Exceptions

• What should int do when the input isn’t an int?
  • Notice that there really isn’t a “Free” integer that could serve as a warning.
  • Could return a tuple, but that would be annoying to use in most cases (i.e., having to look for a problem every time int is called.)

• Solution: Use try/except blocks.

  ~~~~ user_input = ‘’ while user_input != ‘q’: try: weight = int(input(“Enter weight (in pounds): “)) height = int(input(“Enter height (in inches): “))

    bmi = (float(weight) / float(height * height)) * 703
    print(f'BMI: {bmi}')
    print('(CDC: 18.6-24.9 normal)\n')  # Source www.cdc.gov   except:
    print('Could not calculate health info.\n')
  

  user_input = input(“Enter any key (‘q’ to quit): “) ~~~~

  ~~~~ try: f = open(‘notafile.txt’) except: print(“problem opening file”) ~~~~

• Notice that the above approach doesn’t look at what the type of problem is. Sometimes we want to be more specific.
• In general, an Exception is an object that describes a problem: https://docs.python.org/3/library/exceptions.html
  • IndexError: Index out of bounds
  • KeyError: Dictionary key not found
  • ZeroDivisionError: Divide by zero.
  • ValueError: Try to convert a non-int to an int.
  • FileNotFoundError:

• You can look for a specific type of error and handle accordingly:

  try:
     # Code here
  except ValueError:
     # handle the value error
  except FileNotFoundError:
    # handle filenot ofund
  except (KeyError, ZeroDivisionError):
    # Not realistic.  Just an example of how to watch for two errors.
  

• Add exception handling to the “grades” code from in-class 12

• Benefits of this approach:
  • Don’t need to do anything if default behavior (i.e., crashing) is OK.
  • One check can cover multiple lines (e.g., multiple calls to int)
  • Calls will propagate up through the call stack. So, without any extra “work”, you can let an error move up to a point where it can be handled.

  def read_file(filename):
  
     # This is the wrong place to handle a 
     # FileNotFound error
     f = open(filename)
     lines = f.readlines()
     ...
  
  def process_file():
  
     try:
        filename = input("Enter filename")
        data = read_file(filename)
     except FileNotFoundError:
        print(f"File {filename} not found.  Please try again")
        process_file();
  

• Exceptions are objects, so they contain data (often a description of the problem.)

  try:
    int("This is not an int")
  except ValueError as v:
    print(v)
  

• finally
• Custom exceptions