Mocks & Stubs & Exceptions in Ruby
Some of my recent work has been around improving error handling and logging.
We had some tasks that, if they failed to execute correctly, were supposed to raise exceptions, log themselves, and re-queue, but they were not.
The class in which I was working managed in large part API calls to external services, services that our team has no control over. Sometimes the services work great, sometimes they don’t. The tests for this class made heavy use of mocks, stubs, and the VCR gem.
This post was originally going to be about error handling, but then I realized I was getting a bit crossed up just by the mocks and stubs, so I took a quick detour into the topic, in order to build out a mental model of what was going on.
As I often do, I spun up a bare-bones implementation of all of the required pieces, to see how everything was playing together. One guide I found was excellent, but the sample app was a bit over-kill. I didn’t want a whole new rails app - I just wanted a class and test file.
note: This isn’t a tutorial, per se, but it’s just two files and you can easily copy-paste the code into an editor and run the tests. I’ll link to specific commits in a github repo throughout. Clone it down, check out the commit, poke around.
This project took me through:
- raising exceptions
- rescuing exceptions
- testing all of the above
What are mocks and stubs?
I’ll admit - the “standard definition” of mocks and stubs didn’t mean much to me. Quite a few definitions I found referenced “the Fowler article” which, while interesting, doesn’t quite move me forward on testing Rails app today.
- mocks are objects that have a similar interface as something else
- stubs are fake methods and return a specific answer
So, of course, I needed to see them function ‘in the wild’.
Eliminate dependencies on other classes in testing
I decided to make up this
Imagine I want to connect to a few different third-party services. I’ll have a
connection class into which I can pass a third-party service “object”, and it should “connect”.
I will assume I can call
third_party_service.status and get back
I will also assume that sometimes my own service will not function correctly, and so
my_service.status can also be any particular status code, like
I’d like to be able to test that I can raise specific errors if either
third_party_service.status is not
Here’s my sample
Connection class, where I’ve hard-coded the 200 status:
class Connection attr_reader :status_code def initialize @status_code = get_service_status end def get_service_status 200 end def connect_to_external_service(srvc) return srvc.status_code end end
And here’s the test that shows all is groovy:
class ConnectionTest < Minitest::Test def test_initializes_with_response_code c = Connection.new assert_equal 200, c.status_code end end
Now, lets say I’ve got this third party service. Since I’m testing this all locally, and I want to square away mocking and stubbing anyway, I’m getting real fancy and I’m putting this in the same file as my
class Connection attr_reader :status_code def initialize @status_code = get_service_status end def get_service_status 200 end def connect_to_external_service(srvc) return srvc.status_code end end # this is my third-party class that `Connection` might have dependencies upon class Service attr_reader :status_code def initialize @status_code = get_service_status # status_code would be 200, 504, 404, etc end def get_service_status # do complicated stuff here to get status code # return a status code 200 end end
Inside my test, when I call
Connection.new.connect_to_external_service(), I need to pass in a service object, like so:
srvc = Service.new conn = Connection.new conn.connect_to_external_service(srvc)
And I’m off to the races. This is where I want to start raising exceptions if either class returns errors, and where stubbing and mocking comes to be very valuable.
So, this test would pass:
class ConnectionTest < Minitest::Test def test_connection_default_status_code_is_200 c = Connection.new assert_equal 200, c.status_code end def test_external_service_default_status_code_is_200 conn = Connection.new srvc = Service.new assert_equal 200, conn.connect_to_external_service(srvc) end end
But I want to make
srvc.status_code be something besides
Lets figure out how to make
srvc.status_code be 404, and maybe in the process remove the dependency upon the
Service class all together.
Stubbing the service object to force errors
Right now, I want to run a test like this:
def test_raises_error_when_service_returns_4xx_no_mocks srvc = Service.new srvc.status_code = 404 conn = Connection.new assert_raises Connection::ServiceNotFound do conn.connect_to_external_service(srvc) end end
This won’t work because I don’t have an
attr_accessor for the
Service instance variable of
This is the error I get when I run the test:
1) Error: ConnectionTest#test_raises_error_when_service_returns_4xx_no_mocks: NoMethodError: undefined method `status_code=' for #<Service:0x00007f81ce3c34d8 @status_code=200>
And this makes sense, right? I cannot assume ownership of the third party service.
Here’s how I can stub out the method as I want it, using
mocha’s tooling for stubbing:
gem install mocha, and include the following front-matter in the test file:
require './lib/connection' require 'minitest/autorun' require 'mocha/minitest' class ConnectionTest < Minitest::Test def test_raise_error_when_service_returns_4xx_using_stubs serv = Service.new # instantiating a new Service object. if I call # serv.status_code, it would return 200 Service.any_instance.stubs(:status_code).returns(404) # override the method to always return 404 conn = Connection.new assert_raises Connection::ServiceNotFound do conn.connect_to_external_service(serv) end end
The magic is
Stubbing modifies an existing object to coerce it into giving a certain output.
This is cool, but nothing revelatory to me. I’ve been working with stubs for a while. What finally clicked for me was the difference between mocks and stubs.
The above test still requires access to a
Service class. If Service doesn’t exist, or requires any setup that I didn’t pass it, I’m out of luck and will need to do even more test setup.
Mocking the service object
I want to remove all dependencies on
Service. I want to create a service object in my test, assign it variables, and use them in my
Connection class, all without
Connection knowing anything about
Service, or indeed,
Service knowing anything about itself.
Connection under test here, remember. Not
Here’s the commit for the following code: 6b7014c
stub, which I can now make it do anything I want:
def test_rais_error_when_service_returns_4xx_using_mocks conn = Connection.new srvc = stub("srvc is now a stub.") # srvc.class == Mocha::Mock srvc.stubs(:status_code).returns(404) assert_raises Connection::ServiceNotFound do conn.connect_to_external_service(srvc) end end
so I no longer have a
Service object. If I call
srvc.class on that stubbed object, it’s
I then assign it a method of
.status_code, and tell it to return
404. Boom. Done.
So, I’m no longer dependent upon the
Service model for any of my
Connection testing. I’d say thats a win.
never not necessarily see the word ‘mock’ in your code, even when using mocks.
I’m not the sharpest tool in the shed, so I do a lot of pattern matching. Articles about stubbing always had the word ‘stub’ being scattered about the code. That made it easy to see when a stub was being used.
Mocks, though, don’t necessarily get called in the code. Traditionally, you could do
obj = stub('object'), and get your stubbed object that way. Now (I think this is more recent) you can make it a bit more explicit and call
obj = mock('object'), and you’ll be on your way.
stub() are interchangeable:
mocked_object = mock('mocked_object') stubbed_object = stub('stubbed_object')
> mocked_object => #<Mock:mocked_object> > stubbed_object => #<Mock:stubbed_object>
So, if you see
foo = mock(), it’s a mocked object, but if you see
foo = stub(), it’s also a mocked object.
We’ve covered mocking and stubbing, and are ready to dig into exception raising and handling.
Lets say my third party service goes down (404) or times out (504). I’d like to raise these descriptive errors, and do something with it.
This is the test I could create:
def test_raise_service_timeout_if_service_returns_504 @srv.stubs(:status_code).returns(504) assert_raises Connection::ServiceTimeOut do @conn.connect_to_external_service(@srv) end end
And I can update my
def connect_to_external_service(srvc) raise ServiceNotFound if srvc.status_code == 404 raise ServiceTimeOut if srvc.status_code == 504 # do other stuff return srvc.status_code end
This won’t quite pass - for reasons I’m still exploring, we cannot raise a
ServiceNotFound error unless these classes are included in the class:
. . def connect_to_external_service(srvc) raise ServiceNotFound if srvc.status_code == 404 raise ServiceTimeOut if srvc.status_code == 504 # do other stuff return srvc.status_code end class ServiceNotFound < StandardError; end class ServiceTimeOut < StandardError; end end
And these tests pass.
Why do we need
ServiceTimeOut to inherit from
StandardError, and be included in the class?
ServiceNotFound.ancestors => [Connection::ServiceNotFound, StandardError, Exception, Object, Kernel, BasicObject]
StandardError.ancestors => [StandardError, Exception, Object, Kernel, BasicObject]
raise is a Kernal method, it seems.
I’m going to leave this here, for now. The whole problem that led to this blog post I am now fully ready to dig into - I wanted to
rescue some raised exceptions, and when I did that, the standard
assert_raises testing stopped working, because… it seemed the raised error was caught and “squelched” by the rescue, and never bubbled back up to the test.
I’ll write more on that soon.
- Mocking and stubbing in Ruby on Rails(this article is from 2007 and is the 2nd ranked result for googling
mocks stubs rails!!!)
- Testing Network Services in Ruby Is Easier Than You Think
- Mocking in Ruby with Minitest