exceptions - RDoc Documentation

# `Exception` Handling¶ ↑

Exceptions are rescued in a `begin`/`end` block:

    begin
      # code that might raise
    rescue
      # handle exception
    end

If you are inside a method, you do not need to use `begin` or `end` unless you wish to limit the scope of rescued exceptions:

    def my_method
      # ...
    rescue
      # ...
    end

The same is true for a `class`, `module`, and `block`:

    [0, 1, 2].map do |i|
      10 / i
    rescue ZeroDivisionError
      nil
    end
    #=> [nil, 10, 5]

You can assign the exception to a local variable by using `=> variable_name` at the end of the `rescue` line:

    begin
      # ...
    rescue => exception
      warn exception.message
      raise # re-raise the current exception
    end

By default, `StandardError` and its subclasses are rescued. You can rescue a specific set of exception classes (and their subclasses) by listing them after `rescue`:

    begin
      # ...
    rescue ArgumentError, NameError
      # handle ArgumentError or NameError
    end

You may rescue different types of exceptions in different ways:

    begin
      # ...
    rescue ArgumentError
      # handle ArgumentError
    rescue NameError
      # handle NameError
    rescue
      # handle any StandardError
    end

The exception is matched to the rescue section starting at the top, and matches only once. If an `ArgumentError` is raised in the begin section, it will not be handled in the `StandardError` section.

You may retry rescued exceptions:

    begin
      # ...
    rescue
      # do something that may change the result of the begin block
      retry
    end

Execution will resume at the start of the begin block, so be careful not to create an infinite loop.

Inside a rescue block is the only valid location for `retry`, all other uses will raise a `SyntaxError`. If you wish to retry a block iteration use `redo`. See Control Expressions for details.

To always run some code whether an exception was raised or not, use `ensure`:

    begin
      # ...
    rescue
      # ...
    ensure
      # this always runs BUT does not implicitly return the last evaluated statement.
    end

You may also run some code when an exception is not raised:

    begin
      # ...
    rescue
      # ...
    else
      # this runs only when no exception was raised AND return the last evaluated statement
    ensure
      # this always runs.
      # It is evaluated after the evaluation of either the `rescue` or the `else` block.
      # It will not return implicitly.
    end

NB : Without explicit `return` in the `ensure` block, `begin`/`end` block will return the last evaluated statement before entering in the ‘ensure` block.