A raised exception can be rescued to prevent it from crashing your application once it reaches the top of the call stack. In Ruby, we use the rescue
keyword for that.
When rescuing an exception in Ruby, you can specify a specific error class that should be rescued from.
begin
raise 'This exception will be rescued!'
rescue StandardError => e
puts "Rescued: #{e.inspect}"
end
Note: When using raise
without specifying an exception class, Ruby will default to RuntimeError
.
Besides specifying a single exception class to rescue, you can pass an array of exception classes to the rescue
keyword. This will allow you to respond to multiple errors in the same way.
begin
raise 'This exception will be rescued!'
rescue StandardError, AnotherError => e
puts "Rescued: #{e.inspect}"
end
Multiple rescue
blocks can be used to handle different errors in different ways. This can be useful when working with a library that produces different exceptions for different scenarios.
begin
raise 'This exception will be rescued!'
rescue StandardError => e
puts "Rescued: #{e.inspect}"
rescue AnotherError => e
puts "Rescued, but with a different block: #{e.inspect}"
end
The exception hierarchy
Ruby’s exception hierarchy is used to differentiate between different types of errors, while giving you the ability to rescue from a group of errors without specifying all of them.
Although libraries can define their own exception subclasses, the list of built-in exception subclasses on Ruby 2.5 looks like this:
- NoMemoryError
- ScriptError
- LoadError
- NotImplementedError
- SyntaxError
- SecurityError
- SignalException
- Interrupt
- StandardError (default for `rescue`)
- ArgumentError
- UncaughtThrowError
- EncodingError
- FiberError
- IOError
- EOFError
- IndexError
- KeyError
- StopIteration
- LocalJumpError
- NameError
- NoMethodError
- RangeError
- FloatDomainError
- RegexpError
- RuntimeError (default for `raise`)
- SystemCallError
- Errno::*
- ThreadError
- TypeError
- ZeroDivisionError
- SystemExit
- SystemStackError
- fatal (impossible to rescue)
When omitting the exception class in a rescue
block, StandardError
is assumed. Because ArgumentError
and NoMethodError
are subclasses of StandardError
, these are rescued from when they occur in the block.
A good example of how the exception hierarchy works is SystemCallError
, which is a low-level platform-dependent exception class. It’s seen most often when reading or writing to files.
Ruby’s File.read
method will raise an exception if it fails to read the file. That can happen because of a number of reasons, like the file not existing or the program not having the correct permissions to read it.
Since these problems are platform-dependent, Ruby can raise different exceptions depending on what kind of operating system is running on the machine. For low-level errors like this, Ruby implements a different list of Errno::*
-exceptions for each platform.
All of these Errno::*
exceptions are subclasses of SystemCallError
. Although they’re platform-specific, they can still be used in a rescue
block by rescuing from SystemCallError
.
begin
File.read("does/not/exist")
rescue SystemCallError => e
puts "Rescued: #{e.inspect}"
end
Swallowing exceptions
Usually, it’s best to be as specific as possible when rescuing exceptions, to prevent unintentionally swallowing exceptions.
image = nil
begin
File.read(image.filename)
rescue
puts "File can't be read!"
end
In this example, the image
variable is nil
, so it raises a NoMethodError
when we try to call #filename
on it (NoMethodError: undefined method `filename' for nil:NilClass
). Because every StandardError
subclass is rescued from (including NoMethodError
), the exception is swallowed and the “File can’t be read!”-message is printed. This hides a possible bug in the code.
Note: Although it’s possible, using the Exception
superclass in a rescue
block is highly discouraged.
Have any questions about raising or rescuing exceptions in Ruby? Please don't hesitate to let us know at @AppSignal. Of course, we’d love to know how you liked this article, or if you have another subject you’d like to know more about.
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