TEST: Explicitly test that we are exercising w2_thresh

Author: effigies

nibabel/tests/test_quaternions.py

@@ -75,13 +75,28 @@ def test_fillpos_precision(dtype):
     rand = np.random.default_rng()
     # Permit 1 epsilon per value
     w2_thresh = 3 * -np.finfo(dtype).eps
+
+    pos_error = neg_error = False
     for _ in range(50):
         xyz = rand.uniform(size=(3,)).astype(dtype)
         # Normalize to unit vector up to type precision
         xyz /= np.sqrt(xyz @ xyz)
         wxyz = nq.fillpositive(xyz, w2_thresh)
         assert wxyz[0] == 0.0
 
+        # Verify that we exercise the threshold
+        magnitude = xyz @ xyz
+        if magnitude < 1:
+            pos_error = True
+        elif magnitude > 1:
+            neg_error = True
+
+        if pos_error and neg_error:
+            break
+
+    assert pos_error, 'Did not encounter a case where 1 - |xyz| > 0'
+    assert neg_error, 'Did not encounter a case where 1 - |xyz| < 0'
+
 
 def test_conjugate():
     # Takes sequence