exceptions - Documentation for Ruby 4.0 (original) (raw)

Exception Handling

Exceptions are rescued in a begin/end block:

begin

rescue

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

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 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

end

You may rescue different types of exceptions in different ways:

begin

rescue ArgumentError

rescue NameError

rescue

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

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

end

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

begin

rescue

else

ensure

end

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