DEV Community

K-Sato
K-Sato

Posted on

Ruby Exception handling

#ruby #exception #errors

Table of contents

Basic Syntax

The basic syntax of ruby's exception handling looks like this.

begin # Code that potentially raises an error  rescue => e # variable # Execute if there was an error between begin and rescue. end
Enter fullscreen mode Exit fullscreen mode

Example 

begin raise rescue => e p e #=> RuntimeError end
Enter fullscreen mode Exit fullscreen mode

Relevant methods

Method Explanation
Object#class Returns the class of the receiver.
Kernel#raise Raise an error.
Exception#backtrace Returns the backtrace of the error.
Exception#message Returns the error message.

Example 

begin raise StandardError.new("Message") rescue => e p e.class #=> StandardError p e.message #=> "Message" p e.backtrace #=> ["(repl):4:in `<main>'", "/run_dir/repl.rb:41:in `eval'", "/run_dir/repl.rb:41:in `run'", "/run_dir/repl.rb:57:in `handle_eval'", "/run_dir/repl.rb:170:in `start'", "/run_dir/repl.rb:177:in `start'", "/run_dir/repl.rb:181:in `<main>'"] end
Enter fullscreen mode Exit fullscreen mode

Ensure

You can run something regardless of the existence of an error with ensure

Example 

begin "no Error" rescue => e p e.message ensure p "Ensured" #=> Ensured end
Enter fullscreen mode Exit fullscreen mode

Example2 

begin raise StandardError.new('error') rescue => e p e.message #=> error ensure p "Ensured" #=> Ensured end
Enter fullscreen mode Exit fullscreen mode

Retry

Literally re-try the execution.

Example 

file = ARGV[0] begin # Open a file. io = File.open( file ) rescue # If there was an error during opening the file, this part gets executed. sleep( 10 ) # Goes back to begin and re-try the execution. retry end
Enter fullscreen mode Exit fullscreen mode

Rescue modifier

You can write begin ~ rescue in one sentence just like if ~ end in Ruby.

Example 

raise "Error" rescue p "rescued" #=> rescued
Enter fullscreen mode Exit fullscreen mode

Exception in methods

You don't need to write begin and end to do exception handling in methods.

Example 

def method raise 'error' rescue => e p e.message #=> error ensure p 'ensured' #=> ensured end method #=> "error"
Enter fullscreen mode Exit fullscreen mode

Multiple Exceptions

You can write rescue multiple times for each corresponding error.

begin rescue Exception1, Exception2 => e # For Exception1 or Exception2 rescue Exception3 => e # For Exception3 rescue => e # For other errors. end
Enter fullscreen mode Exit fullscreen mode

Example 

begin raise StandardError rescue StandardError, RangeError p 'Standard or Ranage Error' rescue RuntimeError p 'Runtime Error' rescue => e p 'some other error' end #=> "Standard or Ranage Error"
Enter fullscreen mode Exit fullscreen mode

Custom Exception

Simply Follow the steps below.

(1) Make a new Class

class CustomError < StandardError end
Enter fullscreen mode Exit fullscreen mode

(2) Add a message

class CustomError < StandardError def initialize(msg="My Error") super end end
Enter fullscreen mode Exit fullscreen mode

(3) Add custom data attributes to your exception

class CustomError < StandardError attr_reader :atr def initialize(msg="My Error", atr="atrribute") @atr = atr super(msg) end end
Enter fullscreen mode Exit fullscreen mode

Example 

class CustomError < StandardError attr_reader :atr def initialize(msg="My Error", atr="atrribute") @atr = atr super(msg) end end begin raise CustomError.new("CustomMessage", "Custom Attribute") rescue => e p e.message #=> "CustomMessage" p e.atr #=> "Custom Attribute" end
Enter fullscreen mode Exit fullscreen mode

References

Top comments (4)

Collapse
 
tommydiones profile image
TommyDiones •

Too many codes, that I don't understand. One day I'll be able to read those codes and also know what those means. But for now, I'll just focus on my recovery from drug addiction with the help of the Transcend Recovery Community.

Collapse
 
coderzpro profile image
Coderzpro • • Edited

In ruby, exception managing is a technique which describes a way to deal with the error raised in a software. Here ENGLISH LANGUAGE PROGRAM , error manner an unwanted or unexpected event, which takes place at some stage in the execution of a program, i.E. At run time, that disrupts the regular waft of this system’s instructions. So those sorts of errors have been handled via the rescue block.

Collapse
 
the20co_com profile image
The20Co •

Exception handling is one of the important parts of any programming language. The basic features of exception handling have been around in Ruby since version 1.9 However, if you are not used to a strong typing language and protected mode in Ruby, you may easily miss an easier way to catch and handle exceptions. Congratulations Message For New Home

Collapse
 
meave9786 profile image
meave9786 •

It is the best blog to all over read it thanks for share how do you get robux for free this information and start be great fun any time.