Module 12: Quick Reference

Error Handling Cheat Sheet

Common Error Types

Error	Cause	Example
`SyntaxError`	Invalid Python grammar	Missing colon, parenthesis
`NameError`	Undefined variable/function	Typo in variable name
`TypeError`	Wrong data type for operation	`"hello" + 5`
`ValueError`	Right type, wrong value	`int("hello")`
`IndexError`	List index out of range	`list[99]`
`KeyError`	Dictionary key not found	`dict["missing"]`
`FileNotFoundError`	File does not exist	`open("missing.txt")`
`ZeroDivisionError`	Division by zero	`10 / 0`

try / except / else / finally

try:
    # Code that might raise an error
    result = risky_operation()
except SpecificError as e:
    # Runs if SpecificError occurs
    print(f"Error: {e}")
except AnotherError:
    # Catches a different error type
    print("Different error!")
else:
    # Runs ONLY if no error occurred
    print("Success!")
finally:
    # ALWAYS runs (cleanup code)
    print("Done.")

Raising Exceptions

# Raise an exception
raise ValueError("Invalid input")

# Raise in a function
def validate(age):
    if age < 0:
        raise ValueError("Age cannot be negative")

Debugging Checklist

Read the error message (bottom up)
Check the line number in the traceback
Add print() statements to trace values
Comment out code to isolate the bug
Check for = vs ==, indentation, off-by-one errors
Explain your code out loud (rubber duck debugging)
Take a break and come back fresh

Input Validation Pattern

while True:
    try:
        num = int(input("Enter a number: "))
        break  # Exit loop on success
    except ValueError:
        print("Invalid input. Try again.")