scikit-learn
diff --git a/‎doc/whats_new/v1.0.rst
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diff --git a/‎sklearn/ensemble/_forest.py
Lines changed: 15 additions & 3 deletions b/‎sklearn/ensemble/_forest.py
Lines changed: 15 additions & 3 deletions
diff --git a/‎sklearn/ensemble/tests/test_forest.py
Lines changed: 89 additions & 0 deletions b/‎sklearn/ensemble/tests/test_forest.py
Lines changed: 89 additions & 0 deletions
@@ -270,6 +270,10 @@ Changelog
   :class:`ensemble.StackingClassifier` and :class:`ensemble.StackingRegressor`.
   :pr:`19564` by `Thomas Fan`_.
 
+- |Enhancement| Documented and tested support of the Poisson criterion for
+  :class:`ensemble.RandomForestRegressor`. :pr:`19836` by
+  :user:`Brian Sun <bsun94>`.
+
 - |Fix| Fixed the range of the argument max_samples to be (0.0, 1.0]
   in :class:`ensemble.RandomForestClassifier`,
   :class:`ensemble.RandomForestRegressor`, where `max_samples=1.0` is
 
@@ -323,6 +323,14 @@ def fit(self, X, y, sample_weight=None):
             # [:, np.newaxis] that does not.
             y = np.reshape(y, (-1, 1))
 
+        if self.criterion == "poisson":
+            if np.any(y < 0):
+                raise ValueError("Some value(s) of y are negative which is "
+                                 "not allowed for Poisson regression.")
+            if np.sum(y) <= 0:
+                raise ValueError("Sum of y is not strictly positive which "
+                                 "is necessary for Poisson regression.")
+
         self.n_outputs_ = y.shape[1]
 
         y, expanded_class_weight = self._validate_y_class_weight(y)
@@ -1324,16 +1332,20 @@ class RandomForestRegressor(ForestRegressor):
            The default value of ``n_estimators`` changed from 10 to 100
            in 0.22.
 
-    criterion : {"squared_error", "mse", "absolute_error", "mae"}, \
+    criterion : {"squared_error", "mse", "absolute_error", "poisson"}, \
             default="squared_error"
         The function to measure the quality of a split. Supported criteria
         are "squared_error" for the mean squared error, which is equal to
-        variance reduction as feature selection criterion, and "absolute_error"
-        for the mean absolute error.
+        variance reduction as feature selection criterion, "absolute_error"
+        for the mean absolute error, and "poisson" which uses reduction in
+        Poisson deviance to find splits.
 
         .. versionadded:: 0.18
            Mean Absolute Error (MAE) criterion.
 
+        .. versionadded:: 1.0
+           Poisson criterion.
+
         .. deprecated:: 1.0
             Criterion "mse" was deprecated in v1.0 and will be removed in
             version 1.2. Use `criterion="squared_error"` which is equivalent.
 
@@ -27,6 +27,8 @@
 import joblib
 from numpy.testing import assert_allclose
 
+from sklearn.dummy import DummyRegressor
+from sklearn.metrics import mean_poisson_deviance
 from sklearn.utils._testing import assert_almost_equal
 from sklearn.utils._testing import assert_array_almost_equal
 from sklearn.utils._testing import assert_array_equal
@@ -185,6 +187,76 @@ def test_regression(name, criterion):
     check_regression_criterion(name, criterion)
 
 
+def test_poisson_vs_mse():
+    """Test that random forest with poisson criterion performs better than
+    mse for a poisson target."""
+    rng = np.random.RandomState(42)
+    n_train, n_test, n_features = 500, 500, 10
+    X = datasets.make_low_rank_matrix(n_samples=n_train + n_test,
+                                      n_features=n_features, random_state=rng)
+    X = np.abs(X)
+    X /= np.max(np.abs(X), axis=0)
+    # We create a log-linear Poisson model
+    coef = rng.uniform(low=-4, high=1, size=n_features)
+    y = rng.poisson(lam=np.exp(X @ coef))
+    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=n_test,
+                                                        random_state=rng)
+
+    forest_poi = RandomForestRegressor(
+        criterion="poisson",
+        min_samples_leaf=10,
+        max_features="sqrt",
+        random_state=rng)
+    forest_mse = RandomForestRegressor(
+        criterion="squared_error",
+        min_samples_leaf=10,
+        max_features="sqrt",
+        random_state=rng)
+
+    forest_poi.fit(X_train, y_train)
+    forest_mse.fit(X_train, y_train)
+    dummy = DummyRegressor(strategy="mean").fit(X_train, y_train)
+
+    for X, y, val in [(X_train, y_train, "train"), (X_test, y_test, "test")]:
+        metric_poi = mean_poisson_deviance(y, forest_poi.predict(X))
+        # squared_error forest might produce non-positive predictions => clip
+        # If y = 0 for those, the poisson deviance gets too good.
+        # If we drew more samples, we would eventually get y > 0 and the
+        # poisson deviance would explode, i.e. be undefined. Therefore, we do
+        # not clip to a tiny value like 1e-15, but to 0.1. This acts like a
+        # mild penalty to the non-positive predictions.
+        metric_mse = mean_poisson_deviance(
+            y,
+            np.clip(forest_mse.predict(X), 1e-6, None))
+        metric_dummy = mean_poisson_deviance(y, dummy.predict(X))
+        # As squared_error might correctly predict 0 in train set, its train
+        # score can be better than Poisson. This is no longer the case for the
+        # test set. But keep the above comment for clipping in mind.
+        if val == "test":
+            assert metric_poi < metric_mse
+        assert metric_poi < metric_dummy
+
+
+@pytest.mark.parametrize('criterion', ('poisson', 'squared_error'))
+def test_balance_property_random_forest(criterion):
+    """"Test that sum(y_pred)==sum(y_true) on the training set."""
+    rng = np.random.RandomState(42)
+    n_train, n_test, n_fe
F438
atures = 500, 500, 10
+    X = datasets.make_low_rank_matrix(n_samples=n_train + n_test,
+                                      n_features=n_features, random_state=rng)
+
+    coef = rng.uniform(low=-2, high=2, size=n_features) / np.max(X, axis=0)
+    y = rng.poisson(lam=np.exp(X @ coef))
+
+    reg = RandomForestRegressor(criterion=criterion,
+                                n_estimators=10,
+                                bootstrap=False,
+                                random_state=rng)
+    reg.fit(X, y)
+
+    assert np.sum(reg.predict(X)) == pytest.approx(np.sum(y))
+
+
 def check_regressor_attributes(name):
     # Regression models should not have a classes_ attribute.
     r = FOREST_REGRESSORS[name](random_state=0)
@@ -1367,6 +1439,23 @@ def test_min_impurity_decrease():
             assert tree.min_impurity_decrease == 0.1
 
 
+def test_poisson_y_positive_check():
+    est = RandomForestRegressor(criterion="poisson")
+    X = np.zeros((3, 3))
+
+    y = [-1, 1, 3]
+    err_msg = (r"Some value\(s\) of y are negative which is "
+               r"not allowed for Poisson regression.")
+    with pytest.raises(ValueError, match=err_msg):
+        est.fit(X, y)
+
+    y = [0, 0, 0]
+    err_msg = (r"Sum of y is not strictly positive which "
+               r"is necessary for Poisson regression.")
+    with pytest.raises(ValueError, match=err_msg):
+        est.fit(X, y)
+
+
 # mypy error: Variable "DEFAULT_JOBLIB_BACKEND" is not valid type
 class MyBackend(DEFAULT_JOBLIB_BACKEND):  # type: ignore
     def __init__(self, *args, **kwargs):