Handling Exceptions in Ruby

Handling Exceptions in Ruby is Easy!

Exceptions are Ruby’s way of allowing us to attempt to recover from errors. The basic syntax for handling exceptions in Ruby goes something like this:

begin
  do_something_risky
rescue
  recover_from_exception
end

begin

do_something_risky

rescue

recover_from_exception

end

We can gather more information from an exception like so:

begin
  do_something_risky
rescue StandardError => e
  puts e.message
end

begin

do_something_risky

rescue StandardError => e

puts e.message

end

Notice we rescued StandardError and not Exception. There are many types of exception you might not necessarily want to rescue. For example, pressing CTRL+C or sending SIGTERM to the process would not work as expected and you might have to resort to SIGKILL.

Implicit Contexts

It’s not always necessary to explicitly type begin. Class, module, and method definitions and ruby blocks serve as implicit rescue contexts.

def safe_method
  do_something_risky
rescue StandardError => e
  logger.error e.message
  logger.debug e.backtrace.join("\n")
end

def safe_method

do_something_risky

rescue StandardError => e

logger.error e.message

logger.debug e.backtrace.join("\n")

end

Rescuing exceptions is a good use case for a logger.

Multiple Exception Handling

You can handle multiple types of exception differently. Just be sure to put specific exception types before StandardError if rescue it.

begin
  do_something_risky
rescue WhizBangError => e
  handle_specific e
rescue StandardError => e
  handle_generic e
end

begin

do_something_risky

rescue WhizBangError => e

handle_specific e

rescue StandardError => e

handle_generic e

end

Re-raising Exceptions

You can re-raise an exception after doing something with it. raise knows to re-raise the exception in the rescue context.

begin
  do_something_risky
rescue StandardError => e
  logger.error e.message
  raise
end

begin

do_something_risky

rescue StandardError => e

logger.error e.message

raise

end

Retrying

You can retry the error-prone task after rescuing.

def safe_method
  retried = false
  begin
    do_something_risky
  rescue NotLoggedInError
    log_in
    unless retried
      retried = true
      retry
    end
  end
end

def safe_method

retried = false

begin

do_something_risky

rescue NotLoggedInError

log_in

unless retried

retried = true

retry

end

Just be sure to check whether you’ve already retried first to avoid an endless loop if the fix doesn’t work.

Cleaning Up

In some situations there’s code that needs to run whether there’s an exception or not. The most common example is closing a file handle

begin
  file = File.open 'file', 'w'
  write_to_file file
rescue
  handle_file_error
ensure
  file.close unless file.nil?
end

begin

file = File.open 'file', 'w'

write_to_file file

rescue

handle_file_error

ensure

file.close unless file.nil?

end

Defining Your Own Exceptions

class BarlessFooError < StandardError  
end

1 2	class BarlessFooError < StandardError end

Failing

fail BarlessFooError, 'Foo requires a bar.' if foo.bar.nil?

1	fail BarlessFooError, 'Foo requires a bar.' if foo.bar.nil?

Use fail rather than raise to surface an exception unless you are reraising as above.