random function in formulas.py only applies randomness to some da…

…ta, creating visual bug within `policyengine-app` (#135)

Fixes #134

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Bug causing random behaviour differences between situations with and without axes.

policyengine_core/commons/formulas.py

@@ -18,6 +18,8 @@
 from policyengine_core.types import ArrayLike, ArrayType
 from policyengine_core.variables.variable import Variable
 
+import json
+
 T = TypeVar("T")
 
 
@@ -298,10 +300,11 @@ def amount_between(
     return clip(amount, threshold_1, threshold_2) - threshold_1
 
 
-def random(entity, reset=True):
+def random(entity, reset=False):
     if entity.simulation.has_axes:
-        # Don't simulate randomness in simulations with axes.
-        return 0
+        # Generate the same random number for each entity.
+        random_number = np.random.rand(1)[0]
+        return np.array([random_number] * entity.count)
     if reset:
         np.random.seed(0)
     x = np.random.rand(entity.count)

policyengine_core/simulations/simulation.py

@@ -21,6 +21,8 @@
     TracingParameterNodeAtInstant,
 )
 
+import json
+
 if TYPE_CHECKING:
     from policyengine_core.taxbenefitsystems import TaxBenefitSystem
 
@@ -154,6 +156,16 @@ def __init__(
             if len(self.get_holder(variable.name).get_known_periods()) > 0
         ]
 
+        self.situation_input = situation
+        if self.situation_input is not None:
+            original_input = json.loads(json.dumps(self.situation_input))
+            if original_input.get("axes") is not None:
+                original_input["axes"] = {}
+            # Hash the situation input to a random number, so situations with axes behave the
+            # same ways as the same situations without axes.
+            hashed_input = hash(json.dumps(original_input)) % 1000000
+            np.random.seed(hashed_input)
+
     def apply_reform(self, reform: Union[tuple, Reform]):
         if isinstance(reform, tuple):
             for subreform in reform: