ruby/doc/exceptions.md

Exceptions

Ruby code can raise exceptions.

Most often, a raised exception is meant to alert the running program that an unusual (i.e., exceptional) situation has arisen, and may need to be handled.

Code throughout the Ruby core, Ruby standard library, and Ruby gems generates exceptions in certain circumstances:

File.open('nope.txt') # Raises Errno::ENOENT: "No such file or directory"

Raised Exceptions

A raised exception transfers program execution, one way or another.

Unrescued Exceptions

If an exception not rescued (see Rescued Exceptions below), execution transfers to code in the Ruby interpreter that prints a message and exits the program (or thread):

$ ruby -e "raise"
-e:1:in `<main>': unhandled exception

Rescued Exceptions

An exception handler may determine what is to happen when an exception is raised; the handler may rescue an exception, and may prevent the program from exiting.

A simple example:

begin
  raise 'Boom!'                # Raises an exception, transfers control.
  puts 'Will not get here.'
rescue
  puts 'Rescued an exception.' # Control transferred to here; program does not exit.
end
puts 'Got here.'

Output:

Rescued an exception.
Got here.

An exception handler has several elements:

Element	Use
Begin clause.	Begins the handler and contains the code whose raised exception, if any, may be rescued.
One or more rescue clauses.	Each contains "rescuing" code, which is to be executed for certain exceptions.
Else clause (optional).	Contains code to be executed if no exception is raised.
Ensure clause (optional).	Contains code to be executed whether or not an exception is raised, or is rescued.
end statement.	Ends the handler. `

Begin Clause

The begin clause begins the exception handler:

• May start with a begin statement; see also Begin-Less Exception Handlers.
• Contains code whose raised exception (if any) is covered by the handler.
• Ends with the first following rescue statement.

Rescue Clauses

A rescue clause:

• Starts with a rescue statement.
• Contains code that is to be executed for certain raised exceptions.
• Ends with the first following rescue, else, ensure, or end statement.

Rescued Exceptions

A rescue statement may include one or more classes that are to be rescued; if none is given, StandardError is assumed.

The rescue clause rescues both the specified class (or StandardError if none given) or any of its subclasses; see Built-In Exception Class Hierarchy.

begin
  1 / 0 # Raises ZeroDivisionError, a subclass of StandardError.
rescue
  puts "Rescued #{$!.class}"
end

Output:

Rescued ZeroDivisionError

If the rescue statement specifies an exception class, only that class (or one of its subclasses) is rescued; this example exits with a ZeroDivisionError, which was not rescued because it is not ArgumentError or one of its subclasses:

begin
  1 / 0
rescue ArgumentError
  puts "Rescued #{$!.class}"
end

A rescue statement may specify multiple classes, which means that its code rescues an exception of any of the given classes (or their subclasses):

begin
  1 / 0
rescue FloatDomainError, ZeroDivisionError
  puts "Rescued #{$!.class}"
end

Multiple Rescue Clauses

An exception handler may contain multiple rescue clauses; in that case, the first clause that rescues the exception does so, and those before and after are ignored:

begin
  Dir.open('nosuch')
rescue Errno::ENOTDIR
  puts "Rescued #{$!.class}"
rescue Errno::ENOENT
  puts "Rescued #{$!.class}"
end

Output:

Rescued Errno::ENOENT

Capturing the Rescued \Exception

A rescue statement may specify a variable whose value becomes the rescued exception (an instance of Exception or one of its subclasses:

begin
  1 / 0
rescue => x
  puts x.class
  puts x.message
end

Output:

ZeroDivisionError
divided by 0

Global Variables

Two read-only global variables always have nil value except in a rescue clause; there:

• $!: contains the rescued exception.
• $@: contains its backtrace.

Example:

begin
  1 / 0
rescue
  p $!
  p $@
end

Output:

#<ZeroDivisionError: divided by 0>
["t.rb:2:in `/'", "t.rb:2:in `<main>'"]

Cause

In a rescue clause, the method Exception#cause returns the previous value of $!, which may be nil; elsewhere, the method returns nil.

Example:

begin
  raise('Boom 0')
rescue => x0
  puts "Exception: #{x0.inspect};  $!: #{$!.inspect};  cause: #{x0.cause.inspect}."
  begin
    raise('Boom 1')
  rescue => x1
    puts "Exception: #{x1.inspect};  $!: #{$!.inspect};  cause: #{x1.cause.inspect}."
    begin
      raise('Boom 2')
    rescue => x2
      puts "Exception: #{x2.inspect};  $!: #{$!.inspect};  cause: #{x2.cause.inspect}."
    end
  end
end

Output:

Exception: #<RuntimeError: Boom 0>;  $!: #<RuntimeError: Boom 0>;  cause: nil.
Exception: #<RuntimeError: Boom 1>;  $!: #<RuntimeError: Boom 1>;  cause: #<RuntimeError: Boom 0>.
Exception: #<RuntimeError: Boom 2>;  $!: #<RuntimeError: Boom 2>;  cause: #<RuntimeError: Boom 1>.

Else Clause

The else clause:

• Starts with an else statement.
• Contains code that is to be executed if no exception is raised in the begin clause.
• Ends with the first following ensure or end statement.

begin
  puts 'Begin.'
rescue
  puts 'Rescued an exception!'
else
  puts 'No exception raised.'
end

Output:

Begin.
No exception raised.

Ensure Clause

The ensure clause:

• Starts with an ensure statement.
• Contains code that is to be executed regardless of whether an exception is raised, and regardless of whether a raised exception is handled.
• Ends with the first following end statement.

def foo(boom: false)
  puts 'Begin.'
  raise 'Boom!' if boom
rescue
  puts 'Rescued an exception!'
else
  puts 'No exception raised.'
ensure
  puts 'Always do this.'
end

foo(boom: true)
foo(boom: false)

Output:

Begin.
Rescued an exception!
Always do this.
Begin.
No exception raised.
Always do this.

End Statement

The end statement ends the handler.

Code following it is reached only if any raised exception is rescued.

Begin-Less \Exception Handlers

As seen above, an exception handler may be implemented with begin and end.

An exception handler may also be implemented as:

• A method body:

  def foo(boom: false) # Serves as beginning of exception handler.
    puts 'Begin.'
    raise 'Boom!' if boom
  rescue
    puts 'Rescued an exception!'
  else
    puts 'No exception raised.'
  end                  # Serves as end of exception handler.
  

• A block:

  Dir.chdir('.') do |dir| # Serves as beginning of exception handler.
    raise 'Boom!'
  rescue
    puts 'Rescued an exception!'
  end                     # Serves as end of exception handler.
  

Re-Raising an \Exception

It can be useful to rescue an exception, but allow its eventual effect; for example, a program can rescue an exception, log data about it, and then "reinstate" the exception.

This may be done via the raise method, but in a special way; a rescuing clause:

• Captures an exception.
• Does whatever is needed concerning the exception (such as logging it).
• Calls method raise with no argument, which raises the rescued exception:

begin
  1 / 0
rescue ZeroDivisionError
  # Do needful things (like logging).
  raise # Raised exception will be ZeroDivisionError, not RuntimeError.
end

Output:

ruby t.rb
t.rb:2:in `/': divided by 0 (ZeroDivisionError)
        from t.rb:2:in `<main>'

Retrying

It can be useful to retry a begin clause; for example, if it must access a possibly-volatile resource (such as a web page), it can be useful to try the access more than once (in the hope that it may become available):

retries = 0
begin
  puts "Try ##{retries}."
  raise 'Boom'
rescue
  puts "Rescued retry ##{retries}."
  if (retries += 1) < 3
    puts 'Retrying'
    retry
  else
    puts 'Giving up.'
    raise
  end
end

Try #0.
Rescued retry #0.
Retrying
Try #1.
Rescued retry #1.
Retrying
Try #2.
Rescued retry #2.
Giving up.
# RuntimeError ('Boom') raised.

Note that the retry re-executes the entire begin clause, not just the part after the point of failure.

Raising an \Exception

\Method Kernel#raise raises an exception.

Custom Exceptions

To provide additional or alternate information, you may create custom exception classes. Each should be a subclass of one of the built-in exception classes (commonly StandardError or RuntimeError); see Built-In Exception Class Hierarchy.

class MyException < StandardError; end

Messages

Every Exception object has a message, which is a string that is set at the time the object is created; see Exception.new.

The message cannot be changed, but you can create a similar object with a different message; see Exception#exception.

This method returns the message as defined:

• Exception#message.

Two other methods return enhanced versions of the message:

• Exception#detailed_message: adds exception class name, with optional highlighting.
• Exception#full_message: adds exception class name and backtrace, with optional highlighting.

Each of the two methods above accepts keyword argument highlight; if the value of keyword highlight is true, the returned string includes bolding and underlining ANSI codes (see below) to enhance the appearance of the message.

Any exception class (Ruby or custom) may choose to override either of these methods, and may choose to interpret keyword argument highlight: true to mean that the returned message should contain ANSI codes that specify color, bolding, and underlining.

Because the enhanced message may be written to a non-terminal device (e.g., into an HTML page), it is best to limit the ANSI codes to these widely-supported codes:

• Begin font color:

  Color	ANSI Code
  Red	\e[31m
  Green	\e[32m
  Yellow	\e[33m
  Blue	\e[34m
  Magenta	\e[35m
  Cyan	\e[36m

• Begin font attribute:

  Attribute	ANSI Code
  Bold	\e[1m
  Underline	\e[4m

• End all of the above:

  Color	ANSI Code
  Reset	\e[0m

It's also best to craft a message that is conveniently human-readable, even if the ANSI codes are included "as-is" (rather than interpreted as font directives).

Backtraces

A backtrace is a record of the methods currently in the call stack; each such method has been called, but has not yet returned.

These methods return backtrace information:

• Exception#backtrace: returns the backtrace as an array of strings or nil.
• Exception#backtrace_locations: returns the backtrace as an array of Thread::Backtrace::Location objects or nil. Each Thread::Backtrace::Location object gives detailed information about a called method.

An Exception object stores its backtrace value as one of:

• An array of Thread::Backtrace::Location objects; this is the common case: the exception was raised by the Ruby core or the Ruby standard library. In this case:

  • Exception#backtrace_locations returns the array of Thread::Backtrace::Location objects.
  • Exception#backtrace returns the array of their string values (Exception#backtrace_locations.map {|loc| loc.to_s }).

• An array of strings; this is an uncommon case: the user manually set the backtrace to an array of strings; In this case:

  • Exception#backtrace returns the array of strings.
  • Exception#backtrace_locations returns nil.

• nil, in which case both methods return nil.

These methods set the backtrace value:

• Exception#set_backtrace: sets the backtrace value to an array of strings, or to nil.
• Kernel#raise: sets the backtrace value to an array of Thread::Backtrace::Location objects, or to an array of strings.