Add just mean test

sklearn/utils/tests/test_extmath.py

@@ -469,6 +469,22 @@ def test_incremental_weighted_mean_and_variance_simple():
     assert_almost_equal(var, var_exp)
 
 
+def test_incremental_weighted_mean_and_variance_just_mean():
+    rng = np.random.RandomState(42)
+    mult = 10
+    X = rng.rand(1000, 20)*mult
+    sample_weight = rng.rand(X.shape[0]) * mult
+    mean, var, _ = \
+        _incremental_weighted_mean_and_var(X, sample_weight,
+                                           last_weighted_mean=0,
+                                           last_weighted_variance=None,
+                                           last_weight_sum=0)
+
+    mean_exp = np.average(X, weights=sample_weight, axis=0)
+    assert_almost_equal(mean, mean_exp)
+    assert(var is None)
+
+
 def test_incremental_weighted_mean_and_variance():
 
     # Testing of correctness and numerical stability