# 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 'Integer#/'", "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 'Integer#/': 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.

By default, Ruby sets the backtrace of the exception to the location where it was raised.

The developer might adjust this by either providing `backtrace` argument to `Kernel#raise`, or using `Exception#set_backtrace`.

Note that:

- by default, both `backtrace` and `backtrace_locations` represent the same backtrace;

- if the developer sets the backtrace by one of the above methods to an array of `Thread::Backtrace::Location`, they still represent the same backtrace;

- if the developer sets the backtrace to a string or an array of strings:

- by `Kernel#raise`: `backtrace_locations` become `nil`;

- by `Exception#set_backtrace`: `backtrace_locations` preserve the original value;

- if the developer sets the backtrace to `nil` by `Exception#set_backtrace`, `backtrace_locations` preserve the original value; but if the exception is then reraised, both `backtrace` and `backtrace_locations` become the location of reraise.