Working with the OS: os, sys, shutil, platform

Practice that adapts to you

Practise this lesson

10 questions about 11 ideas of this lesson. Each answer is explained at once, and what you get right is kept in this browser.

1. Question 1 of 10 Which of these does a new process receive from the program that starts it?

   Choose every answer that is right.

   Its arguments The parent's Python variables A copy of the environment variables Its working directory A live link to the parent's environment

   Show the answer: Question 1 of 10

   Answer:

   • Its arguments
   • A copy of the environment variables
   • Its working directory

   A parent passes the arguments, a copy of its environment and its working directory; the child hands back one number, its exit status, when it ends.

2. Question 2 of 10 You export SHOP_API_TOKEN in your shell's start-up file. The script works in your terminal and raises KeyError when the scheduler runs it. What explains it?

   Choose one answer.

   The scheduler deletes the variables of the scripts it runs os.environ is read only once a day KeyError means the token's value is wrong The scheduler is not your shell, so it starts the script with its own environment, without your exports

   Show the answer: Question 2 of 10

   Answer: The scheduler is not your shell, so it starts the script with its own environment, without your exports

   The environment comes from the parent. Give the job its settings where the scheduler can pass them, and stop with a clear message when a required one is missing.

3. Question 3 of 10 Which test tells reliably whether the interpreter is Python 3.10 or newer?

   Choose one answer.

   sys.version_info >= (3, 10) sys.version >= "3.10" sys.version_info.minor >= 10 platform.python_version() >= "3.10"

   Show the answer: Question 3 of 10

   Answer: sys.version_info >= (3, 10)

   Compare sys.version_info with a tuple; every other form compares text or only part of the version.

4. Question 4 of 10 A process is pinned to 2 of a machine's 16 CPUs. Which call gives the number to size its process pool by?

   Choose one answer.

   platform.machine() os.process_cpu_count() len(sys.argv) os.cpu_count()

   Show the answer: Question 4 of 10

   Answer: os.process_cpu_count()

   os.process_cpu_count() can be smaller than os.cpu_count(), and it is the one that says how many CPUs the process can really keep busy.

5. Question 5 of 10 What does os.getenv("SHOP_DEBUG") return when SHOP_DEBUG is not set?

   Choose one answer.

   "False" An empty string It raises KeyError None

   Show the answer: Question 5 of 10

   Answer: None

   os.getenv(name) and os.environ.get(name) return None for a missing variable; pass a default as the second argument to get something else.

6. Question 6 of 10 SHOP_DEBUG=0 is set to turn debugging off. What does debug = bool(os.environ.get("SHOP_DEBUG")) give?

   Choose one answer.

   It raises ValueError None True, because "0" is a non-empty string False, because the value is 0

   Show the answer: Question 6 of 10

   Answer: True, because "0" is a non-empty string

   Environment values are text, so turn a flag into True or False on purpose, as the lesson's program does with flag.lower() in {"1", "true", "yes"}.

7. Question 7 of 10 What does cwd_trap.py print?

   What does this program print? Choose one answer.

   """A file opened by a relative name is looked up in the working directory, not next to the script."""
   import os
   from pathlib import Path
   
   HERE = Path(__file__).resolve().parent           # the folder this script lives in
   (HERE / "prices.csv").write_text("item,paise\ntea,1500\n", encoding="utf-8")
   (HERE / "reports").mkdir(exist_ok=True)
   
   def read_relative():
       with open("prices.csv", encoding="utf-8") as f:          # relative to os.getcwd()
           return f.readline().strip()
   
   def read_beside_script():
       return (HERE / "prices.csv").read_text(encoding="utf-8").splitlines()[0]
   
   print("working directory is the script's folder:", Path.cwd() == HERE)
   print("relative name:", read_relative())
   
   os.chdir(HERE / "reports")                       # what a scheduler, an IDE or a cd before the command does
   print("working directory is the script's folder:", Path.cwd() == HERE)
   try:
       print("relative name:", read_relative())
   except FileNotFoundError as e:
       print("relative name: FileNotFoundError:", e.strerror)
   print("next to the script:", read_beside_script())

   working directory is the script's folder: True relative name: item,paise working directory is the script's folder: False relative name: FileNotFoundError: No such file or directory next to the script: item,paise working directory is the script's folder: True relative name: item,paise working directory is the script's folder: False relative name: item,paise next to the script: FileNotFoundError: No such file or directory working directory is the script's folder: True relative name: item,paise working directory is the script's folder: True relative name: item,paise next to the script: item,paise

   Show the answer: Question 7 of 10

   Answer: it prints

   working directory is the script's folder: True
   relative name: item,paise
   working directory is the script's folder: False
   relative name: FileNotFoundError: No such file or directory
   next to the script: item,paise

   After os.chdir(), the relative name "prices.csv" is looked up in the reports folder, where no such file exists. The path built from Path(__file__).resolve().parent does not depend on the working directory, so it still finds the file.

8. Question 8 of 10 A clean-up script calls shutil.rmtree(path) with a folder name read from a configuration file. Which check belongs before the call?

   Choose one answer.

   Resolve the path and confirm that it is inside the folder the script owns None, because rmtree() asks for confirmation None, because rmtree() moves the folder to the recycle bin Check only that the folder exists

   Show the answer: Question 8 of 10

   Answer: Resolve the path and confirm that it is inside the folder the script owns

   shutil.rmtree() deletes a folder and everything in it, with no undo. Resolve the path and check that it is inside the folder the script is meant to clean, for example with Path.is_relative_to(), before calling it.

9. Question 9 of 10 A script calls sys.exit("disk full") inside a try block that has a finally clause. Which statements are true?

   Choose every answer that is right.

   "disk full" is printed on standard error The exit status is 0, because no error was raised The finally clause runs before the program ends The exit status is 1 The program ends at once, without running the finally clause

   Show the answer: Question 9 of 10

   Answer:

   • "disk full" is printed on standard error
   • The finally clause runs before the program ends
   • The exit status is 1

   sys.exit() raises SystemExit, so cleanup in finally clauses and with statements still runs. When the argument is a string, Python prints it on standard error and exits with status 1, which callers read as a failure.

10. Question 10 of 10 By convention, which exit status says that the command line itself was wrong?

    Choose one answer.

    0 1 2 Any negative number

    Show the answer: Question 10 of 10

    Answer: 2

    0 means success, 2 a mistake on the command line, and any other number a failure of its own kind that the caller can act on.