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 a93a6c186f8d803e23e55853c91a69ba5c1e5294..7ce274652d394aa11f30e055caee5524e263d39c 100644 (file)
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -1127,8 +1127,9 @@ 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 5c2c2f05093f7e3a71aa1f6cde8abd2f336dbac5..eecb1e7f253fbaf55e3e60a022d6664ab5720ef2 100644 (file)
--- 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 2d3ab93555b4c3e796a2ae05278c54c5a42ab602..4661d1e25280afcc51c7e1423e631e68f7430bae 100644 (file)
--- 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:

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.