Made it required to have mass > 0 on dynamic bodies when stepping #268

Author: viblo

CHANGELOG.rst

@@ -6,6 +6,7 @@ Changelog
    New feature: Vec2d supports bool to test if zero. (bool(Vec2d(2,3) == True) Note this is a breaking change.
    Added Vec2d.length_squared, and depreacted Vec2d.get_length_sqrd()
    Added Vec2d.get_distance_squared(), and deprecated Vec2d.get_dist_sqrd()
+   A dynamic body must have non-zero mass when calling Space.step (either from Body.mass, or by setting mass or density on a Shape attached to the Body). Its not valid to set mass to 0 on a dynamic body attached to a space. 
    
    Added default do_nothing and always_collide callback functions to the CollisionHandler, so that its clear how to reset and align with other callbacks.
    If in old code you did handler.begin = None, you should now instead to handler.begin = CollisionHandler.always_collide etc.

pymunk/body.py

@@ -254,11 +254,18 @@ def __repr__(self) -> str:
 
     @property
     def mass(self) -> float:
-        """Mass of the body."""
+        """Mass of the body.
+
+        Note that dynamic bodies must have mass > 0 if they are attached to a
+        Space.
+        """
         return lib.cpBodyGetMass(self._body)
 
     @mass.setter
     def mass(self, mass: float) -> None:
+        assert (
+            self._space is None or mass > 0
+        ), "Dynamic bodies must have mass > 0 if they are attached to a Space."
         lib.cpBodySetMass(self._body, mass)
 
     @property
@@ -347,9 +354,9 @@ def angle(self) -> float:
         """Rotation of the body in radians.
 
         When changing the rotation you may also want to call
-        :py:func:`Space.reindex_shapes_for_body` to update the collision 
-        detection information for the attached shapes if plan to make any 
-        queries against the space. A body rotates around its center of gravity, 
+        :py:func:`Space.reindex_shapes_for_body` to update the collision
+        detection information for the attached shapes if plan to make any
+        queries against the space. A body rotates around its center of gravity,
         not its position.
 
         .. Note::
@@ -393,7 +400,7 @@ def rotation_vector(self) -> Vec2d:
     def space(self) -> Optional["Space"]:
         """Get the :py:class:`Space` that the body has been added to (or
         None)."""
-        assert hasattr(self, "_space"), (
+        assert hasattr(self, "_space"), (  # TODO: When can this happen?
             "_space not set. This can mean there's a direct or indirect"
             " circular reference between the Body and the Space. Circular"
             " references are not supported when using pickle or copy and"
@@ -482,12 +489,7 @@ def _set_position_func(self, func: _PositionFunc) -> None:
     @property
     def kinetic_energy(self) -> float:
         """Get the kinetic energy of a body."""
-        # todo: use ffi method
-        # return lib._cpBodyKineticEnergy(self._body)
-
-        vsq: float = self.velocity.dot(self.velocity)
-        wsq: float = self.angular_velocity * self.angular_velocity
-        return (vsq * self.mass if vsq else 0.0) + (wsq * self.moment if wsq else 0.0)
+        return lib.cpBodyKineticEnergy(self._body)
 
     @staticmethod
     def update_velocity(
@@ -598,7 +600,7 @@ def is_sleeping(self) -> bool:
 
     @property
     def body_type(self) -> _BodyType:
-        """The type of a body (:py:const:`Body.DYNAMIC`, 
+        """The type of a body (:py:const:`Body.DYNAMIC`,
         :py:const:`Body.KINEMATIC` or :py:const:`Body.STATIC`).
 
         When changing an body to a dynamic body, the mass and moment of

pymunk/space.py

@@ -143,6 +143,7 @@ def spacefree(cp_space: ffi.CData) -> None:
 
         self._add_later: Set[_AddableObjects] = set()
         self._remove_later: Set[_AddableObjects] = set()
+        self._bodies_to_check: Set[Body] = set()
 
     def _get_self(self) -> "Space":
         return self
@@ -409,6 +410,7 @@ def _add_body(self, body: "Body") -> None:
 
         body._space = weakref.proxy(self)
         self._bodies[body] = None
+        self._bodies_to_check.add(body)
         cp.cpSpaceAddBody(self._space, body._body)
 
     def _add_constraint(self, constraint: "Constraint") -> None:
@@ -437,6 +439,8 @@ def _remove_body(self, body: "Body") -> None:
         """Removes a body from the space"""
         assert body in self._bodies, "body not in space, already removed?"
         body._space = None
+        if body in self._bodies_to_check:
+            self._bodies_to_check.remove(body)
         # During GC at program exit sometimes the shape might already be removed. Then skip this step.
         if cp.cpSpaceContainsBody(self._space, body._body):
             cp.cpSpaceRemoveBody(self._space, body._body)
@@ -546,6 +550,13 @@ def step(self, dt: float) -> None:
 
         :param dt: Time step length
         """
+
+        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}."
+        self._bodies_to_check.clear()
+
         try:
             self._locked = True
             if self.threaded:

pymunk/tests/test_common.py

@@ -134,14 +134,14 @@ def remove_first(arbiter: p.Arbiter, space: p.Space, data: Any) -> None:
     def testX(self) -> None:
         space = p.Space()
 
-        b1 = p.Body()
+        b1 = p.Body(1)
         c1 = p.Circle(b1, 10)
         c1.collision_type = 2
 
-        b2 = p.Body()
+        b2 = p.Body(1)
         c2 = p.Circle(b2, 10)
 
-        b3 = p.Body()
+        b3 = p.Body(1)
         c3 = p.Circle(b3, 10)
 
         # b1.position = 0, 0