5DEBUGGING

Raising Exceptions

You’ve already practiced handling Python’s exceptions with try and except statements so that your program can recover from exceptions you anticipated. But you can also raise your own exceptions. Raising an exception is a way of saying, “Stop running this code and move the program execution to the except statement.” We raise exceptions with a raise statement. Answer the following questions about exceptions, the try and except statements, and raise statements.

1. 1. What happens if you run the following program and press enter instead of entering a name?

   print('Enter your name:')
   name = input()
   if name == '':
       raise Exception('You did not enter a name.')
   else:
       print('Hello,', name)

2. 2. Write the code that raises an Exception error with the error message 'An error happened. This error message is vague and unhelpful.'

3. 3. True or false: A raise statement must be inside a try block.

4. 4. What happens if you run the following program and press enter instead of entering a name?

   def get_name():
       print('Enter your name:')
       name = input()
       if name == '':
           raise Exception('You did not enter a name.')
   
       return name
   
   try:
       name = get_name()
   except:
       name = 'Guido'
   
   print('Hello,', name)

Assertions

An assertion is a sanity check that makes sure your code isn’t doing something obviously wrong. We make assertions with an assert statement. If the condition in an assert statement is False, Python raises the AssertionError exception. The following questions test your knowledge of assert statements and how to use assertions to detect problems.

1. 5. “While exceptions are for user errors, assertions are for ____ errors.”

2. 6. Why is failing fast a good thing?

3. 7. Which command line argument to the Python interpreter suppresses assertion checks when running a program?

4. 8. What does assert False do?

Logging

Logging is a great way to understand what’s happening in your program, and in what order. Python’s logging module makes it easy to create a record of custom messages that you write.

1. 9. Alice writes a program with several print() calls for debugging information instead of using the logging module. After she’s done programming, she starts removing these print() calls. What are two possible mistakes she could make while removing them?

For each of the following events, decide what logging level to use for the corresponding log message. (These can be subjective and may have multiple acceptable answers.)

1. 10. An error causes a failure that makes the program crash with no chance of recovery.

2. 11. A particular function in your program, calculate_my_result(), is called.

3. 12. The program logs the value of a particular variable.

4. 13. The user requests that the program open a file, but the file doesn’t exist.

5. 14. The program detects that a calculation is wrong but is able to continue running.

6. 15. The program starts running and needs to record the time and date at which it started.

7. 16. The program keeps track of how many times a while loop had looped before exiting.

8. 17. The program logs the string the user entered for an input() call.

Mu’s Debugger

The debugger is a tool that can run a single line of code and then wait for you to tell it to continue. By running your program “under the debugger” like this, you can take as much time as you want to examine the values in the variables at any given point during the program’s lifetime, making it a valuable tool for tracking down bugs. It’s also more efficient than debugging your program by sprinkling print() calls throughout your code and rerunning it over and over.

The following questions concern the debugger for the Mu code editor used in Automate the Boring Stuff with Python. If you use a different debugger, try answering these questions for it instead.

1. 18. What do you do if you want the program to run at normal speed, then pause and start the debugger once the execution reaches a particular line of code?

2. 19. If the debugger is currently paused on a line of code within a function and you want it to run the rest of the code in the function at normal speed, then pause once the execution has returned from the function, which debugger button should you press?

3. 20. Which button should you press if the debugger is currently paused on a line of code and you want it to resume running at normal speed?

4. 21. If the debugger is currently paused on a line of code, how can you immediately terminate the program?

5. 22. If the debugger is currently paused on a line of code that is a function call, which debugger button would cause the debugger to pause on the first line in that function?

6. 23. If the debugger is currently paused on a line of code that is a function call, and you want to run all the code inside that function at normal speed, then pause again when the execution has returned from the function, which debugger button should you press?

Buggy Grade-Average Calculator

Copy the following program into your editor or download it from https://autbor.com/buggygradeaverage.py. This program lets the user enter any number of grades until the user enters done. It then displays the average of the entered grades.

def calculate_grade_average(grade_sum, number_of_grades):
    grade_average = int(grade_sum / number_of_grades)
    return grade_average

counter = 0
total = 0
while True:
    print('Enter a grade, or "done" if done entering grades:')
    grade = input()
    if grade == 'done':
        break
    counter = counter + 1
    total = total + int(grade)

avg = calculate_grade_average(counter, total)
print('The grade average is:', avg)

When you run the program and enter 100 and 50, however, it reports the average as 0 instead of 75:

Enter a grade, or "done" if done entering grades:
100
Enter a grade, or "done" if done entering grades:
50
Enter a grade, or "done" if done entering grades:
done
The grade average is: 0

Run this program under a debugger to find out why it doesn’t work, then fix the bug. (Note that if the user enters a response other than done or a number, the program crashes; ignore this bug for now.)

Zero Division Error

Take a look at your corrected version of the previous grade-average program. If you run this program and immediately enter done without entering any grades, the program crashes with a ZeroDivisionError: division by zero error.

Use the debugger to find out why this happens. Add code to the calculate_grade_average() function so that it returns the integer 0 when the user hasn’t entered any grades, instead of crashing.

Leap Year Calculator

Copy the following program into your editor or download it from https://autbor.com/buggyLeapYear.py. This program has an is_leap_year() function that takes an integer year, then returns True if it’s a leap year and False if it isn’t.

def is_leap_year(year):
    if year % 4 == 0:
        if year % 100 == 0:
            if year % 400 == 0:
                return True
            return True
        return True
    return False

while True:
    print('Enter a year or "done":')
    response = input()
    if response == 'done':
        break
    print('Is leap year:', is_leap_year(int(response)))

For example, if you run this program, the output will look like this:

Enter a year or "done":
2000
Is leap year: True
Enter a year or "done":
2001
Is leap year: False
Enter a year or "done":
2004
Is leap year: True
Enter a year or "done":
2100
Is leap year: True
Enter a year or "done":
done

A year is a leap year if it is evenly divisible by 4. An exception to this rule occurs if the year is also evenly divisible by 100, in which case it is not a leap year. There is an exception to that exception too: If the year is also evenly divisible by 400, it is a leap year.

The year 2100 should not be a leap year, but the function call is_leap_year(2100) incorrectly returns True. Run this code under a debugger so that you can see where exactly the bug is, and then write the corrected is_leap_year() function.

Writing Buggy Code on Purpose

Write several short programs that produce the given error message in the following list. If you’re unfamiliar with the error message, search for it on the internet to find bug reports from others who have encountered it. The filename is a hint for writing the program.

• A program named nameError.py that produces the error message NameError: name 'spam' is not defined
• A program named badInt.py that produces the error message ValueError: invalid literal for int() with base 10: 'five'
• A program named badEquals.py that produces the error message SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
• A program named badString.py that produces the error message SyntaxError: unterminated string literal (detected at line x) (where x can be any number)
• A program named badBool.py that produces the error message NameError: name 'true' is not defined. Did you mean: 'True'?
• A program named missingIfBlock.py that produces the error message IndentationError: expected an indented block after 'if' statement on line x (where x can be any number)
• A program named stringPlusInt.py that produces the error message TypeError: can only concatenate str (not "int") to str
• A program named intPlusString.py that produces the error message TypeError: unsupported operand type(s) for +: 'int' and 'str'