Closes #21527: Add default number of workers to ThreadPoolExecutor. (Claudiu Popa.)

Lib/concurrent/futures/thread.py

@@ -10,6 +10,7 @@ from concurrent.futures import _base
 import queue
 import threading
 import weakref
+import os
 
 # Workers are created as daemon threads. This is done to allow the interpreter
 # to exit when there are still idle threads in a ThreadPoolExecutor's thread
@@ -80,13 +81,17 @@ def _worker(executor_reference, work_queue):
         _base.LOGGER.critical('Exception in worker', exc_info=True)
 
 class ThreadPoolExecutor(_base.Executor):
-    def __init__(self, max_workers):
+    def __init__(self, max_workers=None):
         """Initializes a new ThreadPoolExecutor instance.
 
         Args:
             max_workers: The maximum number of threads that can be used to
                 execute the given calls.
         """
+        if max_workers is None:
+            # Use this number because ThreadPoolExecutor is often
+            # used to overlap I/O instead of CPU work.
+            max_workers = (os.cpu_count() or 1) * 5
         if max_workers <= 0:
             raise ValueError("max_workers must be greater than 0")