gh-98966: Handle stdout=subprocess.STDOUT (GH-98967)

Explicitly handle the case where stdout=STDOUT
as otherwise the existing error handling gets
confused and reports hard to understand errors.

Signed-off-by: Paulo Neves <ptsneves@gmail.com>

Lib/subprocess.py

@@ -839,6 +839,9 @@ class Popen:
         if not isinstance(bufsize, int):
             raise TypeError("bufsize must be an integer")
 
+        if stdout is STDOUT:
+             raise ValueError("STDOUT can only be used for stderr")
+
         if pipesize is None:
             pipesize = -1  # Restore default
         if not isinstance(pipesize, int):

Lib/test/test_subprocess.py

@@ -1763,6 +1763,13 @@ class RunFuncTestCase(BaseTestCase):
         self.assertIn(b'BDFL', cp.stdout)
         self.assertIn(b'FLUFL', cp.stderr)
 
+    def test_stdout_stdout(self):
+        # run() refuses to accept stdout=STDOUT
+        with self.assertRaises(ValueError,
+                msg=("STDOUT can only be used for stderr")):
+            self.run_python("print('will not be run')",
+                            stdout=subprocess.STDOUT)
+
     def test_stdout_with_capture_output_arg(self):
         # run() refuses to accept 'stdout' with 'capture_output'
         tf = tempfile.TemporaryFile()

Misc/NEWS.d/next/Library/2024-03-26-11-48-39.gh-issue-98966.SayV9y.rst

@@ -0,0 +1,2 @@
+In :mod:`subprocess`, raise a more informative message when
+``stdout=STDOUT``.