bpo-42388: Fix subprocess.check_output input=None when text=True (GH-23467)

When the modern text= spelling of the universal_newlines= parameter was added
for Python 3.7, check_output's special case around input=None was overlooked.
So it behaved differently with universal_newlines=True vs text=True.  This
reconciles the behavior to be consistent and adds a test to guarantee it.

Also clarifies the existing check_output documentation.

Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>
(cherry picked from commit 64abf373444944a240274a9b6d66d1cb01ecfcdd)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

2 files changed, 27 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 5c2c2f05093..eecb1e7f253 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -406,7 +406,11 @@ def check_output(*popenargs, timeout=None, **kwargs):
     if 'input' in kwargs and kwargs['input'] is None:
         # Explicitly passing input=None was previously equivalent to passing an
         # empty string. That is maintained here for backwards compatibility.
-        kwargs['input'] = '' if kwargs.get('universal_newlines', False) else b''
+        if kwargs.get('universal_newlines') or kwargs.get('text'):
+            empty = ''
+        else:
+            empty = b''
+        kwargs['input'] = empty
 
     return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
                **kwargs).stdout
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 2d3ab93555b..4661d1e2528 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -195,6 +195,28 @@ class ProcessTestCase(BaseTestCase):
                 input=b'pear')
         self.assertIn(b'PEAR', output)
 
+    def test_check_output_input_none(self):
+        """input=None has a legacy meaning of input='' on check_output."""
+        output = subprocess.check_output(
+                [sys.executable, "-c",
+                 "import sys; print('XX' if sys.stdin.read() else '')"],
+                input=None)
+        self.assertNotIn(b'XX', output)
+
+    def test_check_output_input_none_text(self):
+        output = subprocess.check_output(
+                [sys.executable, "-c",
+                 "import sys; print('XX' if sys.stdin.read() else '')"],
+                input=None, text=True)
+        self.assertNotIn('XX', output)
+
+    def test_check_output_input_none_universal_newlines(self):
+        output = subprocess.check_output(
+                [sys.executable, "-c",
+                 "import sys; print('XX' if sys.stdin.read() else '')"],
+                input=None, universal_newlines=True)
+        self.assertNotIn('XX', output)
+
     def test_check_output_stdout_arg(self):
         # check_output() refuses to accept 'stdout' argument
         with self.assertRaises(ValueError) as c: