chore: import upstream snapshot with attribution

python/ray/tune/stopper/function_stopper.py

@@ -0,0 +1,38 @@
+from typing import Callable, Dict
+
+from ray.tune.stopper.stopper import Stopper
+from ray.util.annotations import PublicAPI
+
+
+@PublicAPI
+class FunctionStopper(Stopper):
+    """Provide a custom function to check if trial should be stopped.
+
+    The passed function will be called after each iteration. If it returns
+    True, the trial will be stopped.
+
+    Args:
+        function: Function that checks if a trial
+            should be stopped. Must accept the `trial_id` string  and `result`
+            dictionary as arguments. Must return a boolean.
+
+    """
+
+    def __init__(self, function: Callable[[str, Dict], bool]):
+        self._fn = function
+
+    def __call__(self, trial_id, result):
+        return self._fn(trial_id, result)
+
+    def stop_all(self):
+        return False
+
+    @classmethod
+    def is_valid_function(cls, fn):
+        is_function = callable(fn) and not issubclass(type(fn), Stopper)
+        if is_function and hasattr(fn, "stop_all"):
+            raise ValueError(
+                "Stop object must be ray.tune.Stopper subclass to be detected "
+                "correctly."
+            )
+        return is_function