From 64abf373444944a240274a9b6d66d1cb01ecfcdd Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <greg@krypto.org>
Date: Thu, 24 Dec 2020 20:57:21 -0800
Subject: [PATCH] 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>
---
 Doc/library/subprocess.rst                    |  5 +++--
 Lib/subprocess.py                             |  6 ++++-
 Lib/test/test_subprocess.py                   | 22 +++++++++++++++++++
 .../2020-11-22-11-22-28.bpo-42388.LMgM6B.rst  |  2 ++
 4 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-11-22-11-22-28.bpo-42388.LMgM6B.rst

diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 4ac3f80a319..07b8ad77655 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -1187,8 +1187,9 @@ calls these functions.
    The arguments shown above are merely some common ones.
    The full function signature is largely the same as that of :func:`run` -
    most arguments are passed directly through to that interface.
-   However, explicitly passing ``input=None`` to inherit the parent's
-   standard input file handle is not supported.
+   One API deviation from :func:`run` behavior exists: passing ``input=None``
+   will behave the same as ``input=b''`` (or ``input=''``, depending on other
+   arguments) rather than using the parent's standard input file handle.
 
    By default, this function will return the data as encoded bytes. The actual
    encoding of the output data may depend on the command being invoked, so the
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index e259dc3a8e5..aa107cb60e4 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -420,7 +420,11 @@ 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 2a4c47530e6..70b54f4155a 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -204,6 +204,28 @@ 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:
diff --git a/Misc/NEWS.d/next/Library/2020-11-22-11-22-28.bpo-42388.LMgM6B.rst b/Misc/NEWS.d/next/Library/2020-11-22-11-22-28.bpo-42388.LMgM6B.rst
new file mode 100644
index 00000000000..1b19247e841
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-11-22-11-22-28.bpo-42388.LMgM6B.rst
@@ -0,0 +1,2 @@
+Fix subprocess.check_output(..., input=None) behavior when text=True to be
+consistent with that of the documentation and universal_newlines=True.