RSpec, implicit subject, and exceptions

Question

Is there a way to properly test exception raising with implicit subjects in rspec?

For example, this fails:

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describe 'test' do
  subject {raise 'an exception'}
  it {should raise_exception}
end

But this passes:

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describe 'test' do
  it "should raise an exception" do
    lambda{raise 'an exception'}.should raise_exception
  end
end

Why is this?

Answer 1

subject accepts a block which returns the subject of the remainder.

What you want is this:

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describe 'test' do
  subject { lambda { raise 'an exception' } }
  it { should raise_exception }
end

Edit: clarification from comment

This:

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describe 'test' do
   subject { foo }
   it { should blah_blah_blah }
end

is more or less equivalent to

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(foo).should blah_blah_blah

Now, consider: without the lambda, this becomes:

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(raise 'an exception').should raise_exception

See here that the exception is raised when the subject is evaluated (before should is called at all). Whereas with the lambda, it becomes:

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lambda { raise 'an exception' }.should raise_exception

Here, the subject is the lambda, which is evaluated only when the should call is evaluated (in a context where the exception will be caught).

While the "subject" is evaluated anew each time, it still has to evaluate to the thing you want to call should on.