bpo-31756: subprocess.run should alias universal_newlines to text (#4049)

Improve human friendliness of the Popen API: Add text=False as a
keyword-only argument to subprocess.Popen along with a Popen
attribute .text_mode and set this based on the
encoding/errors/universal_newlines/text arguments.

The universal_newlines parameter and attribute are maintained for
backwards compatibility.

Doc/library/subprocess.rst

@@ -61,7 +61,7 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
 
    The *input* argument is passed to :meth:`Popen.communicate` and thus to the
    subprocess's stdin.  If used it must be a byte sequence, or a string if
-   *encoding* or *errors* is specified or *universal_newlines* is true.  When
+   *encoding* or *errors* is specified or *text* is true.  When
    used, the internal :class:`Popen` object is automatically created with
    ``stdin=PIPE``, and the *stdin* argument may not be used as well.
 
@@ -70,10 +70,11 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
    exception hold the arguments, the exit code, and stdout and stderr if they
    were captured.
 
-   If *encoding* or *errors* are specified, or *universal_newlines* is true,
+   If *encoding* or *errors* are specified, or *text* is true,
    file objects for stdin, stdout and stderr are opened in text mode using the
    specified *encoding* and *errors* or the :class:`io.TextIOWrapper` default.
-   Otherwise, file objects are opened in binary mode.
+   The *universal_newlines* argument is equivalent  to *text* and is provided
+   for backwards compatibility. By default, file objects are opened in binary mode.
 
    Examples::
 
@@ -95,6 +96,10 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
 
       Added *encoding* and *errors* parameters
 
+   .. versionchanged:: 3.7
+
+      Added the *text* parameter, as a more understandable alias of *universal_newlines*
+
 .. class:: CompletedProcess
 
    The return value from :func:`run`, representing a process that has finished.
@@ -114,8 +119,8 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
    .. attribute:: stdout
 
       Captured stdout from the child process. A bytes sequence, or a string if
-      :func:`run` was called with an encoding or errors. ``None`` if stdout was not
-      captured.
+      :func:`run` was called with an encoding, errors, or text=True.
+      ``None`` if stdout was not captured.
 
       If you ran the process with ``stderr=subprocess.STDOUT``, stdout and
       stderr will be combined in this attribute, and :attr:`stderr` will be
@@ -124,8 +129,8 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
    .. attribute:: stderr
 
       Captured stderr from the child process. A bytes sequence, or a string if
-      :func:`run` was called with an encoding or errors. ``None`` if stderr was not
-      captured.
+      :func:`run` was called with an encoding, errors, or text=True.
+      ``None`` if stderr was not captured.
 
    .. method:: check_returncode()
 

Lib/subprocess.py

@@ -320,8 +320,11 @@ def check_output(*popenargs, timeout=None, **kwargs):
     ...              input=b"when in the course of fooman events\n")
     b'when in the course of barman events\n'
 
-    If universal_newlines=True is passed, the "input" argument must be a
-    string and the return value will be a string rather than bytes.
+    By default, all communication is in bytes, and therefore any "input"
+    should be bytes, and the return value wil be bytes.  If in text mode,
+    any "input" should be a string, and the return value will be a string
+    decoded according to locale encoding, or by "encoding" if set. Text mode
+    is triggered by setting any of text, encoding, errors or universal_newlines.
     """
     if 'stdout' in kwargs:
         raise ValueError('stdout argument not allowed, it will be overridden.')
@@ -384,15 +387,17 @@ def run(*popenargs, input=None, timeout=None, check=False, **kwargs):
     exception will be raised.
 
     There is an optional argument "input", allowing you to
-    pass a string to the subprocess's stdin.  If you use this argument
+    pass bytes or a string to the subprocess's stdin.  If you use this argument
     you may not also use the Popen constructor's "stdin" argument, as
     it will be used internally.
 
-    The other arguments are the same as for the Popen constructor.
+    By default, all communication is in bytes, and therefore any "input" should
+    be bytes, and the stdout and stderr will be bytes. If in text mode, any
+    "input" should be a string, and stdout and stderr will be strings decoded
+    according to locale encoding, or by "encoding" if set. Text mode is
+    triggered by setting any of text, encoding, errors or universal_newlines.
 
-    If universal_newlines=True is passed, the "input" argument must be a
-    string and stdout/stderr in the returned object will be strings rather than
-    bytes.
+    The other arguments are the same as for the Popen constructor.
     """
     if input is not None:
         if 'stdin' in kwargs:
@@ -513,7 +518,7 @@ def getstatusoutput(cmd):
     (-15, '')
     """
     try:
-        data = check_output(cmd, shell=True, universal_newlines=True, stderr=STDOUT)
+        data = check_output(cmd, shell=True, text=True, stderr=STDOUT)
         exitcode = 0
     except CalledProcessError as ex:
         data = ex.output
@@ -565,8 +570,10 @@ class Popen(object):
 
       env: Defines the environment variables for the new process.
 
-      universal_newlines: If true, use universal line endings for file
-          objects stdin, stdout and stderr.
+      text: If true, decode stdin, stdout and stderr using the given encoding
+          (if set) or the system default otherwise.
+
+      universal_newlines: Alias of text, provided for backwards compatibility.
 
       startupinfo and creationflags (Windows only)
 
@@ -587,10 +594,10 @@ class Popen(object):
     def __init__(self, args, bufsize=-1, executable=None,
                  stdin=None, stdout=None, stderr=None,
                  preexec_fn=None, close_fds=_PLATFORM_DEFAULT_CLOSE_FDS,
-                 shell=False, cwd=None, env=None, universal_newlines=False,
+                 shell=False, cwd=None, env=None, universal_newlines=None,
                  startupinfo=None, creationflags=0,
                  restore_signals=True, start_new_session=False,
-                 pass_fds=(), *, encoding=None, errors=None):
+                 pass_fds=(), *, encoding=None, errors=None, text=None):
         """Create new Popen instance."""
         _cleanup()
         # Held while anything is calling waitpid before returncode has been
@@ -642,10 +649,16 @@ class Popen(object):
         self.stderr = None
         self.pid = None
         self.returncode = None
-        self.universal_newlines = universal_newlines
         self.encoding = encoding
         self.errors = errors
 
+        # Validate the combinations of text and universal_newlines
+        if (text is not None and universal_newlines is not None
+            and bool(universal_newlines) != bool(text)):
+            raise SubprocessError('Cannot disambiguate when both text '
+                                  'and universal_newlines are supplied but '
+                                  'different. Pass one or the other.')
+
         # Input and output objects. The general principle is like
         # this:
         #
@@ -677,25 +690,25 @@ class Popen(object):
             if errread != -1:
                 errread = msvcrt.open_osfhandle(errread.Detach(), 0)
 
-        text_mode = encoding or errors or universal_newlines
+        self.text_mode = encoding or errors or text or universal_newlines
 
         self._closed_child_pipe_fds = False
 
         try:
             if p2cwrite != -1:
                 self.stdin = io.open(p2cwrite, 'wb', bufsize)
-                if text_mode:
+                if self.text_mode:
                     self.stdin = io.TextIOWrapper(self.stdin, write_through=True,
                             line_buffering=(bufsize == 1),
                             encoding=encoding, errors=errors)
             if c2pread != -1:
                 self.stdout = io.open(c2pread, 'rb', bufsize)
-                if text_mode:
+                if self.text_mode:
                     self.stdout = io.TextIOWrapper(self.stdout,
                             encoding=encoding, errors=errors)
             if errread != -1:
                 self.stderr = io.open(errread, 'rb', bufsize)
-                if text_mode:
+                if self.text_mode:
                     self.stderr = io.TextIOWrapper(self.stderr,
                             encoding=encoding, errors=errors)
 
@@ -735,6 +748,16 @@ class Popen(object):
 
             raise
 
+    @property
+    def universal_newlines(self):
+        # universal_newlines as retained as an alias of text_mode for API
+        # compatability. bpo-31756
+        return self.text_mode
+
+    @universal_newlines.setter
+    def universal_newlines(self, universal_newlines):
+        self.text_mode = bool(universal_newlines)
+
     def _translate_newlines(self, data, encoding, errors):
         data = data.decode(encoding, errors)
         return data.replace("\r\n", "\n").replace("\r", "\n")
@@ -805,12 +828,16 @@ class Popen(object):
         reached.  Wait for process to terminate.
 
         The optional "input" argument should be data to be sent to the
-        child process (if self.universal_newlines is True, this should
-        be a string; if it is False, "input" should be bytes), or
-        None, if no data should be sent to the child.
-
-        communicate() returns a tuple (stdout, stderr).  These will be
-        bytes or, if self.universal_newlines was True, a string.
+        child process, or None, if no data should be sent to the child.
+        communicate() returns a tuple (stdout, stderr).
+
+        By default, all communication is in bytes, and therefore any
+        "input" should be bytes, and the (stdout, stderr) will be bytes.
+        If in text mode (indicated by self.text_mode), any "input" should
+        be a string, and (stdout, stderr) will be strings decoded
+        according to locale encoding, or by "encoding" if set. Text mode
+        is triggered by setting any of text, encoding, errors or
+        universal_newlines.
         """
 
         if self._communication_started and input:
@@ -1533,7 +1560,7 @@ class Popen(object):
 
             # Translate newlines, if requested.
             # This also turns bytes into strings.
-            if self.encoding or self.errors or self.universal_newlines:
+            if self.text_mode:
                 if stdout is not None:
                     stdout = self._translate_newlines(stdout,
                                                       self.stdout.encoding,
@@ -1553,8 +1580,7 @@ class Popen(object):
             if self.stdin and self._input is None:
                 self._input_offset = 0
                 self._input = input
-                if input is not None and (
-                    self.encoding or self.errors or self.universal_newlines):
+                if input is not None and self.text_mode:
                     self._input = self._input.encode(self.stdin.encoding,
                                                      self.stdin.errors)
 

Lib/test/test_subprocess.py

@@ -845,8 +845,9 @@ class ProcessTestCase(BaseTestCase):
         self.assertEqual(stdout, b"bananasplit")
         self.assertStderrEqual(stderr, b"")
 
-    def test_universal_newlines(self):
-        p = subprocess.Popen([sys.executable, "-c",
+    def test_universal_newlines_and_text(self):
+        args = [
+            sys.executable, "-c",
             'import sys,os;' + SETBINARY +
             'buf = sys.stdout.buffer;'
             'buf.write(sys.stdin.readline().encode());'
@@ -863,10 +864,12 @@ class ProcessTestCase(BaseTestCase):
             'buf.flush();'
             'buf.write(b"\\nline7");'
             'buf.flush();'
-                              'buf.write(b"\\nline8");'],
-                             stdin=subprocess.PIPE,
-                             stdout=subprocess.PIPE,
-                             universal_newlines=1)
+            'buf.write(b"\\nline8");']
+
+        for extra_kwarg in ('universal_newlines', 'text'):
+            p = subprocess.Popen(args, **{'stdin': subprocess.PIPE,
+                                          'stdout': subprocess.PIPE,
+                                          extra_kwarg: True})
             with p:
                 p.stdin.write("line1\n")
                 p.stdin.flush()

Misc/NEWS.d/next/Library/2017-10-20-12-57-52.bpo-31756.IxCvGB.rst

+Add a ``subprocess.Popen(text=False)`` keyword argument to `subprocess`
+functions to be more explicit about when the library should attempt to
+decode outputs into text. Patch by Andrew Clegg.