MNT synchronize forest with scikit-learn (#622)

* MNT add max_features parameter

* MNT add ccp_alpha parameter

* DOC whats new

Author: glemaitre

doc/whats_new/v0.6.rst

@@ -32,7 +32,11 @@ Maintenance
   :pr:`617` by :user:`Guillaume Lemaitre <glemaitre>`.
 
 - Synchronize :mod:`imblearn.pipeline` with :mod:`sklearn.pipeline`.
-  :pr:`617` by :user:`Guillaume Lemaitre <glemaitre>`.
+  :pr:`620` by :user:`Guillaume Lemaitre <glemaitre>`.
+
+- Synchronize :class:`imblearn.ensemble.BalancedRandomForestClassifier` and add
+  parameters `max_samples` and `ccp_alpha`.
+  :pr:`621` by :user:`Guillaume Lemaitre <glemaitre>`.
 
 Deprecation
 ...........

imblearn/ensemble/_forest.py

@@ -17,6 +17,7 @@
 from sklearn.base import clone
 from sklearn.ensemble import RandomForestClassifier
 from sklearn.ensemble._base import _set_random_states
+from sklearn.ensemble._forest import _get_n_samples_bootstrap
 from sklearn.ensemble._forest import _parallel_build_trees
 from sklearn.exceptions import DataConversionWarning
 from sklearn.tree import DecisionTreeClassifier
@@ -44,6 +45,7 @@ def _local_parallel_build_trees(
     n_trees,
     verbose=0,
     class_weight=None,
+    n_samples_bootstrap=None
 ):
     # resample before to fit the tree
     X_resampled, y_resampled = sampler.fit_resample(X, y)
@@ -59,7 +61,7 @@ def _local_parallel_build_trees(
         n_trees,
         verbose=verbose,
         class_weight=class_weight,
-        n_samples_bootstrap=X_resampled.shape[0],
+        n_samples_bootstrap=n_samples_bootstrap,
     )
     return sampler, tree
 
@@ -195,6 +197,27 @@ class BalancedRandomForestClassifier(RandomForestClassifier):
         Note that these weights will be multiplied with sample_weight (passed
         through the fit method) if sample_weight is specified.
 
+
+    ccp_alpha : non-negative float, optional (default=0.0)
+        Complexity parameter used for Minimal Cost-Complexity Pruning. The
+        subtree with the largest cost complexity that is smaller than
+        ``ccp_alpha`` will be chosen. By default, no pruning is performed. See
+        :ref:`minimal_cost_complexity_pruning` for details.
+
+        .. versionadded:: 0.22
+           Added in `scikit-learn` in 0.22
+
+    max_samples : int or float, default=None
+        If bootstrap is True, the number of samples to draw from X
+        to train each base estimator.
+            - If None (default), then draw `X.shape[0]` samples.
+            - If int, then draw `max_samples` samples.
+            - If float, then draw `max_samples * X.shape[0]` samples. Thus,
+              `max_samples` should be in the interval `(0, 1)`.
+
+        .. versionadded:: 0.22
+           Added in `scikit-learn` in 0.22
+
     Attributes
     ----------
     estimators_ : list of DecisionTreeClassifier
@@ -281,6 +304,8 @@ def __init__(
         verbose=0,
         warm_start=False,
         class_weight=None,
+        ccp_alpha=0.0,
+        max_samples=None,
     ):
         super().__init__(
             criterion=criterion,
@@ -299,6 +324,8 @@ def __init__(
             max_features=max_features,
             max_leaf_nodes=max_leaf_nodes,
             min_impurity_decrease=min_impurity_decrease,
+            ccp_alpha=ccp_alpha,
+            max_samples=max_samples,
         )
 
         self.sampling_strategy = sampling_strategy
@@ -414,6 +441,12 @@ def fit(self, X, y, sample_weight=None):
             else:
                 sample_weight = expanded_class_weight
 
+        # Get bootstrap sample size
+        n_samples_bootstrap = _get_n_samples_bootstrap(
+            n_samples=X.shape[0],
+            max_samples=self.max_samples
+        )
+
         # Check parameters
         self._validate_estimator()
 
@@ -479,6 +512,7 @@ def fit(self, X, y, sample_weight=None):
                     len(trees),
                     verbose=self.verbose,
                     class_weight=self.class_weight,
+                    n_samples_bootstrap=n_samples_bootstrap,
                 )
                 for i, (s, t) in enumerate(zip(samplers, trees))
             )

imblearn/ensemble/tests/test_forest.py

@@ -134,3 +134,45 @@ def test_balanced_random_forest_grid_search(imbalanced_dataset):
         brf, {"n_estimators": (1, 2), "max_depth": (1, 2)}, cv=3
     )
     grid.fit(*imbalanced_dataset)
+
+
+def test_little_tree_with_small_max_samples():
+    rng = np.random.RandomState(1)
+
+    X = rng.randn(10000, 2)
+    y = rng.randn(10000) > 0
+
+    # First fit with no restriction on max samples
+    est1 = BalancedRandomForestClassifier(
+        n_estimators=1,
+        random_state=rng,
+        max_samples=None,
+    )
+
+    # Second fit with max samples restricted to just 2
+    est2 = BalancedRandomForestClassifier(
+        n_estimators=1,
+        random_state=rng,
+        max_samples=2,
+    )
+
+    est1.fit(X, y)
+    est2.fit(X, y)
+
+    tree1 = est1.estimators_[0].tree_
+    tree2 = est2.estimators_[0].tree_
+
+    msg = "Tree without `max_samples` restriction should have more nodes"
+    assert tree1.node_count > tree2.node_count, msg
+
+
+def test_balanced_random_forest_pruning(imbalanced_dataset):
+    brf = BalancedRandomForestClassifier()
+    brf.fit(*imbalanced_dataset)
+    n_nodes_no_pruning = brf.estimators_[0].tree_.node_count
+
+    brf_pruned = BalancedRandomForestClassifier(ccp_alpha=0.015)
+    brf_pruned.fit(*imbalanced_dataset)
+    n_nodes_pruning = brf_pruned.estimators_[0].tree_.node_count
+
+    assert n_nodes_no_pruning > n_nodes_pruning