Ruby JSON parse method has a instance variable quirk

Question

So basically, sometimes when I use JSON.parse and set the returned value to a variable, later in my app that variable will turn out to be NilClass.

What?

Take a look at this: Doing

require 'rest-client'

class Foo
  attr_reader :response
  @response = JSON.parse RestClient.get "http://path/to/api/?params"
end

foo = Foo.new
puts foo.response.class

prints out NilClass. Funky, right? On top of that, this means that all of the data within @response is rendered useless, as its inaccessible being of NilClass. Yet, just printing out foo.response will actually give all of data. Why is that? I made a workaround where within my method I set the Hash to a local variable and return that instead of a instance variable, but that's rather inconvenient.

You're not setting @response on any instance of Foo, but on the class Foo itself. What is your goal? — Mark Reed
– Mark Reed, Commented Jan 30, 2016 at 4:06

sbs · Accepted Answer · 2016-01-30 04:01:08Z

2

From the code given above, the @response is not a instance variable, it's a class instance variable.

foo.instance_variables   # => []
Foo.instance_variables   # => [:@response]

You can change the code like this

class Foo
  attr_reader :response

  def initialize
    @response = JSON.parse RestClient.get "http://path/to/api/?params"
  end
end

edited Jan 30, 2016 at 4:01

answered Jan 30, 2016 at 3:52

sbs

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3 Comments

dude0faw3 Over a year ago

Ok, so what if I change the class to instead be a module? Will the same problem still persist?

Dave Newton Over a year ago

@dude0faw3 What's your actual purpose? Why do you want it to be a module? How will you be trying to use this functionality?

sbs Over a year ago

@dude0faw3 If you change the class to a module, then you cannot initiate a instance, but you can use the module methods or mixin the module to some other class. Depending on how you want to use it in your code.

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