Python Error Handling

 # Goal: Handle errors gracefully

# Topics:

# • try-except blocks

# • Common exceptions (ValueError, FileNotFoundError)

# • finally clause

# • Basic debugging with print statements

# ── 1. Basic try-except ───────────────────────────────────────────────────────

try:

x = int("abc") # raises ValueError

except ValueError:

print("Cannot convert 'abc' to int.") # Cannot convert 'abc' to int.

# ── 2. ValueError ─────────────────────────────────────────────────────────────

def get_age(value):

try:

age = int(value)

print(f"Age is {age}.") # Age is 25. (if valid)

except ValueError:

print(f"'{value}' is not a valid age.") # 'twenty' is not a valid age.

get_age("25") # Age is 25.

get_age("twenty") # 'twenty' is not a valid age.

# ── 3. FileNotFoundError ──────────────────────────────────────────────────────

try:

with open("missing.txt", "r") as f: # file does not exist

content = f.read()

except FileNotFoundError:

print("File not found. Check the filename.") # File not found. Check the filename.

# ── 4. Multiple except Clauses ────────────────────────────────────────────────

def divide(a, b):

try:

result = a / b

print(f"{a} / {b} = {result}") # 10 / 2 = 5.0

except ZeroDivisionError:

print("Cannot divide by zero.") # Cannot divide by zero.

except TypeError:

print("Both values must be numbers.") # Both values must be numbers.

divide(10, 2) # 10 / 2 = 5.0

divide(10, 0) # Cannot divide by zero.

divide(10, "a") # Both values must be numbers.

# ── 5. finally Clause ─────────────────────────────────────────────────────────

# finally always runs — whether an exception occurred or not

def read_file(filename):

try:

f = open(filename, "r")

print(f.read()) # prints file content if found

except FileNotFoundError:

print(f"'{filename}' not found.") # 'data.txt' not found.

finally:

print("Attempted file operation.") # always prints

read_file("data.txt")

# 'data.txt' not found.

# Attempted file operation.

# ── 6. else Clause ────────────────────────────────────────────────────────────

# else runs only if NO exception was raised

try:

num = int("99") # succeeds

except ValueError:

print("Conversion failed.")

else:

print(f"Conversion succeeded: {num}") # Conversion succeeded: 99

# ── 7. Basic Debugging with print ────────────────────────────────────────────

def calculate_discount(price, discount):

print(f"[DEBUG] price={price}, discount={discount}") # [DEBUG] price=200, discount=0.1

try:

result = price - (price * discount)

print(f"[DEBUG] result={result}") # [DEBUG] result=180.0

return result

except TypeError as e:

print(f"[ERROR] {e}") # [ERROR] unsupported operand...

return None

final = calculate_discount(200, 0.1)

print(f"Final price: {final}") # Final price: 180.0

calculate_discount(200, "ten") # [ERROR] unsupported operand type(s)...

# ── 8. Mini Project: Safe Number Input ───────────────────────────────────────

print("\n--- Safe Number Input ---")

def get_number(prompt):

while True:

try:

value = float(input(prompt)) # raises ValueError if not a number

return value

except ValueError:

print("Invalid input. Please enter a number.") # Invalid input. Please enter a number.

a = get_number("Enter first number : ")

b = get_number("Enter second number: ")

try:

print(f"\n{a} / {b} = {a / b:.4f}") # e.g. 10.0 / 4.0 = 2.5000

except ZeroDivisionError:

print("Cannot divide by zero.") # Cannot divide by zero.

finally:

print("Done.") # Done. (always prints)