Python - Errors & Exceptions

Overview

Estimated time: 35–45 minutes

Handle errors robustly with Python’s exception system. Learn the EAFP vs LBYL philosophy and when to log vs raise.

Learning Objectives

• Use try/except/else/finally to manage errors.
• Choose EAFP or LBYL appropriately.
• Decide when to log and when to raise exceptions.

Prerequisites

• Python - Files & I/O

try/except/else/finally

try:
    n = int(input("Number: "))
except ValueError:
    print("Not a number")
else:
    print(n * 2)
finally:
    print("done")

Sample Run: Number: x\nNot a number\ndone

EAFP vs LBYL

# LBYL (Look Before You Leap)
s = input("Enter integer: ")
if s.isdigit():
    print(int(s) * 2)
else:
    print("Not a number")

# EAFP (Easier to Ask Forgiveness than Permission)
try:
    print(int(s) * 2)
except ValueError:
    print("Not a number")

Common Pitfalls

• Broad except clauses hiding bugs; prefer specific exceptions.
• Logging and re-raising the same exception multiple times (duplicate signals).

Checks for Understanding

1. What is the difference between EAFP and LBYL?
2. Why avoid bare except:?

Exercises

1. Open a file given by the user and handle the case where it does not exist.
2. Convert a list of strings to integers using EAFP; skip invalid entries with a warning.