Lesson 35/48 · 🌍 Real World Python

Python Foundations🌍 Real World PythonLesson 35/48

Phase 7 · Real World Python22 min

Error Handling, try / except

Write programs that survive mistakes instead of crashing

Every real program will encounter errors, a file that doesn't exist, invalid user input, a network timeout. Exception handling lets your program deal with these situations gracefully instead of crashing.

How would you approach this? Think first, then expand.

Basic try/exceptpython

# Without error handling, crashes on bad input
number = int("abc")   # ValueError: invalid literal for int()

# With error handling
try:
    number = int("abc")
    print(f"Got: {number}")
except ValueError:
    print("That wasn't a valid number!")
# Output: That wasn't a valid number!

🔍Code TracerStep 1 / 4

trace.py

1try:←

2 x = int("hello")

3 print("no error")

4except ValueError as e:

5 print(f"Caught: {e}")

Variables

No variables yet

Step 1 / 4

How it works:

Python tries to run the try block

If an exception occurs, it jumps to the matching except block

If no exception occurs, the except block is skipped

Catching specific exception types
Always catch the most specific exception you can, it makes bugs easier to diagnose:

Multiple except clausespython

def safe_divide(a, b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        print("Error: cannot divide by zero")
        return None
    except TypeError:
        print("Error: both arguments must be numbers")
        return None

print(safe_divide(10, 2))   # 5.0
print(safe_divide(10, 0))   # Error: cannot divide by zero
print(safe_divide(10, "x")) # Error: both arguments must be numbers

🤔Quick Check

What is the correct order for except clauses when catching both ValueError and Exception?

else and finally

else: runs only if NO exception occurred

finally: ALWAYS runs, regardless of whether an exception occurred (used for cleanup)

else and finallypython

def read_number():
    try:
        n = int(input("Enter a number: "))
    except ValueError:
        print("Not a valid number")
    else:
        print(f"You entered: {n}")   # only if no exception
    finally:
        print("Done.")               # always runs

# Simulating with a fixed value:
try:
    n = int("42")
except ValueError:
    print("Not a valid number")
else:
    print(f"You entered: {n}")
finally:
    print("Done.")
# You entered: 42
# Done.

raise, trigger an exception yourself
Use raise to signal that something is wrong in your own code:

Raising exceptionspython

def set_age(age):
    if age < 0 or age > 150:
        raise ValueError(f"Age {age} is not realistic")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(f"Error: {e}")   # Error: Age -5 is not realistic

🤔Quick Check

When does the finally block execute?

🌍In the Real World...

Your pipeline script must handle: API timeouts, missing config files, invalid YAML, permission denied errors, network failures. try/except is the difference between a script that crashes and one that recovers.

Practice Exercises

0/3 solved

Exercise 1 of 3easyGuided

⏱ 00:00

Safe Integer Input

Wrap the int() conversion in a try/except so that invalid input prints an error message instead of crashing.
Given: user_input = "hello"Expected output: Error: "hello" is not a valid integer

solution.py

1 / 3

Exercise 2 of 3easy

⏱ 00:00

Safe Division

Write a function divide(a, b) that returns a / b but prints "Cannot divide by zero" and returns None if b is 0.
- print(divide(10, 2)) → 5.0- divide(10, 0) → prints Cannot divide by zero, returns None

solution.py

2 / 3

Exercise 3 of 3medium

⏱ 00:00

Validate Age

Write a function validate_age(age) that:- Raises ValueError if age is not an integer- Raises ValueError if age < 0 or > 120- Returns age if valid
Call it with age = -5 and catch the error:Expected output: Invalid age: -5 is out of range

solution.py

3 / 3

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