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  • Exception Handling in Rails using begin rescue

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    Exception Handling in Rails using begin rescue

    Exception is a condition, that occurs when something goes wrong in a code. Normally in that case the program gets terminated. In rails, an object containing the information of the error is an instance of Exception class, or one of class Exception's children. A sample hierarchy for Exception class is as given below in the diagram:


    Now as we don't want the program to halt in any case, so we need to handle the exception. Rails provide an easy and effective way of handling exceptions. One of the way is using begin..rescue..end:

    begin..rescue..end

    In this approach whatever code we want to write we write it inside begin and rescue and we handle it inside rescue and end. beside rescue we can give the class of the exception or its superclass, then the rescue will handle only that kind of exception. A common syntax can be:

    begin 
        # some code 
    rescue OneTypeException
        # some code 
    rescue OtherTypeException 
        # some
    else 
        # some other exceptions
    ensure
        # It will always get executed
    end

    Lets understand the above syntax, any code written between begin and rescue is monitored, so if the exception relates to the class specified with rescue class or its subclass then the thrown exception is handled. If there is no exception then the else block gets executed. The code inside ensure block always gets executed.

    Example:

    begin
      file = open("no_file_exist")
    rescue StandardError => e
      print "Inside Exception Class\n"
      print e + "\n"
      print "\n"
    else
        print "Inside else block\n"
    ensure
        print "Inside ensure block\n"
    end
    
    #=> Output will be
    
    Inside Exception Class
    No such file or directory @ rb_sysopen - no_file_exist
    Inside ensure block
    

    Thus the call in the above example, first came to StandardError rescue block and also went to ensure block. Lets see if there is no exception then what will happen

    
    begin
      x = 10 + 5
      print "Output is : " + x.to_s + "\n"
    rescue StandardError => e
      print "Inside Excetion Class\n"
      print e
      print "\n"
    else
        print "Inside else block\n"
    ensure
        print "Inside ensure block\n"
    end
    
    # Output will be:
    Output is : 15
    Inside else block
    Inside ensure block

    Thus we can see that here the whole code inside the begin-rescue block got executed but the rescue part got ommitted and again the else part and the ensure part got executed.

    Hope you liked reading this blog.

    Exception Handling in Rails using begin rescue

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