[RAILS] [Ruby] Exception handling, give back if given

For example, suppose you want to parse certain data and store only specific data in the database. Among them, if there is an extension that cannot be parsed, an error will occur, processing will be interrupted, and a bug will occur.

Therefore, exception handling is convenient, and it is possible to perform processing in the case of an exception case without interrupting the processing.

Indeed, it is possible to realize if given, give back, give back.

This time, let's take a simple calculation as an example and look at exception handling.

Processing when an error occurs begin / rescue

In Ruby, when I try to divide 10 by 0, I get an error.

python

puts 10 / 0
puts "Hello"

divided by 0 (ZeroDivisionError)

The process is interrupted in the middle.

Therefore, --Enclose the part that is likely to be the target of the error with begin. --Describe the processing when an error occurs in rescue.

python

begin
  10 / 0
rescue
  p "Does not break at 0"
end

puts "Hello"

"Does not break at 0"
"Hello"

The processing was not interrupted, and both the error processing and the normal processing were executed.

Abbreviation

Rescue without begin

python

puts 10 / 0 rescue 0
puts 10 / nil rescue 0 

0
0

Store the error content in a variable rescue =>

python

begin
  10 / 0
rescue => e
  puts e
end

puts "Hello"

divided by 0
Hello

The error object can be stored in the variable e and output.

Change the processing for each error rescue "error message object"

python

begin
  10 / 0
rescue NoMethodError
  puts "There is no such method"
rescue ZeroDivisionError
  puts "Does not break at 0"
end

Does not break at 0

Since it corresponds to the second error message, it responds to the rescue.

important point

If the parent of the target exception class is described first, that is processed first.

python

begin
  10 / 0
rescue StandardError
  puts "Basic error"
rescue ZeroDivisionError
  puts "Does not break at 0"
end

Basic error

Raise to generate an explicit error and interrupt the process

The intended use is

1. When the parameters are not what you expected
2. When unauthorized access comes

python

begin
  raise NoMethodError
rescue => e
  p e
end

NoMethodError

Raise your own error

Let's inherit the exception class (StandardError class).

python

class Hoge < StandardError
end

begin
  raise Hoge
rescue => e
  p e
end

#<Hoge: Hoge>

Even if an error occurs, try again from the beginning.

python

num = 0

begin
  puts 10 / num
rescue ZeroDivisionError => e
  puts e
  num = 1
  retry
end

puts "Finished"

divided by 0
10
Finished

In the first loop, there was an error In the second loop, processing is performed normally.

Processing to be performed with or without an error ensure

python

begin
  puts "No exception"
rescue => e
  puts e
ensure
  puts "Hello"
end

No exception
Hello

ensure is always performed at any time.

What is an extra error object?

python

begin
  10 / 0
rescue => e
  puts e.class
  puts e.class.superclass
  puts e.class.superclass.superclass
  puts e.class.superclass.superclass.superclass
end

ZeroDivisionError
StandardError
Exception
Object

Exception is the base class and Object is its parent