refactor: EOM is instantiated when the trajectory is computed. no mor…

…e post_init

frispy/disc.py

@@ -1,7 +1,7 @@
 """Disc class."""
 
 from dataclasses import dataclass
-from typing import Dict, Optional, Tuple
+from typing import Dict, Tuple, Type
 
 import numpy as np
 from scipy.integrate import solve_ivp
@@ -64,20 +64,7 @@ class Disc:
     air_density: float = 1.225  # kg / m ^ 3
     g: float = 9.81  # m / s ^ 2
     model: Model = Model()
-    eom: Optional[EOM] = None
-
-    def __post_init__(self) -> None:
-        """Sets the ``eom`` (equations of motion) attribute."""
-        self.eom = self.eom or EOM(
-            model=self.model,
-            area=self.area,
-            I_xx=self.I_xx,
-            I_zz=self.I_zz,
-            mass=self.mass,
-            air_density=self.air_density,
-            g=self.g,
-        )
-        return
+    eom_class: Type = EOM
 
     def compute_trajectory(
         self,
@@ -118,9 +105,20 @@ def compute_trajectory(
             "t_eval", np.linspace(t_span[0], t_span[1], n_times)
         )
 
+        # Instantiate the equations of motion
+        eom = self.eom_class(
+            model=self.model,
+            area=self.area,
+            I_xx=self.I_xx,
+            I_zz=self.I_zz,
+            mass=self.mass,
+            air_density=self.air_density,
+            g=self.g,
+        )
+
         # Call the solver
         result = solve_ivp(
-            fun=self.eom.compute_derivatives,
+            fun=eom.compute_derivatives,
             t_span=t_span,
             y0=[
                 self.x,

tests/test_disc.py

@@ -11,14 +11,13 @@ def test_smoke():
 def test_disc_has_properties():
     d = Disc()
     assert hasattr(d, "model")
-    assert hasattr(d, "eom")
+    assert hasattr(d, "eom_class")
 
 
 def test_physical_attribute_kwarg():
     d = Disc(mass=12345, area=0.1234)
     assert d.mass == 12345
     assert d.area == 0.1234
-    assert d.eom.diameter == 2 * np.sqrt(d.area / np.pi)
 
 
 def test_compute_trajectory_basics():