fix logic for handling seed values in Formulae ctor (seed=0 was treated as seed=None) (#1368)

Author: slayoo

.codecov.yml

@@ -2,4 +2,4 @@ coverage:
   status:
     project:
       default:
-        threshold: 0.01%
+        threshold: 0.1%

PySDM/formulae.py

@@ -119,7 +119,7 @@ def __init__(  # pylint: disable=too-many-locals
             **constants_defaults
         )
         self.constants = constants
-        self.seed = seed or physics.constants.default_random_seed
+        self.seed = seed if seed is not None else physics.constants.default_random_seed
         self.fastmath = fastmath
         self.handle_all_breakups = handle_all_breakups
         dimensional_analysis = physics.impl.flag.DIMENSIONAL_ANALYSIS

tests/smoke_tests/box/srivastava_1982/test_eq_10.py

@@ -19,7 +19,7 @@
 ASSERT_PROD = SimProducts.Computed.mean_drop_volume_total_volume_ratio.name
 N_STEPS = 32
 N_REALISATIONS = 5
-SEEDS = list(range(N_REALISATIONS))
+SEEDS = list(range(1, N_REALISATIONS + 1))
 
 
 def test_pysdm_coalescence_is_close_to_analytic_coalescence(

tests/unit_tests/test_formulae.py

@@ -131,3 +131,14 @@ def test_raise_error_on_unknown_constant():
     @staticmethod
     def test_derived_constant_overridable():
         Formulae(constants={"Mv": np.nan})
+
+    @staticmethod
+    def test_seed_zero():
+        # arrange
+        seed = 0
+
+        # act
+        sut = Formulae(seed=seed)
+
+        # assert
+        assert sut.seed == seed