How to write an exception for nil or empty string in Ruby

Question

Started learning Ruby a week ago. The most frustrating part so far has been exception handling. Been looking for an example for what I want to do for hours.

I am looping through an array of API codes, grabbing text via Net::Http. Sometimes the grab returns nil or empty, and I'm trying to test for it in a way that will let me retry the grab on that exception until it works.

I am fairly certain I need to do something like

array.each do |api_key|
  begin
   result = # the code to grab the page via the API key

   if result.empty or result.nil
     raise SomeKindOfExceptionThing
   end

  rescue SomeKindOfExceptionThing
    puts "some error message"
    retry

  else
    # Act on the valid return for result
  end
end

I have no idea how to properly form this so it does what I want. Most of the documentation I've found on exceptions is for handling predefined types of errors or just a generic rescue.

Please, read the documentation carefully. The methods are named nil? and empty? not nil and empty. — Jörg W Mittag
– Jörg W Mittag, Commented Feb 3, 2016 at 22:16

SteveTurczyn · Accepted Answer · 2016-02-04 09:38:35Z

4

As @eugen says your code will work. Not sure I see much benefit in raising the exception, though, in that particular example.

array.each do |api_key|
  result = # the code to grab the page via the API key 
  if result.empty || result.nil
    puts "some error message"
    redo
  end
  # Act on the valid return for result
end

edited Feb 4, 2016 at 9:38

answered Feb 3, 2016 at 21:04

SteveTurczyn

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5 Comments

Stefan Over a year ago

I think you confuse retry and redo.

Amadan Over a year ago

Indeed; retry will go through all api keys again in case of an error, redo will only repeat the failed ones. +1 for doubting exceptions: they are the mechanism for non-local error handling. Instead of raising an exception and immediately rescuing it, if is much easier, shorter, and more performant.

SteveTurczyn Over a year ago

@Stefan absolutely. I copied OP's code and didn't spot the mistake. Answer edited.

SteveTurczyn Over a year ago

@Amadan absolutely. I copied OP's code and didn't spot the mistake. Answer edited.

Skroo Over a year ago

I didn't know you could repeat a loop iteration without an exception. This is great, thanks.

eugen · Accepted Answer · 2016-02-03 20:51:56Z

0

Your code looks fine, you just need to define your exception class. Something like

class SomeKindOfExceptionThing < StandardError
end

should be enough (you might need a better name, though).

answered Feb 3, 2016 at 20:51

eugen

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