Raspberry Pi Emulation on a Mac

I use a Raspberry Pi as a controller for our X-Carve CNC machine, and it recently stopped booting (my guess is due to SD card corruption from being improperly shut down). I wanted to test out some configuration changes to allow for the SD card to be mounted completely read only, but I didn’t want to have to keep on rebooting the Pi. So I decided to look into emulation (running the Pi in a VM).

The Pi uses an ARM processor, which means that traditional virtual machine approaches like VirtualBox or VMWare wouldn’t be suited. I found a few gists online of people who have done similar things on their Macs that I thought could be useful as a starting point.

I used this as my basis: https://gist.github.com/thomasweng15/af0929114efce3524d55d10f170ff30d

First, one-time setup procedure (I put this in a file setup_qemu.sh)

#!/usr/bin/env bash

# # install qemu
# brew install qemu

# # download the latest stretch kernel and the necessary dtb file
# $ curl -OL https://github.com/dhruvvyas90/qemu-rpi-kernel/raw/master/kernel-qemu-4.14.79-stretch
# $ curl -OL https://github.com/dhruvvyas90/qemu-rpi-kernel/raw/master/versatile-pb.dtb

# # download and unzip the latest raspbian lite
# $ curl -o raspbian_lite_latest.zip -L https://downloads.raspberrypi.org/raspbian_lite_latest
# $ unzip raspbian_lite_latest.zip

export QEMU=$(which qemu-system-arm)
export RPI_KERNEL=kernel-qemu-4.14.79-stretch
export RPI_FS=2019-04-08-raspbian-stretch-lite.img
export RPI_FS_ZIP=2019-04-08_raspbian_lite_stretch.zip
export PTB_FILE=versatile-pb.dtb

# First time setup
# wipe old img file (uncomment this if you are share that's what you want to do!)
if [ -f $RPI_FS ]; then
    rm $RPI_FS
if [ ! -f $RPI_FS ]; then
    unzip $RPI_FS_ZIP

$QEMU -kernel ${RPI_KERNEL} \
    -cpu arm1176 \
    -m 256 \
    -M versatilepb \
    -dtb ${PTB_FILE} \
    -no-reboot \
    -serial mon:stdio \
    -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw init=/bin/bash" \
    -drive "file=${RPI_FS},index=0,media=disk,format=raw"

# Paste the line below (after the comment). press enter, then ctrl-d when it is done booting
# sed -i -e 's/^/#/' /etc/ld.so.preload; sed -i -e 's/^/#/' /etc/ld.so.conf; sed -i -e 's/^/#/' /etc/fstab

Next, the script to run QEMU (I put this in run_qemu.sh):

#!/usr/bin/env bash

export QEMU=$(which qemu-system-arm)
export RPI_KERNEL=kernel-qemu-4.14.79-stretch
export RPI_FS=2019-04-08-raspbian-stretch-lite.img
export PTB_FILE=versatile-pb.dtb

$QEMU -kernel ${RPI_KERNEL} \
    -cpu arm1176 \
    -m 256 \
    -M versatilepb \
    -dtb ${PTB_FILE} \
    -serial mon:stdio \
    -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" \
    -drive "file=${RPI_FS},index=0,media=disk,format=raw" \
    -net user,hostfwd=tcp::5022-:22 \
    -net nic

pylint_django error – NoSuchChecker

I just spent an inordinate amount of time tracking down an issue with pylint-django that I had a hard time finding any clues on the internet about, so I’m documenting it here.

I use pylint and pylint-django to perform automated checks on my Django projects and ensure a certain code quality is maintained. This combination has proven very useful. Recently, though, I ran into an issue where one project would fail to validate with a strange error:

$ pylint --rcfile pylintrc myproject
Using config file /app/pylintrc
Traceback (most recent call last):
  File "/usr/bin/pylint", line 11, in <module>
  File "/usr/lib/python3.6/site-packages/pylint/__init__.py", line 16, in run_pylint
  File "/usr/lib/python3.6/site-packages/pylint/lint.py", line 1312, in __init__
  File "/usr/lib/python3.6/site-packages/pylint/lint.py", line 495, in load_plugin_modules
  File "/usr/lib/python3.6/site-packages/pylint_django/plugin.py", line 18, in register
    name_checker = get_checker(linter, NameChecker)
  File "/usr/lib/python3.6/site-packages/pylint_plugin_utils/__init__.py", line 30, in get_checker
    raise NoSuchChecker(checker_class)
pylint_plugin_utils.NoSuchChecker: <class 'pylint.checkers.base.NameChecker'>


I thought maybe this had to do with some incompatibilities between pylint, pylint-django, and possibly astroid, but strangely, the versions in this project were exactly the same as the other project. So I dug into the code where the exception was being reported.

get_checker is a part of pylint_plugin_utils, and it is used by pylint-django to augment the base PyLint checkers. It was trying to find pylint.checkers.base.NameChecker in the list of registered “checkers” for pylint. The file pylint/checkers/base.py" did exist in thesite-packagesfolder, but strangely, it was being registered assite_packages.pylint.checkers.base.NameChecker`.

Pylint has a register_plugins function that it uses to register all the default plugins. It does this by calling modutils.load_module_from_file on each of the files it finds starting from the checkers directory included with the library.

load_module_from_file figures out the proper import path using a function modpath_from_file, which in turn uses modpath_from_file_with_callback to check that each path it traverses has a __init__.py file. It looks at each path in sys.path in order to determine if the file attempting to be loaded has a valid import path.

What was throwing things off was the presence of __init__.py in the site-packages directory. /usr/lib/python3.6 was in sys.path before /usr/lib/python3.6/site-packages, and because there was a __init__.py file in site-packages, the import mechanism thought that site-packages was itself a module in the /usr/lib/python3.6 directory. It was therefore assigning this as the module name. Now where was that coming from?


It turns out it was a rogue package that stuck that empty file there (singletons). I removed that file (and thankfully had control over the project where that file originated and removed it from the source), and that fixed the issue.

Cloud9 as a Development Environment

I first encountered Cloud9 (the IDE, not the professional gamers) a couple years ago, when I was looking for a nice HTML/CSS editor for the students in my Exploring Computer Science course to use on their own laptops. I settled on it because it had good highlighting, didn’t require them to understand anything on the command line, but still focused on raw code as opposed to WYSIWYG stuff.

Since I stopped teaching high school, I hadn’t thought much about it. As a boot camp instructor, it was important to me that I teach my students how to use “real” tools, and so we focused on local tools.

At the moment though, I find myself temporarily without a dev machine, and my mind turned back to Cloud9. I’m mainly working in libraries (hurrah for free access to computers!), and while I can thankfully count on them to have Google Chrome, I can’t count on much else.

So, how can I do some real development without a real dev machine? I started up a new (free) account to find out.

Continue reading “Cloud9 as a Development Environment”

Markdown in WordPress

Markdown-mark.svgI’ve been writing a lot of prose over the past year – lesson plans, online content, blog posts, etc. As more of my prose has also involved code samples, I’ve gravitated toward Markdown for writing my text for a few reasons:

  • It’s dead simple to incorporate code blocks
  • It’s cleaner to look at than raw HTML
  • There are plenty of great tools that support it (for me those include Jupyter Notebook and The Iron Yard’s online learning platform)

(Yes, there are also reasons not to use Markdown)

But I never made the jump to using it on my blog. Until today!

Continue reading “Markdown in WordPress”