Python Exception Handling

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Exception Handling

Exception Handling is used to manage runtime errors and prevent programs from crashing.

Exceptions occur when:

  • User enters invalid input
  • File not found
  • Network connection fails
  • Wrong operations (divide by zero)

Basic Structure

try:
    # Code that may throw error
except:
    # Code to run if error occurs

Basic Example

try:
    a = 10 / 0
except:
    print("Cannot divide by zero!")

Catching Specific Exceptions


try:
    a = int("abc")
except ValueError:
    print("Value Error occurred: Invalid conversion to integer")

Multiple Except Blocks

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ZeroDivisionError:
    print("You cannot divide by zero.")
except ValueError:
    print("Only numbers are allowed.")

Using else Block

Runs only when no exception occurs.

try:
    x = int(input("Enter age: "))
except ValueError:
    print("Invalid input.")
else:
    print("Valid age:", x)

Using finally Block

Always runs — even if an error occurs.

try:
    f = open("data.txt")
except FileNotFoundError:
    print("File not found!")
finally:
    print("Program finished.")

Raise an Exception Yourself

Use raise for rules/validation.

age = -5
if age < 0:
    raise ValueError("Age cannot be negative")

Custom Exception

class InvalidAgeError(Exception):
    pass

age = 15
if age < 18:
    raise InvalidAgeError("You must be 18+ to sign up")

File Handling with Exception

try:
    f = open("notes.txt", "r")
    print(f.read())
except FileNotFoundError:
    print("File does not exist.")
finally:
    print("Closing file if opened.")

Multiple Exceptions in One Line

try:
    x = 10 / int("a")
except (ZeroDivisionError, ValueError):
    print("Error: Invalid operation")

Catch All Exceptions 

try:
    a = 10 / 0
except Exception as e:
    print("Error occurred:", e)

Common Built-in Exceptions

ExceptionWhen it Occurs
ZeroDivisionErrorDivide by zero
ValueErrorWrong data type in conversion
TypeErrorUnsupported operations between types
FileNotFoundErrorFile does not exist
KeyErrorKey not found in dictionary
IndexErrorIndex out of range
NameErrorVariable not defined
ImportErrorModule not found

Real-Time Examples

User Input Validation

while True:
    try:
        amount = float(input("Enter amount: "))
        break
    except ValueError:
        print("Enter a valid number!")

Retry File Opening Automatically

for _ in range(3):
    try:
        f = open("config.json")
        print("File opened!")
        break
    except FileNotFoundError:
        print("File missing, retrying...")

API Request Error Handling (Concept)

try:
    data = fetch_api_data()
except ConnectionError:
    print("Network issue!")

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