A few times a year, I have the job of teaching a bunch of people who have never written code before how to program from scratch. The nature of programming being what it is, the same error crop up every time in a very predictable pattern. I usually encourage my students to go through a step-by-step troubleshooting process when trying to fix misbehaving code, in which we go through these common errors one by one and see if they could be causing the problem. Today, I decided to finally write this troubleshooting process down and turn it into a flowchart in non-threatening colours.
Behold, the “my code isn’t working” step-by-step troubleshooting guide! Follow the arrows to find the likely cause of your problem – if the first thing you reach doesn’t work, then back up and try again.
It’s intended for programmers who are new to Python, but even experienced Pythonistas sometimes get distracted and stuck on simple things. I’m keeping a copy handy.
Once again, here’s the weekly list of events for events for Tampa Bay techies, entrepreneurs, and nerds. Every week, on GlobalNerdy.com and on the mailing list, I scour the announcements for events that are interesting to or useful for those of you who are building the future here in “The Other Bay Area, on The Other West Coast”.
This list covers events from Monday, August 31 through Sunday, September 6, 2020.
I’ve opted to list only those events that I can confirm are happening online. I’m not yet listing in-person events, as we’re still in the middle of a pandemic in one of the hardest-hit states in one of the hardest-hit countries in the world. We’re also seeing the mandated return of students to schools, which will likely exacerbate the situation.
Events — especially virtual, online ones — can pop up at the last minute. I add them to the list as I find out about them. Come back and check this article from time to time, as you might find a new listing that wasn’t there before!
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(Make sure to use the comma, and spaces correctly)
The first part of my solution was turning those numbers into a list. Copy the numbers into a text editor, stick 0b in front of each one, and then turn the sequence into a Python list:
The next step is to convert those numbers into letters. Once again, the Unicode/ASCII value for “A” is 65, so the trick is to add 64 to each number and convert the resulting number into a character.
I could write a whole article — and I probably should — based on just that single line of code, but in the meantime, I thought I’d post an easier, more Pythonic solution.
>>> characters = [chr(number + 64) for number in numbers]
>>> characters
['W', 'O', 'W', 'Y', 'O', 'U', 'A', 'R', 'E', 'R', 'I', 'G', 'H', 'T']
Most programming languages don’t have list comprehensions. In those languages, if you want to perform some operation on every item in an array, you use a mapping function, typically named map(), but sometimes collect() or select().
Hence my original solution with lambda and map() — it’s force of habit from working in JavaScript, Kotlin, Ruby, and Swift, which don’t have Python’s nifty list comprehensions.
In computing “Capture the Flag” events, the flag isn’t a physical one, but some kind of challenge. Sometimes, it’s something you need to retrieve from a program, website, or even a piece of hardware with an intentionally built-in vulnerability that you must exploit. Sometimes it’s a problem or puzzle you must solve. It may also be a trivia challenge.
Solving each challenge earns you a specified number of points, with the tougher challenges being worth more points. The player with the most points wins.
Since it wasn’t scheduled as a day of actual class — the last day of class was on Wednesday — I’d booked a doctor’s appointment for that morning. A plumbing problem also required me to be at home for a little bit.
By the bye, if you’re looking for a great plumber in Tampa, I highly recommend Joshua Tree Plumbing.
The challenges
Still, since most of the challenges were posted online and since I’d never participated in a CTF before, I decided to try anyway. I decided to treat my schedule as if it was a golfer’s handicap. Since some of the challenges were just questions where you’d either select an answer or type one in, I did them on my phone while waiting for the doctor.
In between a couple of car trips, I managed to eke out a little over an hour and a half of time in the CTF, so I think I placed rather well, all things considered:
Here’s a sampling of some of the challenges:
Who’s on 80? (300 points):
Scan the host at (IP=10.10.1.1) and enumerate the service running on open port, 80.Use the following syntax for your answer: nmap [scan type] [ options] [target]
The Big Kahuna, part 1 (1200 points):
Using the Linux OS and boot method of your choice (VM or live boot):Add the “Kali Linux Headless” Repository to your repository list. Download and install the Kali Tools Headless package to your Linux operating system. Get the Metasploit Framework running. Show one of the staff when you’re finished.
Don’t cross the streams! (500 points):
An attacker got onto a machine and created a rogue user. Dig through the attached PCAP file and identify the rogue user.The flag is the user name. This flag IS case sensitive.
Execution is everything! (400 points): What are the four different execution policies for Powershell?
All the numbers were between 1 and 26 inclusive, suggesting letters of the alphabet.
The ASCII/Unicode value for “A” is 65. If you offset the numbers by adding 64 to each, and then convert each number to a character, you should get the message:
Remembering the instructions to “use the comma, and spaces correctly,” the answer is:
WOW, YOU ARE RIGHT
The big kahuna part 2 (700 points)
Using the Linux OS and boot method of your choice (VM or live boot):
Create a folder. In that folder, create 100 directories that are uniquely named incrementally (ergo directory1, directory2, etc.). Inside each of those 100 directories, create 100 directories that are uniquely named incrementally. Inside each of those 100 directories, create 100 files named incrementally (file1, file2, file3, etc.). The contents of each file should include the lyrics to the “Battle Hymn of the Republic” by Julia Ward Howe.
When complete, show a staff member.
Cochise (artist’s conception).
This challenge is phrased in such a way that it could only have been written by our Linux instructor Cochise (pictured to the right).
Creating those 100 directories in Linux is a one-liner:
mkdir directory{1..100}
The rest of the task calls for some scripting.
I’m terrible at shell scripting. I’m perfectly comfortable with using the shell interactively, in that classic enter-a-line/get-a-response fashion. However, once I have to deal with those half-baked control structures, I tend to walk away and say “Forget this — I’m doing it in Python.”
Here’s a cleaned-up, easier to read version of my solution to the challenge. It assumes that there’s a file called battle.txt in the same directory, and that the file contains the lyrics to the Battle Hymn of the Republic:
import os
import shutil
import sys
for directory_number in range (1, 101):
# Create the directory.
directory_name = f"directory{directory_number}"
try:
os.mkdir(directory_name)
except:
error = sys.exc_info()[0]
print(f"Failed to create directory {directory_name}.\n{error}")
quit()
# Go into the newly-created directory.
os.chdir(directory_name)
# Create the files within the directory
# by copying battle.txt from the directory above
# 100 times, naming them file1...file100.
for file_number in range(1, 101):
filename = f"file{file_number}"
try:
shutil.copy("../battle.txt", f"file{file_number}")
except:
error = sys.exc_info()[0]
print(f"Failed to create file {filename}.\n{error}")
quit()
# Let’s go back up one directory level,
# so that we can create the next directory.
os.chdir("..")
I had a lot of fun on my first CTF, even if I got to take part in a fraction of it. I’ll have to join The Undercroft’s next one!
Dr. Martin Jones, the author behind the book and site Python for Biologists, has come up with a chart to help Python programmers when their code doesn’t work and they can’t figure out why.
