[Python-checkins] bpo-33109: argparse subparsers are once again not required by default (GH-6919)

[Ned Deily]

https://github.com/python/cpython/commit/8ebf5ceb0f5408d1ebc26c19702ac0762ef5ea04
commit: 8ebf5ceb0f5408d1ebc26c19702ac0762ef5ea04
branch: master
author: Ned Deily <nad at python.org>
committer: GitHub <noreply at github.com>
date: 2018-05-23T21:55:15-04:00
summary:

bpo-33109: argparse subparsers are once again not required by default (GH-6919)

bpo-26510 in 3.7.0a2 changed the behavior of argparse to make
subparsers required by default, returning to the behavior of 2.7
and 3.2. The behavior was changed in 3.3 to be no longer required.
While it might make more sense to have the default to required,
compatibility with 3.3 through 3.6 is probably less disruptive
than trying to reintroduce compatibility with 2.7 at this point.
This change restores the 3.6 behavior.

files:
A Misc/NEWS.d/next/Library/2018-05-16-14-57-58.bpo-33109.nPLL_S.rst
M Doc/library/argparse.rst
M Lib/argparse.py
M Lib/test/test_argparse.py
M Misc/NEWS.d/3.7.0a2.rst

diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index 3f327c0cfa77..260e5cbf7000 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -1577,7 +1577,7 @@ Sub-commands
      stored; by default ``None`` and no value is stored
 
    * required_ - Whether or not a subcommand must be provided, by default
-     ``True``.
+     ``False``.
 
    * help_ - help for sub-parser group in help output, by default ``None``
 
diff --git a/Lib/argparse.py b/Lib/argparse.py
index e3da7f0443cf..e0e367bf20ca 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1077,7 +1077,7 @@ def __init__(self,
                  prog,
                  parser_class,
                  dest=SUPPRESS,
-                 required=True,
+                 required=False,
                  help=None,
                  metavar=None):
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index c4440e4df7c1..bcf15ce123e4 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1932,7 +1932,9 @@ def test_required_subparsers_default(self):
         parser = ErrorRaisingArgumentParser()
         subparsers = parser.add_subparsers(dest='command')
         subparsers.add_parser('run')
-        self._test_required_subparsers(parser)
+        # No error here
+        ret = parser.parse_args(())
+        self.assertIsNone(ret.command)
 
     def test_optional_subparsers(self):
         parser = ErrorRaisingArgumentParser()
diff --git a/Misc/NEWS.d/3.7.0a2.rst b/Misc/NEWS.d/3.7.0a2.rst
index 190038d5dd12..621de6d78a76 100644
--- a/Misc/NEWS.d/3.7.0a2.rst
+++ b/Misc/NEWS.d/3.7.0a2.rst
@@ -477,6 +477,8 @@ module now requires sqlite version at least 3.3.9.
 argparse subparsers are now required by default.  This matches behaviour in
 Python 2. For optional subparsers, use the new parameter
 ``add_subparsers(required=False)``. Patch by Anthony Sottile.
+(As of 3.7.0rc1, the default was changed to not required as had been the case
+since Python 3.3.)
 
 ..
 
diff --git a/Misc/NEWS.d/next/Library/2018-05-16-14-57-58.bpo-33109.nPLL_S.rst b/Misc/NEWS.d/next/Library/2018-05-16-14-57-58.bpo-33109.nPLL_S.rst
new file mode 100644
index 000000000000..be731f99f7f0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-05-16-14-57-58.bpo-33109.nPLL_S.rst
@@ -0,0 +1,2 @@
+argparse subparsers are once again not required by default, reverting the
+change in behavior introduced by bpo-26510 in 3.7.0a2.