title: passing fixture as another fixture's decorator params
body: <p>trying to use the pytest's feature to pass params=[] to fixture's decorator expecting it to be invoked individually for each of the passed values. as a param I want to pass another fixtures (just a demo code):</p>
  
  <pre><code>import pytest
  
  @pytest.fixture
  def rates():
      return (1.12, 1.15, 1.37)
  
  @pytest.fixture
  def expires():
      return (102, 105, 107)
  
  @pytest.fixture(params=[rates, expires])
  def data(request):
      return request.param
  
  def test_vol(data):
      assert len(data) == 3
  </code></pre>
  
  <p>this just fails complaining with:</p>
  
  <pre><code>TypeError: object of type 'function' has no len()
  </code></pre>
  
  <p>is there a way to do that?</p>