diff --git a/src/uproot/source/chunk.py b/src/uproot/source/chunk.py
index 2d9b0d08c..398ccec22 100644
--- a/src/uproot/source/chunk.py
+++ b/src/uproot/source/chunk.py
@@ -10,8 +10,8 @@
 :doc:`uproot.source.chunk.Source`, the primary types of the "physical layer."
 """
 
-
 import numbers
+import queue
 
 import numpy
 
@@ -58,7 +58,7 @@ def chunk(self, start, stop):
         :doc:`uproot.source.chunk.Chunk`.
         """
 
-    def chunks(self, ranges, notifications):
+    def chunks(self, ranges, notifications: queue.Queue):
         """
         Args:
             ranges (list of (int, int) 2-tuples): Intervals to fetch
@@ -162,7 +162,7 @@ def chunk(self, start, stop):
         self._executor.submit(future)
         return chunk
 
-    def chunks(self, ranges, notifications):
+    def chunks(self, ranges, notifications: queue.Queue):
         self._num_requests += 1
         self._num_requested_chunks += len(ranges)
         self._num_requested_bytes += sum(stop - start for start, stop in ranges)
@@ -207,13 +207,6 @@ def __exit__(self, exception_type, exception_value, traceback):
         self._executor.__exit__(exception_type, exception_value, traceback)
 
 
-def notifier(chunk, notifications):
-    def notify():
-        notifications.put(chunk)
-
-    return notify
-
-
 class Chunk:
     """
     Args:
@@ -457,3 +450,19 @@ def remainder(self, start, cursor, context):
                 context,
                 self._source.file_path,
             )
+
+
+def notifier(chunk: Chunk, notifications: queue.Queue):
+    """
+    Returns a function that puts the chunk on the notifications queue when called.
+    The function has a 'future' argument to be compatible with the `concurrent.futures.Future.add_done_callback` method.
+
+    Args:
+        chunk (:doc:`uproot.source.chunk.Chunk`): The chunk to put on the queue.
+        notifications (``queue.Queue``): The notifications queue.
+    """
+
+    def notify(future=None):
+        notifications.put(chunk)
+
+    return notify
diff --git a/src/uproot/source/futures.py b/src/uproot/source/futures.py
index d74ad2f62..584be65b2 100644
--- a/src/uproot/source/futures.py
+++ b/src/uproot/source/futures.py
@@ -21,7 +21,6 @@
 These classes implement a *subset* of Python's Future and Executor interfaces.
 """
 
-
 import os
 import queue
 import sys
@@ -172,8 +171,9 @@ def run(self):
 class ThreadPoolExecutor:
     """
     Args:
-        num_workers (None or int): The number of workers to start. If None,
-            use ``os.cpu_count()``.
+        max_workers (None or int): The maximum number of workers to start.
+        In the current implementation this is exactly the number of workers.
+        If None, use ``os.cpu_count()``.
 
     Like Python 3 ``concurrent.futures.ThreadPoolExecutor`` except that it has
     only the subset of the interface Uproot needs and is available in Python 2.
@@ -182,18 +182,18 @@ class ThreadPoolExecutor:
     class.
     """
 
-    def __init__(self, num_workers=None):
-        if num_workers is None:
+    def __init__(self, max_workers=None):
+        if max_workers is None:
             if hasattr(os, "cpu_count"):
-                num_workers = os.cpu_count()
+                self._max_workers = os.cpu_count()
             else:
                 import multiprocessing
 
-                num_workers = multiprocessing.cpu_count()
+                self._max_workers = multiprocessing.cpu_count()
 
         self._work_queue = queue.Queue()
         self._workers = []
-        for _ in range(num_workers):
+        for _ in range(self._max_workers):
             self._workers.append(Worker(self._work_queue))
         for worker in self._workers:
             worker.start()
@@ -204,7 +204,14 @@ def __repr__(self):
         )
 
     @property
-    def num_workers(self):
+    def max_workers(self) -> int:
+        """
+        The maximum number of workers.
+        """
+        return self._max_workers
+
+    @property
+    def num_workers(self) -> int:
         """
         The number of workers.
         """
@@ -263,6 +270,9 @@ def __init__(self, task):
         self._notify = None
 
     def _set_notify(self, notify):
+        """
+        Set the ``notify`` function that is called when this task is complete.
+        """
         self._notify = notify
 
     def _set_excinfo(self, excinfo):