Debugging Principles and Techniques

Lesson Objectives

After this lesson, you will be able to…

Troubleshoot common types of errors.
Implement basic exception mitigation.
Troubleshoot logic errors.

Discussion: Error Messages

Have you found a shiny red error message before? What do you think has happened here?

Making Errors Into Friends

On the surface, errors are frustrating! However, we’ll walk through some common ones. You’ll see:

Errors sometimes say exactly what’s wrong.
Some errors have very common causes.
Errors may say exactly how to fix the issue.
Python errors are very helpful and have clear messages.

With that in mind - what’s the problem with this code?

We Do: IndexError

Let’s debug this code together.

Protip: Index errors typically happen when you attempt to access a list index that doesn’t exist.

Teaching Tips:

This is a demo, not an exercise!
Teach the students how to read the line number where the error occurred. Remind them that the error often happens on the line above the line that threw the error.
Make sure that students come up with the solution themselves, but ask leading questions. For example, “Obviously this code isn’t working, but by looking at it, can you tell what my intention was? Which element do you think I was trying to access?”

Talking Points: - “Remember that lists are indexed starting at zero!” (maybe show a reminder)

Repl.it Note: This replit has

race_runners = ["Yuna", "Bill", "Hyun"]

first_place = race_runners[1]
second_place = race_runners[2]
third_place = race_runners[3]

print("The winners are:", first_place, second_place, third_place)

You Do: Fix a NameError

Directions: Fix it!

Hints: - Run the code to get the error. - What kind of error is it? What is the error message?

Teaching Tips:

Give them just a minute or two; then go over it. This is a new error, but it’s an easy one.

Talking Points: - “Let’s go back to an error we encountered earlier today!” - We most commonly get a NameError if we use a variable: * Without defining it. * Before defining it

Repl.it Note: This replit has

# Get a number between 2 and 8.
my_nums = 5

# Print the number
print(my_num)

KeyError

Accessing a key in a dictionary that doesn’t exist.

Commonly caused by: - A misspelling. - Mixing uppercase and lowercase.

The error message tells you exactly what key is missing!

Teaching tip:

The repl.it is for you to demo this, not for an exercise.
Ask students what they think most commonly causes KeyErrors. Good answers might include case sensitivity in the key names, a typo in the key name, or simply not remembering what keys are available.

Talking Points:

“KeyError happens when you try to use a key in a dictionary that doesn’t exist”
“The error message tells you exactly what key threw the error”

Repl.it Note: It’s long; you might want to open it in a new tab. The replit has:

my_favorites = {
  "Food": "Lobster Rolls",
  "Song": "Bohemian Rhapsody",
  "Flower": "Iris",
  "Band": "Tom Petty & the Heartbreakers",
  "Color": "Green",
  "Movie": "The Princess Bride",
  "Programming Language": "Python"
}

# This is okay!
print("My favorite color is", my_favorites["Color"])

# This is NOT okay! (Case sensitivity!)
print("My favorite color is", my_favorites["color"])

# This is NOT okay! (Key doesn't exist)
print("My favorite restaurant is", my_favorites["Restaurant"])

AttributeError

More general than KeyError, but the same idea.
Accessing an attribute (e.g., function or property) that doesn’t exist

Teaching tip:

The repl.it is for you to demo this, not for an exercise.

Talking Points: - “AttributeError is more general than KeyError (which only applies to dict keys), but the same general idea.”

Repl.it Note: The replit has:

    class Dog():
  def __init__(self, name):
    self.name = name

  def bark(self):
    print("Bark!")

# Declare a new dog instance
my_dog = Dog("Fido")

# Call the bark method
my_dog.bark() # OK!

# Call the run method
my_dog.run() # AttributeError!

Discussion: SyntaxError

Let’s run the code together. What happens? How can we fix it?

Teaching Tips:

See if they can pick up the answer.
Python is different (and better) than other languages in this regard, but it’s important to emphasize that = and == are for different purposes!

Talking Points: - “In any other language, (take JavaScript for example), if you accidentally use a single equals when you mean to use a double equals, the variable would be reassigned while inside that if statement and your 13 year old would be having a beer! Luckily the designers of Python knew this and made the choice to throw an error!”

Repl.it Note: The replit is

my_age = 13

if my_age = 18:
    print("I may vote.")
else:
    print("I may not vote.")

Discussion: TypeError

TypeError and its message tell us:

my_num = 5 + "10"
print(my_num)
# TypeError: unsupported operand type(s) for +: 'int' and 'str'

What do we learn from this error message? Have you learned a way to fix this?

Fun Fact: Some languages, like JavaScript, let this code run (breaking something!).

IndentationError

May be caused by:

Notenoughindentation
Mismatched indentation
Mixing tabs and spaces!

Teaching Tips:

This isn’t an exercise - just code to help you demo.
This is so common for Python beginners, so it is important to get practice recognizing this!
Have students come up with some examples of what might cause an error like this. Answers might include mixing tabs and spaces or indenting too much/too little.

Talking Points: - “IndentationError comes up for any indentation errors, whether it’s too little or just mismatching in some way”

Repl.it Note: this replit has

my_age = 13

if my_age == 16:
  print("I may drive.")
else:
print("I may not drive.")

ValueError

Most commonly caused by trying to convert a bad string into a number.

# This is okay!
my_num = int("10")

# This throws a ValueError
my_num = int("Moose")

RuntimeError

The worst error to see!

When no other error type fits.
You need to rely on the error message content.
May be used for custom errors.

Example: RuntimeError is like if I said to you:

Please eat the piano

You can understand what’s being asked, but can’t actually do that!

Quick Review

There are many types of errors in Python!

Usually, the error has a name or description that says exactly what’s wrong.

Think about IndentationError or IndexError - what went wrong?

Sometimes, you’ll see RuntimeError. Python throws us this if something is broken but it can’t say specifically what - like Please eat the piano. Revisit your code and see what might have happened.

Next Up: A list of common errors, then ways to prevent errors.

List of Common Errors

This chart’s for you to refer to later - don’t memorize it now!

Error Type	Most Common Cause
`AttributeError`	Attempting to access a non-existent attribute
`KeyError`	Attempting to access a non-existent key in a dict
`ImportError`	A module you tried to import doesn’t exist
`IndexError`	You attempted to access a list element that doesn’t exist
`IndentationError`	Indenting code in an invalid way
`IOError`	Accessing a file that doesn’t exist
`NameError`	Attempting to use a module you haven’t imported/installed
`OverflowError`	You made a number larger than the maximum size
`RuntimeError`	The error doesn’t fit into any other category
`SyntaxError`	A typo, such as forgetting a colon
`TypeError`	Using two different types in an incompatible way
`ValueError`	When you are trying to convert bad keyboard input to a number
`ZeroDivisionError`	Dividing By Zero

Discussion: Throwing Errors

Sometimes, we might have code that we expect to throw an error.

# The user might not give us a number!
my_num = int(input("Please give me a number:"))

What if the user types a string like “Moose”?

This causes a ValueError - we’ll be trying to make an int out of a string “Moose”.
We can anticipate and prepare for it!

Try-Except

A Try-Except block is the way we can catch errors in Python. We can catch:

One error (except ValueError:)
Multiple errors (except (ValueError, KeyError):)
Any/every error (except:)

Always try to specify the error, if possible!

Teaching Tips:

Try / except is new; do this with them. Make this a teaching slide, not an exercise.
Demonstrate some example output - strings, ints, floats, etc.

Talking Points:

Call out that we specifically say “ValueError”, and err is just a random keyword. Change it to demo. Add other error catches; take out the ValueError specifically.
“A Try-Except block is the way we can catch errors in Python.”
“We can catch one error (as we see in the code), we can catch multiple errors, or we can just catch any/every error by leaving it blank.”
“You can catch every possible error by leaving the specified error blank, however, this is generally not a great practice because it says very little about how you were thinking.”

Replit Note: The repl.it has

my_num = None

while my_num is None:
  try:
      my_num = int(input("Please give me a number:"))
  except ValueError as err:
      print("That was not good input, please try again!")
      print("Error was", err)

print("Thanks for typing the number", my_num)

Discussion: Switching Gears

Not every programming error is caught by an error message!

Can anyone say what is wrong with this code?
What might happen if you run it?

Do not try to run the below code.

my_num = 1

while my_num < 10:
    print(my_num)
    my_num + 1

Teaching Tips:

See if anyone can tell you the problem (it’s an infinite loop).
Emphasize that an itty-bitty typo - just one character in this case - caused an enormous problem!

Talking Points:

“Errors and error messages are really helpful when we’ve got bad syntax or are trying to access something that doesn’t exist. However, not every possible programming error is able to be caught by error messages like this.”
“For example, consider the code here”
“Can you tell what the original intention of the code was? What went wrong? Why did the code go into an infinite loop? How can we fix this code?”
“There are many errors like this where there is nothing syntactically wrong with the code, but an error in the logic, or a typo that changes the meaning of the code as we saw with the infinite loop.”

Discussion: Another Infinite Loop

It’s easy to accidentally make an infinite loop. What’s the problem here?

am_hungry = True
fridge_has_food = True

while am_hungry or fridge_has_food:
  print("Opening the fridge!")
  am_hungry = False

Infinite Infinite Loops!

Most common infinite loops are a result of:

A while loop’s condition never becomes False.
Forgetting to increment a counter variable.
Logic inside the loop that restarts the loop.
Bad logic in a while loop’s condition (e.g., putting or instead of and)

Be careful to check your end conditions!

If you find your program running endlessly, hit control-c in the terminal window to stop it!

Teaching Tips:

Stress that these are common, and they really have to watch out for it. Show control-c so that they’re all clear on what it does. You can use this code:

am_hungry = True
fridge_has_food = True

while am_hungry or fridge_has_food:
  print("Opening the fridge!")
  am_hungry = False

Talking Points: - “There are many, many ways to accidentally create an infinite loop!”

Discussion: Logic Error

Here, we want to find the average of 8 and 10. The answer should be 9, because 8 + 10 == 18, then 18 / 2 == 9

What happened and why?

Teaching Tips:

Again, this is a teaching slide, not an exercise. Can they find the error?
When they do, write PEMDAS on the board and discuss the order of operations:

Parentheses
Exponents
Multiplication
Division
Addition
Subtraction

Replit Note: The code is:

x = 8
y = 10
average = x + y / 2
print(average)

Quick Review: Common Errors

If you expect an error, use a try/except block:

my_num = None

while my_num is None:
  try:
      my_num = int(input("Please give me a number:"))
  except ValueError as err:
      print("That was not good input, please try again!")
      print("Error was", err)

print("Thanks for typing the number", my_num)

Logic problems are common but won’t throw a helpful error. Always check end conditions on your while loops!

Print Statements for Sanity Checks

Pro Tip: If something is wonky and you don’t know why, starting printing.

Use print statements on each line to peek at the values.
Remember to remove debugging statements once the problem is solved!

x = 8
y = 10
get_average = x + y / 2
print("get_average is", get_average) # Print out what this equals (it's wrong!)
testing_sum = x + y # To figure out why, break it down.
print("testing_sum is", testing_sum) # Print out each step.
testing_average = testing_average / 2
print("testing_average is", testing_average) # The individual math test works
# We know there must be a problem with the logic in "average"

When your programs become very complex, adding print statements will be a great help.

You Do: Wrapping it Up

Can you fix the code below?

5 minutes.

Teaching Tips:

It’s quite long; they probably want to open the repl.it in a new window. Remind them how to do that.
After 5 minutes or so (pending time), go over it with them.

Repl.it Note:: The code is:

new_phone = Phone(5214)

class Phone:
  def __init__(self, phone_number):
    self.number = phone_number

  def call(self, other_number):
    print("Calling from" self.number, "to", other_number)

  def text(self,  other_number, msg):
    print("Sending text from", self.number, "to", other_number
  print(msg)

test_phone = Phone()
test_phone.call(515)
test_phone.text(int("text 141"), "Hi!")"

Summary and Q&A

Python has many common built-in errors.
Use try-except syntax to catch an expected error.
Logic issues don’t throw errors, so be careful!
Use print statements to walk through your code line-by-line.

Additional Resources

List of Built-In Errors
Error Flowchart PDF
Try-Except Documentation
A deep dive into try/except clauses
To get advanced, add logging to your code.
To get very advanced, include unit tests; the pytest module is great.