Reading Errors and Fixing Bugs

Syntax Errors vs. Exceptions

Errors in Python generally fall into two categories. First are Syntax Errors. Syntax errors, also known as parsing errors. This happens before the code is even run, because Python cannot parse what you wrote.

print('Missing parenthesis'

SyntaxError

Second are Exceptions. Errors detected during execution. These happen while the code is running, even if the syntax is perfectly valid.

result = 10 / 0
print(result)

ZeroDivisionError

Reading a Traceback

When an exception occurs, Python prints a traceback. This shows the context where the exception occurred.

def buggy_function():
    return 'string' + 5

buggy_function()

TypeError

When you see a traceback, don’t panic! Always look at the bottom first. The last line of the error message indicates what happened.

print(unknown_variable)

NameError

Built-in Exceptions

The first word on the last line of an error is the exception type. These exception names are built-in identifiers. Some common ones you will encounter include ValueError, TypeError, NameError, and IndexError.

print(int('hello'))

ValueError

items = [1, 2, 3]
print(items[5])

IndexError

Visualizing Exceptions

You can watch an exception halt execution and propagate using the visualizer:

try:
    def buggy_function():
        return 'string' + 5
    
    buggy_function()
except TypeError as e:
    print(f"Caught exception: {type(e).__name__}")