HPR3067: Getting my Python3 code working in Python2

I have a raspberry Pi model B with the original Pifacecad add on board

I recently upgraded the operating system on my raspberry pi from Wheezy (Debian 7) to Jessie (Debian 8), all seemed fine till I tried to run my Python3 project. It reported that the pifacecad module wasn’t present. I tried to install the module using the standard apt-get command given in the Pifacecad documentation. It installed OK for python 2 but It would not install for python 3 as there was a dependency issue with the python-lirc library. I’m guessing the upgrade broke something.

I installed a fresh minimal installation of raspbian Stretch (Debian 9) onto a new SD card and ran the standard installation command in the Pifacecad documentation. It all installed but there was a warning about Spi which I had forgotten to enable. I enabled this using the raspi-config util and rebooted the pi.

I then tested the installation by running the sys-info.py example that is mentioned in the installation documentation that comes with the pifacecad board.

Running this example gave a warning saying that the pifacecad hardware could not be found at this point I gave up and decided instead to run my project in python 2.

Of course at first the code wouldn’t run using python 2 and I had to do a bit of digging around to find out what was going wrong.

You may find this useful in the unlikely event that you have a project written in python 3 that you want to run in python 2, it might also be useful if you’re going in the other direction.

One other thing that I briefly stumbled upon was that I believe there is a tool available that attempts to convert python 2 code to python 3 I don’t know anything about it and didn’t bother looking to see if there is a tool to go in the opposite direction which is what I needed. I’ve included a link to the tool below https://docs.python.org/2/library/2to3.html

I’ve got the two versions of code loaded into the excellent graphical diff tool meld and I’ll just briefly cover the things that I had to change.

Link to meld diff tool: https://meldmerge.org/

After some mucking about I got it partially working by installing Piface module for Python 2.7

Had to do the following changes to get the code working in Python 2.7:

• Changed first line of code from python3 to python2 so that code was run using the correct interpreter

• Print statement fixed by see
  https://stackoverflow.com/questions/32032697/how-to-use-from-future-import-print-function

• Python 3 uses the urllib.request command. For Python 3 I used

    urllib.request.urlopen(url).read()

https://docs.python.org/3/library/urllib.request.html

• Python 2 uses the urllib2 command. For Python 2 I used the following

    urllib2.urlopen(url).read()

https://docs.python.org/2/library/urllib2.html

• hostname --all has different output on this version of Debian, now includes mac address which I did not want.

• Getting the wifi ESSID information. I used iwconfig to get this information. The path to iwconfig command changed in this version of Debian and I now had to give the full path to get it to work.

• Python 2 and 3 seemed to handle strings differently when converting from an array to a plain string.

• Python 3 handles input different from python 2

• If python 2 comes across a non numeric value it quits with an exception

  • so for python 2 I used the command

        raw_input()

  • for python 3 I used the command

        input()

Two article about incorporating future statements in Python 2
https://docs.python.org/3/library/__future__.html

https://simeonvisser.com/posts/how-does-from-future-import-work-in-python.html

Related shows about my PiFaceCAD add on board for the raspberry pi

• hpr2933 :: A walk through my PifaceCAD Python code – Part 1
• hpr2951 :: A walk through my PifaceCAD Python code – Part 2
• hpr2963 :: A walk through my PifaceCAD Python code – Part 3
• hpr2976 :: A walk through my PifaceCAD Python code – Part 4

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