pause, toggle between play and pause on that, that was
really all I needed to achieve. And that was probably the very first thing that I managed.
You know, mock P can be told, I think it's a dash capital G, I think, mock P is based
dash G, dash capital G, toggle between play and pause. And that's all you needed to send
to the PI upstairs, and that would, that would do it. And click to the Raspberry PI upstairs
as a set of cordless headphones. I've probably explained all this before, so maybe I have
to go into all that. So anyway, I've got a section at the front here, which starts things
to do, and I hope I have stuff in there, which, if you're bored, you can read over. Because
I'm quite a long time, actually, I've got a section code starts here, at line 140.
So the really clever thing is, is a PI face, CAD module, which come with the PI face board.
That's what all the clever stuff is, really. I'm just taking advantage of this module
that was written by the PI face people. And of course, there's a new version, I think
of the newer version of PI. And this is, of course, some of the weaknesses, you know,
the distribution updates, and then the drivers don't work in things, and then you lose compatibility
and all this sort of thing. So, that's a problem. It's getting a bit long in the tooth now.
And I've barely had the will and inclination to get it working after the last deb being
upgrade, but that's another story. So, yeah, so the whole pilot module is, I import into
the Python project. I won't be rushing through that. The most recent thing I did was below
that, I've got a section for global variables. And I had littered throughout the code, I had
the address for my internal IP address, the Raspberry Pi, which I was controlling. And
all littered throughout the code was, you know, Pi at 192.168.1.Whatever, you know. And what
I really should have had was a global variable called remote, well, a global variable basically
so that you could just refer to that, and I would say, if you ever needed to change that
IP address, you would change it in one place and not 15 places or whatever. So, that was
I think I did very recently, and I called that variable remote device. So, remote device
equals, in my case, Pi at 192.168.1.Thirteen. So, I probably rely, well, I definitely rely
too heavily on global variables. And I'm not, I don't think I've been terribly consistent
with global variables either, because, from documentation I read online, I was under
the understanding that you needed to use a command to allow you to use global variables
in a function. Now, what I'm just trying to think now, you know, of course, I'm, it's
not long since I've done any programming and Python, and I can't even know what the command
was. Let me just see now. What was that command to allow you? Oh, global, that was it. So,
for example, I've got, I've got a line here, it says global space menu. So, that allows
you to use the global variable menu. Now, what I didn't actually understand was that if
you, I, I, and I'm still not, I haven't done enough, I haven't done any experiments to
really prove this, but it must be the case that if you don't use, if you've got a global
variable, so you've got, maybe, okay, so the difference between that, a private variable
and a global, private variable and not a normal variable and a global variable is that a
normal variable is a scope. If you've got a normal variable inside a, a function in you,
then other functions outside, outside that can't see that, that variable, picking this
very complicated answer, but a global variable of it's at the top of the code, then outside
all the functions in every day, all the functions can see that global variable. But the way
it works is, I think is if you don't use the command global, then you can still access
that global variable, you just can't change it. So, in some cases, I've got, I'm using
the command global institutions where I don't actually need to change that global variable.
So in reality, for safety, I shouldn't be using the word global, so you just ignore it
altogether and just, I can read the contents of the global variable, but I don't need to
change it. I think that's how that basically works. How did I get into that? I don't know.
Anyway, so I've got too many global variables. You should really, my understanding is that
you should really feed values in and out of functions rather than pass values into functions
rather than relying on global variables. I remember a tutor explaining to me many, many years
ago when I was doing a course on what was it? It's not Python, what was it? Pascal, that
was it. I need to say, well, it's a bit like, I imagine you're outside a field and you
want to cross a field to get to the other side and there's a bull in the middle of the field
as well. You can either walk around outside edge where it's nice and safe or you can jump
over the fence and try and run to the other side and I say, well, it's a bit like that.
If you're using global variables, then another function could come go in and change that
of variable to something else that you're not aware of and mark up your code, basically,
whereas if you're passing the value in then you're not affecting the global variable.
Anyway, whatever that will come from. I better watch my tingle how long has been going for?
15 minutes. Oh, this is a disaster. This could be far too long. Okay, I think that's about
enough for this episode. I think it's a good place to stop it. If you want to contact me,
then I can be contacted at MrX at hpr.googlemail.com. That's mrx80hpr.com. So tune in next time for
an exciting episode of Hacker Public Radio.
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