Add test for uniform weights to ensure consistency with unweighted ATE

Author: mackzoor

tests/test_debiased_ate.py

@@ -57,3 +57,27 @@ def test_debiased_ate_bad_weight_shapes(Debiaser, no_noise_data):
     # wrong-shaped weight for control
     with pytest.raises(ValueError):
         deb.debiased_ate(treated, control, iptw_control=np.ones(5))
+
+
+@pytest.mark.parametrize("Debiaser", [TweedieDebiaser, LccDebiaser])
+def test_debiased_ate_uniform_weights_equal_unweighted(Debiaser, no_noise_data):
+    """If IPTW weights are uniform (all equal), result should equal unweighted ATE."""
+    preds, targets = no_noise_data
+    deb = Debiaser().fit(preds, targets)
+
+    treated = preds[:40]
+    control = preds[40:90]
+
+    # uniform weights (all ones) and a constant non-one weight should both match unweighted
+    ones_t = np.ones(len(treated))
+    ones_c = np.ones(len(control))
+
+    const_t = np.full(len(treated), 2.0)
+    const_c = np.full(len(control), 2.0)
+
+    unweighted = deb.debiased_ate(treated, control)
+    ones_weighted = deb.debiased_ate(treated, control, iptw_treated=ones_t, iptw_control=ones_c)
+    const_weighted = deb.debiased_ate(treated, control, iptw_treated=const_t, iptw_control=const_c)
+
+    assert np.isclose(unweighted, ones_weighted)
+    assert np.isclose(unweighted, const_weighted)