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>

diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 382ce85516dfbe2b5bd08c58b7fa6ede865840a9..3150aa60700ee709f4490bac39f0ee5600c86064 100644 (file)
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -1163,8 +1163,9 @@ 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 f1d829a6f1724ef0eafd8240f2e57bca9992261c..ddf1128fdd1c78ec32690e78502d3afc405fb378 100644 (file)
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -415,7 +415,11 @@ 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 7373fe29c47d810be3dbc9d8eb6c34ca5ddd9df3..e8f9699ef763504703f1e40c86bb94e27c2345ca 100644 (file)
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -196,6 +196,28 @@ 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 (file)
index 0000000..1b19247
--- /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.