Assert body.moment > 0 when added to space

Author: viblo

CHANGELOG.rst

@@ -94,7 +94,10 @@ Other improvements:
   GCed.
 - Improved documentation in many places (``Vec2d``, ``Poly``, ``Shape`` and 
   more)
-- Improved assert for ``body.mass`` sanity check.
+- Improved assert for ``body.mass`` sanity check (``0 < mass < math.inf``) for
+  dynamic bodies added to a space.
+- Improved assert for ``body.moment`` sanity check (``0 < moment``) for dynamic
+  bodies added to a space.
 - Internal cleanup of code
 
 

pymunk/body.py

@@ -1,5 +1,6 @@
 __docformat__ = "reStructuredText"
 
+import math
 import weakref
 from collections.abc import KeysView
 from typing import TYPE_CHECKING, Any, Callable, ClassVar, Optional  # Literal,
@@ -252,21 +253,26 @@ def mass(self) -> float:
 
     @mass.setter
     def mass(self, mass: float) -> None:
-        assert self.space is None or 0 < mass < float(
-            "inf"
+        assert (
+            self.space is None or 0 < mass < math.inf
         ), "Dynamic bodies must have mass > 0 if they are attached to a Space."
         lib.cpBodySetMass(self._body, mass)
 
     @property
     def moment(self) -> float:
         """Moment of inertia (MoI or sometimes just moment) of the body.
 
-        The moment is like the rotational mass of a body.
+        The moment is like the rotational mass of a body. Note that it is
+        valid to set moment to float('inf'). This will make a body that cannot
+        rotate.
         """
         return lib.cpBodyGetMoment(self._body)
 
     @moment.setter
     def moment(self, moment: float) -> None:
+        assert (
+            self.space is None or moment > 0
+        ), "Dynamic bodies must have moment > 0 if they are attached to a Space"
         lib.cpBodySetMoment(self._body, moment)
 
     def _set_position(self, pos: tuple[float, float]) -> None:

pymunk/space.py

@@ -562,8 +562,8 @@ def step(self, dt: float) -> None:
 
         for b in self._bodies_to_check:
             assert b.body_type != Body.DYNAMIC or (
-                b.mass > 0 and b.mass < math.inf
-            ), f"Dynamic bodies must have a mass > 0 and < inf. {b} has mass {b.mass}."
+                b.mass > 0 and b.mass < math.inf and b.moment > 0
+            ), f"Dynamic bodies must have a mass > 0 and < inf and moment > 0. {b} has mass {b.mass}, moment {b.moment}."
         self._bodies_to_check.clear()
 
         try:

pymunk/tests/test_body.py

@@ -1,3 +1,4 @@
+import math
 import pickle
 import unittest
 
@@ -182,7 +183,7 @@ def test_mass(self) -> None:
             b.mass = 0
 
         with self.assertRaises(AssertionError):
-            b.mass = float("inf")
+            b.mass = math.inf
 
         s.remove(b)
         b.mass = 0
@@ -202,6 +203,39 @@ def test_mass(self) -> None:
         c.density = 10
         s.step(1)
 
+    def test_moment(self) -> None:
+        s = p.Space()
+        b = p.Body()
+
+        b.mass = 2
+        b.moment = 3
+        s.add(b)
+
+        # Cant set 0 moment on Body in Space
+        with self.assertRaises(AssertionError):
+            b.moment = 0
+
+        # inf moment is fine
+        b.moment = math.inf
+
+        s.remove(b)
+        b.moment = 0
+        s.add(b)
+        # Cant add 0 moment Body to Space and run step
+        with self.assertRaises(AssertionError):
+            s.step(1)
+
+        c = p.Circle(b, 1)
+        s.add(c)
+
+        # Same with a Shape
+        with self.assertRaises(AssertionError):
+            s.step(1)
+
+        # Setting the Shape density should fix it
+        c.density = 10
+        s.step(1)
+
     def test_mass_moment_from_shape(self) -> None:
         s = p.Space()