A couple of rspec mocking gotchas

Just a couple of things that have caused a bit of head-scratching lately when writing RSpec specs using the built-in mocking framework.

Catching StandardError

Watch out if the code you’re testing catches StandardError (of course you’re not catching Exception, right?). Try this:

require 'rubygems'
require 'spec'
 
class Foo
  def self.foo
    Bar.bar
  rescue StandardError
    # do something here and don't re-raise
  end
end
 
class Bar
  def self.bar
  end
end
 
describe 'Calling a method that catches StandardError' do
  it 'calls Bar.bar' do
    Bar.should_receive :bar
    Foo.foo
  end
end

Nothing particularly exciting there. Let’s run it and check that it passes:

$ spec foo.rb 
.

Finished in 0.001862 seconds

1 example, 0 failures

However, what if we change the example to test the opposite behaviour?

describe 'Calling a method that catches StandardError' do
  it 'does NOT call Bar.bar' do
    Bar.should_not_receive :bar
    Foo.foo
  end
end

$ spec foo.rb 
.

Finished in 0.001865 seconds

1 example, 0 failures

Wait, surely they can’t both pass? Let’s take out the rescue and see what’s going on:

class Foo
  def self.foo
    Bar.bar
  end
end

$ spec foo.rb 
F

1)
Spec::Mocks::MockExpectationError in 'Calling a method that catches StandardError does NOT call Bar.bar'
 expected :bar with (no args) 0 times, but received it once
./foo.rb:6:in `foo'
./foo.rb:18:

Finished in 0.002276 seconds

1 example, 1 failure

That’s more like it.

Of course, what’s really happening here is that Spec::Mocks::MockExpectationError is a subclass of StandardError, so is being caught and silently discarded by our method under test.

If you’re doing TDD properly, this won’t result in a useless test (at least not immediately), but it might cause you to spend a while trying to figure out how to get a failing test before you add the call to Foo.foo (assuming the method with the rescue already existed). Generally you can solve the problem by making the code a bit more selective about which exception class(es) it catches, but I wonder whether RSpec exceptions are special cases which ought to directly extend Exception.

Checking receive counts on previously-stubbed methods

It’s quite common to stub a method on a collaborator in a before block, then check the details of the call to the method in a specific example. This doesn’t work quite as you would expect if for some reason you want to check that the method is only called a specific number of times:

require 'rubygems'
require 'spec'

class Foo
  def self.foo
    Bar.bar
    Bar.bar
  end
end

class Bar
  def self.bar
  end
end

describe 'Checking call counts for a stubbed method' do
  before do
    Bar.stub! :bar
  end

  it 'only calls a method once' do
    Bar.should_receive(:bar).once
    Foo.foo
  end
end

$ spec foo.rb 
.

Finished in 0.001867 seconds

1 example, 0 failures

I think what’s happening here is that the mock object would normally receive an unexpected call, causing the expected :bar with (any args) once, but received it twice error that you’d expect. Unfortunately the second call to the method is handled by the stub, so never triggers the error.

You can fix it, but it’s messy:

it 'only calls a method once' do
  Bar.send(:__mock_proxy).reset
  Bar.should_receive(:bar).once
  Foo.foo
end

$ spec foo.rb 
F

1)
Spec::Mocks::MockExpectationError in 'Checking call counts for a stubbed method only calls a method once'
 expected :bar with (any args) once, but received it twice
./foo.rb:23:

Finished in 0.002542 seconds

1 example, 1 failure

Does anyone know a better way?

The full example code is in this gist.