[rspec-users] Unexpected behavior with Matchers::Change and empty collections

John Feminella johnf.pub at distb.net
Fri Jun 18 15:29:13 EDT 2010

The behavior of Spec::Matchers::Change in rspec 1.3.0 appears to
overlook a small corner case. When the initial value is `{}` or `[]`
but the `from` clause is `nil`, rspec will report that the initial
value is correct. Of course, `nil` is not equal to either `{}` or
`[]`. Here's a simple test case:

    describe "Simple hash" do
      before :each do
        @c = Class.new do
          attr_accessor :apple

        @c.apple = {} # or []

      it "should change values with []= calls" do
        # False positive! The initial value is {} (or []), not nil.
        ->{ @c.apple = 100 }.should change(@c, :apple).from(nil).to(100)

Running this produces no errors. However, if you switch things so that
the initial value of `@c.apple` is `nil` and the `from` clause is `[]`
or `{}`, rspec's behavior is now what you'd expect to see:

'Simple hash should change values with []= calls' FAILED
apple should have initially been {}, but was nil
/home/johnf/.rvm/gems/ruby-1.9.1-p378 at standard/gems/rspec-1.3.0/lib/spec/expectations/fail_with.rb:41:in
/home/johnf/.rvm/gems/ruby-1.9.1-p378 at standard/gems/rspec-1.3.0/lib/spec/expectations/handler.rb:21:in
/home/johnf/.rvm/gems/ruby-1.9.1-p378 at standard/gems/rspec-1.3.0/lib/spec/expectations/extensions/kernel.rb:27:in

Is this behavior by design?
John Feminella
Principal Consultant, Distilled Brilliance

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