Exception Handling

🚨 Exception Handling

🛟 begin / rescue / end

Copybegin
  10 / 0
rescue ZeroDivisionError => e
  puts "Can't divide by zero: #{e.message}"
end

begin...end wraps the risky code, and rescue SpecificError => e catches a matching exception, binding it to the local variable e. Multiple rescue clauses are allowed, checked top to bottom, exactly like PHP or Python's multiple catch/except blocks:

Copybegin
  risky_operation
rescue ZeroDivisionError => e
  puts "Math problem: #{e.message}"
rescue ArgumentError => e
  puts "Bad argument: #{e.message}"
rescue => e   # bare rescue -- catches StandardError and its subclasses, see below
  puts "Something else went wrong: #{e.message}"
end

🔒 ensure — Always Runs

Copybegin
  puts "Trying..."
  raise "Something broke"
rescue => e
  puts "Rescued: #{e.message}"
ensure
  puts "Cleaning up, no matter what happened"   # always runs
end

ensure runs whether an exception was raised or not, and even if the rescue block itself re-raises — the direct equivalent of PHP and Python's finally, just with a different keyword. Common use: closing a file handle or a database connection regardless of what happened above it.

⚠️ raise — Throwing Your Own Exceptions

Copyraise "Something went wrong"              # raises a plain RuntimeError with this message
raise ArgumentError, "Age can't be negative"   # raises a specific exception class

A bare raise "message" is a convenient shorthand — Ruby wraps it in a RuntimeError automatically. For anything beyond a quick script, prefer the two-argument form (or a custom class, below) so callers can rescue a specific, meaningful error type instead of every possible RuntimeError in the codebase.

🔁 retry — Ruby-Specific, No Direct PHP/Python Equivalent

Copyattempts = 0

begin
  attempts += 1
  puts "Attempt #{attempts}"
  raise "Simulated failure" if attempts < 3
  puts "Succeeded!"
rescue => e
  retry if attempts < 3   # jumps back to the start of begin, NOT to rescue
  puts "Giving up: #{e.message}"
end

🏗️ Custom Exception Classes

Copyclass InsufficientFundsError < StandardError
  def initialize(msg = "Not enough funds for this transaction")
    super
  end
end

def withdraw(balance, amount)
  raise InsufficientFundsError if amount > balance
  balance - amount
end

begin
  withdraw(50, 100)
rescue InsufficientFundsError => e
  puts e.message   # => Not enough funds for this transaction
end

Custom exceptions inherit from StandardError (not the bare Exception class — see the tip box below), pass a default message to super, and can then be rescued specifically by name — exactly like extending PHP's Exception class or Python's Exception subclassing.

➰ The Modifier rescue

Echoing Chapter 3's modifier if/unless, there's a compact inline form for simple cases:

Copyvalue = Integer("not a number") rescue 0
puts value   # => 0  -- falls back instead of crashing

Useful for a one-line fallback, but avoid it for anything where you'd actually want to know which exception occurred — the modifier form rescues everything indiscriminately, with no way to distinguish error types.

📜 Error Handling, Side by Side

💻 Coding Challenges

🎯 What's Next

Next chapter: Metaprogramming Basics — method_missing, define_method, send, and respond_to?, the tools behind Ruby's reputation for letting code write code.