Add the NoDrift dummy drift detector

docs/releases/unreleased.md

@@ -8,6 +8,10 @@ River's mini-batch methods now support pandas v2. In particular, River conforms
   - Made `score_one` method of `anomaly.LocalOutlierFactor` stateless
   - Defined default score for uninitialized detector
 
+## covariance
+
+- Added `_from_state` method to `covariance.EmpiricalCovariance` to warm start from previous knowledge.
+
 ## clustering
 
 - Add fixes to `cluster.DBSTREAM` algorithm, including:
@@ -22,13 +26,13 @@ River's mini-batch methods now support pandas v2. In particular, River conforms
 
 - Added `datasets.WebTraffic`, which is a dataset that counts the occurrences of events on a website. It is a multi-output regression dataset with two outputs.
 
-## forest
+## drift
 
-- Simplify inner the structures of `forest.ARFClassifier` and `forest.ARFRegressor` by removing redundant class hierarchy. Simplify how concept drift logging can be accessed in individual trees and in the forest as a whole.
+- Add `drift.NoDrift` to allow disabling the drift detection capabilities of models. This detector does nothing and always returns `False` when queried whether or not a concept drift was detected.
 
-## covariance
+## forest
 
-- Added `_from_state` method to `covariance.EmpiricalCovariance` to warm start from previous knowledge.
+- Simplify inner the structures of `forest.ARFClassifier` and `forest.ARFRegressor` by removing redundant class hierarchy. Simplify how concept drift logging can be accessed in individual trees and in the forest as a whole.
 
 ## proba
 

river/drift/__init__.py

@@ -12,6 +12,7 @@
 from .adwin import ADWIN
 from .dummy import DummyDriftDetector
 from .kswin import KSWIN
+from .no_drift import NoDrift
 from .page_hinkley import PageHinkley
 from .retrain import DriftRetrainingClassifier
 
@@ -22,6 +23,7 @@
     "DriftRetrainingClassifier",
     "DummyDriftDetector",
     "KSWIN",
+    "NoDrift",
     "PageHinkley",
     "PeriodicTrigger",
 ]

river/drift/no_drift.py

@@ -0,0 +1,76 @@
+from __future__ import annotations
+
+from river import base
+from river.base.drift_detector import DriftDetector
+
+
+class NoDrift(base.DriftDetector):
+    """Dummy class used to turn off concept drift detection capabilities of adaptive models.
+    It always signals that no concept drift was detected.
+    Examples
+    --------
+
+    >>> from river import drift
+    >>> from river import evaluate
+    >>> from river import forest
+    >>> from river import metrics
+    >>> from river.datasets import synth
+
+    >>> dataset = synth.ConceptDriftStream(
+    ...     seed=8,
+    ...     position=500,
+    ...     width=40,
+    ... ).take(700)
+
+    We can turn off the warning detection capabilities of Adaptive Random Forest (ARF) or
+    other similar models. Thus, the base models will reset immediately after identifying a drift,
+    bypassing the background model building phase:
+
+    >>> adaptive_model = forest.ARFClassifier(
+    ...     leaf_prediction="mc",
+    ...     warning_detector=drift.NoDrift(),
+    ...     seed=8
+    ... )
+
+    We can also turn off the concept drift handling capabilities completely:
+
+    >>> stationary_model = forest.ARFClassifier(
+    ...     leaf_prediction="mc",
+    ...     warning_detector=drift.NoDrift(),
+    ...     drift_detector=drift.NoDrift(),
+    ...     seed=8
+    ... )
+
+    Let's put that to test:
+
+    >>> for x, y in dataset:
+    ...     adaptive_model = adaptive_model.learn_one(x, y)
+    ...     stationary_model = stationary_model.learn_one(x, y)
+
+    The adaptive model:
+
+    >>> adaptive_model.n_drifts_detected()
+    2
+
+    >>> adaptive_model.n_warnings_detected()
+    0
+
+    The stationary one:
+
+    >>> stationary_model.n_drifts_detected()
+    0
+
+    >>> stationary_model.n_warnings_detected()
+    0
+
+    """
+
+    def __init__(self):
+        super().__init__()
+
+    def update(self, x: int | float) -> DriftDetector:
+        return self
+
+    @property
+    def drift_detected(self):
+        return False

river/forest/adaptive_random_forest.py

@@ -10,7 +10,7 @@
 import numpy as np
 
 from river import base, metrics, stats
-from river.drift import ADWIN
+from river.drift import ADWIN, NoDrift
 from river.tree.hoeffding_tree_classifier import HoeffdingTreeClassifier
 from river.tree.hoeffding_tree_regressor import HoeffdingTreeRegressor
 from river.tree.nodes.arf_htc_nodes import (
@@ -32,8 +32,8 @@ def __init__(
         n_models: int,
         max_features: bool | str | int,
         lambda_value: int,
-        drift_detector: base.DriftDetector | None,
-        warning_detector: base.DriftDetector | None,
+        drift_detector: base.DriftDetector,
+        warning_detector: base.DriftDetector,
         metric: metrics.base.MultiClassMetric | metrics.base.RegressionMetric,
         disable_weighted_vote,
         seed,
@@ -50,31 +50,32 @@ def __init__(
 
         self._rng = random.Random(self.seed)
 
-        self._warning_detectors: list[base.DriftDetector] = (
-            None  # type: ignore
-            if self.warning_detector is None
-            else [self.warning_detector.clone() for _ in range(self.n_models)]
-        )
-        self._drift_detectors: list[base.DriftDetector] = (
-            None  # type: ignore
-            if self.drift_detector is None
-            else [self.drift_detector.clone() for _ in range(self.n_models)]
-        )
+        self._warning_detectors: list[base.DriftDetector]
+        self._warning_detection_disabled = True
+        if not isinstance(self.warning_detector, NoDrift):
+            self._warning_detectors = [self.warning_detector.clone() for _ in range(self.n_models)]
+            self._warning_detection_disabled = False
+
+        self._drift_detectors: list[base.DriftDetector]
+        self._drift_detection_disabled = True
+        if not isinstance(self.drift_detector, NoDrift):
+            self._drift_detectors = [self.drift_detector.clone() for _ in range(self.n_models)]
+            self._drift_detection_disabled = False
 
         # The background models
         self._background: list[BaseTreeClassifier | BaseTreeRegressor | None] = (
-            None if self.warning_detector is None else [None] * self.n_models  # type: ignore
+            None if self._warning_detection_disabled else [None] * self.n_models  # type: ignore
         )
 
         # Performance metrics used for weighted voting/aggregation
         self._metrics = [self.metric.clone() for _ in range(self.n_models)]
 
         # Drift and warning logging
         self._warning_tracker: dict = (
-            collections.defaultdict(int) if self.warning_detector is not None else None  # type: ignore
+            collections.defaultdict(int) if not self._warning_detection_disabled else None  # type: ignore
         )
         self._drift_tracker: dict = (
-            collections.defaultdict(int) if self.drift_detector is not None else None  # type: ignore
+            collections.defaultdict(int) if not self._drift_detection_disabled else None  # type: ignore
         )
 
     @property
@@ -101,12 +102,10 @@ def _drift_detector_input(
     def _new_base_model(self) -> BaseTreeClassifier | BaseTreeRegressor:
         raise NotImplementedError
 
-    def n_warnings_detected(self, tree_id: int | None = None) -> int | None:
+    def n_warnings_detected(self, tree_id: int | None = None) -> int:
         """Get the total number of concept drift warnings detected, or the number on an individual
         tree basis (optionally).
 
-        If warning detection is disabled, will return `None`.
-
         Parameters
         ----------
         tree_id
@@ -119,20 +118,18 @@ def n_warnings_detected(self, tree_id: int | None = None) -> int | None:
 
         """
 
-        if self.warning_detector is None:
-            return None
+        if self._warning_detection_disabled:
+            return 0
 
         if tree_id is None:
             return sum(self._warning_tracker.values())
 
         return self._warning_tracker[tree_id]
 
-    def n_drifts_detected(self, tree_id: int | None = None) -> int | None:
+    def n_drifts_detected(self, tree_id: int | None = None) -> int:
         """Get the total number of concept drifts detected, or such number on an individual
         tree basis (optionally).
 
-        If drift detection is disabled, will return `None`.
-
         Parameters
         ----------
         tree_id
@@ -145,8 +142,8 @@ def n_drifts_detected(self, tree_id: int | None = None) -> int | None:
 
         """
 
-        if self.drift_detector is None:
-            return None
+        if self._drift_detection_disabled:
+            return 0
 
         if tree_id is None:
             return sum(self._drift_tracker.values())
@@ -171,13 +168,13 @@ def learn_one(self, x: dict, y: base.typing.Target, **kwargs):
 
             k = poisson(rate=self.lambda_value, rng=self._rng)
             if k > 0:
-                if self.warning_detector is not None and self._background[i] is not None:
+                if not self._warning_detection_disabled and self._background[i] is not None:
                     self._background[i].learn_one(x=x, y=y, sample_weight=k)  # type: ignore
 
                 model.learn_one(x=x, y=y, sample_weight=k)
 
                 drift_input = None
-                if self.drift_detector is not None and self.warning_detector is not None:
+                if not self._warning_detection_disabled:
                     drift_input = self._drift_detector_input(i, y, y_pred)
                     self._warning_detectors[i].update(drift_input)
 
@@ -189,7 +186,7 @@ def learn_one(self, x: dict, y: base.typing.Target, **kwargs):
                         # Update warning tracker
                         self._warning_tracker[i] += 1
 
-                if self.drift_detector is not None:
+                if not self._drift_detection_disabled:
                     drift_input = (
                         drift_input
                         if drift_input is not None
@@ -198,7 +195,7 @@ def learn_one(self, x: dict, y: base.typing.Target, **kwargs):
                     self._drift_detectors[i].update(drift_input)
 
                     if self._drift_detectors[i].drift_detected:
-                        if self.warning_detector is not None and self._background[i] is not None:
+                        if not self._warning_detection_disabled and self._background[i] is not None:
                             self.data[i] = self._background[i]
                             self._background[i] = None
                             self._warning_detectors[i] = self.warning_detector.clone()