pythonbddpytest-bdd

Passing parameter from WHEN to a THEN


How do I pass a parameter from a WHEN TO a THEN in pytest bdd?
For example, if I have the following code:

@when('<n1> is a number divisible by 10')
def n1_is_a_number_divisible_by_10(n1):
  assert (n1 % 10) == 0
  newN1 = n1/10
  return newN1

@then('the result will also be divisible by 3')
def the_result_will_also_be_divisible_by_3(newN1):
  assert newN1 % 3 == 0

How do I pass newN1 from the when to the then?

(I have tried making newN1 a global variable...this works but making things global is often frowned upon in python).


Solution

  • You can't pass anything from "when" to "then" by returning. Generally you want to avoid this situation. But if you have to, use a pytest fixture as a message box in both steps:

    @pytest.fixture(scope='function')
    def context():
        return {}
    
    @when('<n1> is a number divisible by 10')
    def n1_is_a_number_divisible_by_10(n1, context):
      assert (n1 % 10) == 0
      newN1 = n1 / 10
      context['partial_result'] = newN1
    
    @then('the result will also be divisible by 3')
    def the_result_will_also_be_divisible_by_3(context):
      assert context['partial_result'] % 3 == 0
    

    Result of the "context" fixture is passed to the "when" step, populated, then passed to the "then" part. It isn't recreated each time.

    As a side note, it's suboptimal to test your tests - you're doing so in the "when" part, asserting the divisibility. But this is just some sample code I guess.