Allow to periodically save with any function (#362)

* Allow to periodically save with pandas

* Generalize saving

* add future annotations

* Fix defaults

* remove typing_extensions again

* rename

adaptive/runner.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import abc
 import asyncio
 import concurrent.futures as concurrent
@@ -11,11 +13,15 @@
 import traceback
 import warnings
 from contextlib import suppress
+from typing import TYPE_CHECKING, Any, Callable
 
 import loky
 
 from adaptive.notebook_integration import in_ipynb, live_info, live_plot
 
+if TYPE_CHECKING:
+    from adaptive import BaseLearner
+
 try:
     import ipyparallel
 
@@ -663,15 +669,26 @@ def elapsed_time(self):
             end_time = time.time()
         return end_time - self.start_time
 
-    def start_periodic_saving(self, save_kwargs, interval):
+    def start_periodic_saving(
+        self,
+        save_kwargs: dict[str, Any] | None = None,
+        interval: int = 30,
+        method: Callable[[BaseLearner], None] | None = None,
+    ):
         """Periodically save the learner's data.
 
         Parameters
         ----------
         save_kwargs : dict
             Key-word arguments for ``learner.save(**save_kwargs)``.
+            Only used if ``method=None``.
         interval : int
             Number of seconds between saving the learner.
+        method : callable
+            The method to use for saving the learner. If None, the default
+            saves the learner using "pickle" which calls
+            ``learner.save(**save_kwargs)``. Otherwise provide a callable
+            that takes the learner and saves the learner.
 
         Example
         -------
@@ -681,11 +698,19 @@ def start_periodic_saving(self, save_kwargs, interval):
         ...     interval=600)
         """
 
-        async def _saver(save_kwargs=save_kwargs, interval=interval):
+        def default_save(learner):
+            learner.save(**save_kwargs)
+
+        if method is None:
+            method = default_save
+            if save_kwargs is None:
+                raise ValueError("Must provide `save_kwargs` if method=None.")
+
+        async def _saver():
             while self.status() == "running":
-                self.learner.save(**save_kwargs)
+                method(self.learner)
                 await asyncio.sleep(interval)
-            self.learner.save(**save_kwargs)  # one last time
+            method(self.learner)  # one last time
 
         self.saving_task = self.ioloop.create_task(_saver())
         return self.saving_task