title: Provide default argument value for py.test fixture function
body: <p>Is there a better way for me to provide default value for argument <code>pytest.fixture</code> function? </p>
  
  <p>I have a couple of testcases that requires to run <code>fixture_func</code> before testcase and I would like to use default value for argument in fixture if none of them is provided. The only code I can come up with is as follows. Any better way for me to do it?</p>
  
  <pre><code>@pytest.fixture(scope="function")
  def fixture_func(self, request):
      argA = request.param['argA'] if request.param.get('argA', None) else 'default value for argA'
      argB = request.param['argB'] if request.param.get('argB', None) else 'default value for argB'
      # do something with fixture
  
  @pytest.mark.parametrize('fixture_func', [dict(argA='argA')], indirect=['fixture_func'])
  def test_case_1(self, fixture_func):
      #do something under testcase
      pass
  
  @pytest.mark.parametrize('fixture_func', [dict()], indirect=['fixture_func'])
  def test_case_2(self, fixture_func):
      #do something under testcase
      pass
  </code></pre>
  
  <p>i wna to use as</p>
  
  <pre><code>def test_case_3(self, fixture_func):
      #do something under testcase
      pass
  </code></pre>