TST: subtract_drift with no particle column

trackpy/tests/test_motion.py

@@ -61,13 +61,18 @@ def setUp(self):
                      for i in range(P)]
         self.many_walks = conformity(pandas_concat(particles))
 
+        self.unlabeled_walks = self.many_walks.copy()
+        del self.unlabeled_walks['particle']
+
         a = DataFrame({'x': np.arange(N), 'y': np.zeros(N),
                        'frame': np.arange(N), 'particle': np.zeros(N)})
         b = DataFrame({'x': np.arange(1, N), 'y': Y + np.zeros(N - 1),
                        'frame': np.arange(1, N), 'particle': np.ones(N - 1)})
         self.steppers = conformity(pandas_concat([a, b]))
-        self.unlabeled_steppers = self.steppers.copy()
-        del self.unlabeled_steppers['particle']
+
+        # Single-particle trajectory with no particle label
+        self.single_stepper = conformity(a)
+        del self.single_stepper['particle']
 
     def test_no_drift(self):
         N = 10
@@ -121,10 +126,15 @@ def test_subtract_constant_drift(self):
         actual = tp.subtract_drift(add_drift(self.steppers, drift), drift)
         assert_traj_equal(actual, self.steppers)
 
+        actual = tp.subtract_drift(add_drift(self.single_stepper, drift), drift)
+        assert_traj_equal(actual, self.single_stepper)
+
         # Test that subtract_drift is OK without particle labels.
-        actual = tp.subtract_drift(add_drift(self.unlabeled_steppers, drift),
+        # In principle, Series.sub() may raise an error because
+        # the 'frame' index is duplicated.
+        # Don't check the result since we can't compare unlabeled trajectories!
+        actual = tp.subtract_drift(add_drift(self.unlabeled_walks, drift),
                                    drift)
-        assert_traj_equal(actual, self.unlabeled_steppers)
 
 
 class TestMSD(StrictTestCase):