gh-133089: Use original timeout value for TimeoutExpired when the func subprocess.run is called with a timeout

Doc/library/subprocess.rst

@@ -1525,6 +1525,30 @@ handling consistency are valid for these functions.
 Notes
 -----
 
+.. _subprocess-timeout-behavior:
+
+Timeout Behavior
+^^^^^^^^^^^^^^^^
+
+When using the ``timeout`` parameter in functions like :func:`run`,
+:meth:`Popen.wait`, or :meth:`Popen.communicate`,
+users should be aware of the following behaviors:
+
+Process Creation Delay
+^^^^^^^^^^^^^^^^^^^^^^
+
+The initial process creation itself cannot be interrupted on many platform APIs.
+This means that even when specifying a timeout, you are not guaranteed to see a timeout exception
+until at least after however long process creation takes.
+
+Extremely Small Timeout Values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Setting very small timeout values (such as a few milliseconds) may result
+in almost immediate :exc:`TimeoutExpired` exceptions because
+process creation and system scheduling inherently require time.
+
+
 .. _converting-argument-sequence:
 
 Converting an argument sequence to a string on Windows

Lib/subprocess.py

@@ -2145,8 +2145,10 @@ def _communicate(self, input, endtime, orig_timeout):
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
                             self._fileobj2output[key.fileobj].append(data)
-
-            self.wait(timeout=self._remaining_time(endtime))
+            try:
+                self.wait(timeout=self._remaining_time(endtime))
+            except TimeoutExpired:
+                raise TimeoutExpired(self.args, orig_timeout)
 
             # All data exchanged.  Translate lists into strings.
             if stdout is not None:

Misc/NEWS.d/next/Library/2025-04-29-02-23-04.gh-issue-133089.8Jy1ZS.rst

@@ -0,0 +1,2 @@
+Use origional timeout value for :exc:`TimeoutExpired`
+when the func :meth:`subprocess.run` is called with a timeout