fix(robot): qlim and printing work for a robot with symbolic parameters

src/roboticstoolbox/robot/BaseRobot.py

@@ -1017,14 +1017,12 @@ def qlim(self) -> NDArray:
 
         for link in self.links:
             if link.isrevolute:
-                if (
-                    link.qlim is None
-                    or link.qlim[0] is None
-                    or np.any(np.isnan(link.qlim))
-                ):
+                # a symbolic model holds SymPy numbers, which np.isnan rejects
+                qlim = None if link.qlim is None else np.asarray(link.qlim, dtype=float)
+                if qlim is None or np.any(np.isnan(qlim)):
                     v = [-np.pi, np.pi]
                 else:
-                    v = link.qlim
+                    v = qlim
             elif link.isprismatic:
                 if link.qlim is None:
                     raise ValueError("Undefined prismatic joint limit")

tests/test_symbolic_qlim.py

@@ -0,0 +1,25 @@
+"""The joint limits and the display of a robot with symbolic parameters."""
+
+import numpy as np
+import numpy.testing as nt
+
+import roboticstoolbox as rtb
+
+
+def test_symbolic_robot_qlim():
+    robot = rtb.models.DH.Puma560(symbolic=True)
+    numeric = rtb.models.DH.Puma560()
+
+    nt.assert_allclose(robot.qlim, numeric.qlim)
+
+
+def test_symbolic_robot_can_be_printed():
+    robot = rtb.models.DH.Puma560(symbolic=True)
+
+    assert "DHRobot" in str(robot)
+
+
+def test_unset_revolute_limits_are_the_default():
+    robot = rtb.DHRobot([rtb.RevoluteDH(a=1.0)], name="one")
+
+    nt.assert_allclose(robot.qlim, [[-np.pi], [np.pi]])