## ![](https://s3.amazonaws.com/python-ga/images/GA_Cog_Medium_White_RGB.png) {.separator}

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? ![](https://s3.amazonaws.com/ga-instruction/assets/python-fundamentals/ZeroDivisionError.png) --- ## 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? ![](https://s3.amazonaws.com/ga-instruction/assets/python-fundamentals/ZeroDivisionError.png) --- ## 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 ```python 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? --- ## 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: ```python 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 --- ## Discussion: SyntaxError Let's run the code together. What happens? How can we fix it? --- ## Discussion: TypeError `TypeError` and its message tell us: ```python 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! --- ## ValueError Most commonly caused by trying to convert a bad string into a number. ```python # 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. ```python # 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 ```python 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**. ```python 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? ```python 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! --- ## 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? --- ## Quick Review: Common Errors - If you expect an error, use a try/except block: ```python 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 `print`ing. * Use `print` statements on each line to peek at the values. * Remember to remove debugging statements once the problem is solved! ```python 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? --- ## 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](https://www.tutorialspoint.com/python/standard_exceptions.htm) * [Error Flowchart PDF](https://www.dropbox.com/s/cqsxfws52gulkyx/drawing.pdf) * [Try-Except Documentation](http://www.pythonforbeginners.com/error-handling/python-try-and-except) - [A deep dive into try/except clauses](https://jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/) - To get advanced, add [logging](https://fangpenlin.com/posts/2012/08/26/good-logging-practice-in-python/) to your code. - To get very advanced, include [unit tests](http://www.diveintopython.net/unit_testing/index.html); the [pytest module](http://pythontesting.net/framework/pytest/pytest-introduction/) is great.