pythongh-98966: Handle stdout=subprocess.STDOUT

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 <[email protected]>

Author: ptsneves

Lib/subprocess.py

@@ -834,6 +834,9 @@ def __init__(self, args, bufsize=-1, executable=None,
         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

@@ -1695,6 +1695,13 @@ def test_capture_output(self):
         self.assertIn(b'BDFL', cp.stdout)
         self.assertIn(b'FLUFL', cp.stderr)
 
+    def test_stdout_stdout(self):
+        # run() refuses to accetp stdout=STDOUT
+        with self.assertRaises(ValueError,
+                msg=("STDOUT can only be used for stderr")) as c:
+                output = 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()