intermediatePython Framework • Core Concepts

Errors & Exceptions

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Introduction to Errors

Errors fall into two categories: Syntax Errors (parsing issues, like missing colons) and Runtime Errors (exceptions that occur during execution, like dividing by zero). We focus here on handling Runtime Errors.

Python
# Syntax Error (code won't run)
# if True print("Hello")  # Missing colon

# Runtime Error (crashes while running)
num = int("abc")  # ValueError

Basic Try & Except

Use a try-except block to 'catch' errors and prevent your program from crashing. If an error occurs in the 'try' block, execution jumps immediately to the 'except' block.

Python
try:
    x = int("not a number")
    print("This won't print")
except ValueError:
    print("Invalid conversion caught!")

print("Program continues...")

Flow of handled error

Start
↓
int('abc')
↓
if
ValueError?
Yes
↓
Print 'Invalid'
↓
Next line
No
↓
Next line
Start/End
Condition
Action

Catching Multiple Exceptions

You can handle different errors differently using multiple except blocks. Code execution enters the first matching block.

Python
try:
    num = int(input("Enter number: "))
    print(10 / num)
except ValueError:
    print("That's not a number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")
except Exception as e:
    print(f"Unknown error: {e}")

Exception Routing

Try
↓
if
ValueError?
Yes
↓
Handle Value
↓
Continue
No
↓
if
ZeroDivision?
Yes
↓
Handle Zero
↓
Continue
No
↓
Continue
Start/End
Condition
Action

Else and Finally

'else' runs if NO exception occurs. 'finally' runs ALWAYS, regardless of errors—perfect for cleanup like closing files.

Python
try:
    f = open("data.txt")
except FileNotFoundError:
    print("File missing")
else:
    print("File opened!")
    content = f.read()
finally:
    print("Closing cleanup...")
    # f.close() if valid

Execution Flow with Finally

Start
↓
Try Block
↓
if
Error?
Yes
↓
Except
↓
Finally (Always)
↓
End
No
↓
Else
↓
Finally (Always)
↓
End
Start/End
Condition
Action

Raising Exceptions

You can intentionally trigger an error using the 'raise' keyword. This is useful for enforcing rules in your functions.

Python
def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    print(f"Age set to {age}")

try:
    set_age(-5)
except ValueError as e:
    print(e)  # "Age cannot be negative"

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Visual Lab

Try/Except Flow

Start
↓
Try Block Code
↓
if
Error Occurred?
Yes
↓
Except Block
↓
Continue Execution
No
↓
Continue Execution
Start/End
Condition
Action

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