1 Overview

This short unit will introduce us to the idea of error handling in Python programs and bash scripts.

In Python, errors that can happen when running a program are called exceptions. We will see how the try/except construct can be used to prevent your program from crashing when an error occurs. Instead, you can write any code you like to handle the error in some nice way.

In Bash, errors don’t crash the shell (usually), but they typically cause some error message to be printed to the terminal and change the exit status to some non-zero value. This can be used in bash’s if statements and with tools like grep to conditionally execute parts of a bash script.

2 Resources

2.1 Errors and try/except in Python

• P4E, Chapter 3, section on “Catching exceptions”

  A quick overview of the try/except syntax in Python.

• P4DA, Section 3.2, last sub-section on “Errors and Exception Handling”

  How to catch specific exception types and use the more advanced try/except/else/finally syntax.

• Python Tutorial Chapter 8: Errors and Exceptions

  The Python language guide to what exceptions are, how to handle them, and how to raise them in your own functions.

2.2 Exit codes and if statements in bash

• TLCL Chapter 27: Branching with If.

  This is all good info, but the part to focus on for our purposes is just the first two sections on if statements and exit status.

• Advanced Bash Scripting Guide chapters on exit status and if statements.

  A good but terse discussion of exit status on the command line, and then some examples of using tests and command exit statuses in bash if statements.

  Look for “the very useful if-grep construct” — this is something we should be able to make use of!

3 Examples

• Python program that tries repeatedly to open a file until someone enters a valid filename, and then counts how many lines are in that file.

  handle = None
  while handle is None:
      fname = input("Filename: ")
      try:
          handle = open(fname)
      except FileNotFoundError:
          print("Sorry, I couldn't open that file")
  
  count = 0
  for line in handle:
      count += 1
  
  print(f"File {fname} has {count} lines.")

• Python program that tries to get today’s weather forecast from the USNA weather page. Remember BeautifulSoup from last semester?

  from bs4 import BeautifulSoup
  import requests
  from datetime import datetime
  
  # Get current day of week, e.g. "Wednesday"
  today = datetime.now().strftime('%A')
  
  # Load the weather page, check connection errors
  try:
      page = requests.get('https://www.usna.edu/Weather/index.phpp')
  except:
      print("Couldn't load the webpage. Maybe you aren't connected to the internet?")
      exit(1)
  
  # Note: not all errors are exceptions!
  # Sometimes a normal if statement is still good.
  if page.status_code != 200:
      print("Got an error page from the USNA webserver")
      exit(1)
  
  # Note: BeautifulSoup never returns an error if you pass it any string
  soup = BeautifulSoup(page.text, 'lxml')
  
  try:
      # Will raise an error if the find() returned None
      print(soup.find(text=today).parent.parent.text)
  except AttributeError:
      print(f"Couldn't find the weather for {today} in the page")
      exit(1)