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

gpshead webhook-mailer at python.org
Thu Dec 24 23:57:26 EST 2020

commit: 64abf373444944a240274a9b6d66d1cb01ecfcdd
branch: master
author: Gregory P. Smith <greg at krypto.org>
committer: gpshead <greg at krypto.org>
date: 2020-12-24T20:57:21-08:00

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 at ispras.ru>

A Misc/NEWS.d/next/Library/2020-11-22-11-22-28.bpo-42388.LMgM6B.rst
M Doc/library/subprocess.rst
M Lib/subprocess.py
M Lib/test/test_subprocess.py

diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 4ac3f80a319f8..07b8ad77655db 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 e259dc3a8e538..aa107cb60e461 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,
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 2a4c47530e6a1..70b54f4155a9a 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -204,6 +204,28 @@ def test_check_output_input_arg(self):
         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 0000000000000..1b19247e84148
--- /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.

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