Debugging

Debugging#

Errors
- Syntax
- Exceptions
Debugging Process
- Stack trace
- Using outside resources
try/except

Debugging is the process of finding and fixing errors in a computer program.

Errors#

Errors are problems with code definition or execution that interrupt running Python code.

Syntax Errors#

Syntax Errors
Indentation Errors

Syntax & Indentation Errors reflect code that doesn't follow Python structure, and will necessarily fail.

Syntax Error Examples#

# will produce a syntax error
if True
    print('Yep.')

Python does its best to tell you:

what type of error it is
and where it thinks it occurred (^)

# will produce a syntax error
# and specifically an indentation error
if 3 < 5:
print('It\'s smaller!')

Python gives you a readout about what it was expecting and where you appear to have gone wrong.

Exceptions#

Exceptions are errors that occur when a code is executed.

For these, there’s nothing wrong with the syntax or structure of the code, but in your specific case and how you’re trying to use it, Python says ‘no’.

ZeroDivisionError#

ZeroDivisionError occurs when you try to divide by zero.

# produces ZeroDivisionError
1 / 0

Python specifies:

the Exception and specific type / error
points you to where the error occurred

NameError#

NameError occurs when you try to access a name that Python does not know.

# Define a variable
variable = 12

# If you typo a name, you will get a NameError
varaible

While it’s annoying, it’s helpful that Python doesn’t just guess that you meant ‘variable’….because sometimes Python would guess wrong. It’s better for Python to just give us the error.

# You also get a name error if you try to use the wrong operator for assignment
new_variable == 1

IndexError#

IndexError occurs when you try to access an index that doesn’t exist.

my_string = 'COGS18'
my_string[6]

# Relatedly, 'KeyError' occurs if you ask for a dictionary key that doesn't exist
my_dictionary = {'name1' : 1, 'name2' : 2}
my_dictionary['name3']

ValueError#

ValueError occurs when you try to use an illegal value for something.

int('cat')

TypeError#

TypeError occurs when you try to do something with a variable type that python cannot interpret.

'a_string' + 12

Error Recap#

Syntax Errors
- SyntaxError
- IndentationError
Exceptions
- ZeroDivisionError
- NameError
- Index Errors
  - IndexError
  - KeyError
- ValueError
- TypeError

Class Question #1#

What type of error will the following code produce?

if num > 0
    print("Greater than 0")

A) Syntax
B) Name
C) Type
D) Index
E) Value

Class Question #2#

What type of error will the following code produce?

if num > 0:
    print("Greater than 0")

A) Syntax
B) Name
C) Type
D) Index
E) Value

Stack Trace#

Read (and understand!) your error messages!

The trace (log) of what Python did as it went through your code. Gets printed out if Python runs into an error.

For example…say you’re trying to write a function that takes a character as input, turns it into its unicode code point…with an offset of 500, turns that back into a character and returns the output…Your first attempt (which has errors) is below:

def encode_char(char):
    # turn into code point
    ord(char)
    # add offset
    char + 500 = offset
    # turn into character
    chr(offset)

What to look for and think about:

What kind of error is it?
What line of code/where in the line of code is the error pointing to?
What does it mean? How do I fix this?

Sometimes these get really complex. We’re here to get better at interpreting these traces. Note that if external functions are being used, these will get longer.

Sometimes you even have assert messages to guide you…how should you use assert messages?

Read them; understand them
Revisit the part of your code most likely associated with the assert
Think about what you would need to change to fix the issue (do not just guess wildly trying to pass the assert - will waste your time)
Fix the code; re-execute; re-run the assert

assert callable(encode_char)
assert type(encode_char('c')) == str
assert encode_char('c') == 'ɗ'

Helping yourself when debugging#

Pause and think
You aren’t sure how to approach a problem: Google it
You don’t understand an error message:
- Use print() statements
- Google the error message…but then you need to understand what you’re reading

Unsure how to approach a problem?#

“Write code that will check whether the value stored in my_val is both positive and even. If it is, store the integer 1 in the variable output”

…How do I check whether a variable stores an even value?

Google: “How to check whether a variable stores an even value in python”

ChatGPT: How would I write a function in python to determine whether a variable stores an even value?

Don’t understand the error message?#

So you try to accomplish the task…below is your first attempt

# this code has errors
# we're going to debug together in class

if my_val > 0 and my_val / 2 = 0
output == 1

Try / Except#

Exceptions do not necessarily have to lead to breaking the program - they can be programmatically dealt with, using try and except.

For this example, I’m going to introduce input(), a function which allows us to get input from the user.

# Example: we want to get an input number from the user

my_num = input("Please type a number: ")

print('\nmy_num is: ', my_num)

`try` / `except` Block#

try:
    int(input('Number: '))
    print("A valid number was inputted")

except:
    print("That was not a valid number")

Raising Errors#

You can also write code to raise an Exception if something unexpected happens.

Raise Exception Examples#

raise is a keyword that tells Python you want to create your own error.

my_int = input('An integer please: ')
if not my_int.isnumeric():
    raise ValueError('I wanted a number! :(')
    
print('My integer is: ', my_int) 

Class Question #3#

Edit the code below (replacing --- with either values or variable names) so that when executed, this cell prints None.

num1 = ---
num2 = ---

try:
    output = num1 / num2
except ZeroDivisionError:
    output = None
    
print(output)

A) I did it!
B) I think I did it…
C) I’m totally lost.