From bca0e40d5f4be8228acc158e07fbd43697c79f5f Mon Sep 17 00:00:00 2001
From: Zac-HD <zac.hatfield.dodds@gmail.com>
Date: Sun, 23 Jun 2019 13:26:39 +1000
Subject: [PATCH] bpo-26967: fix argparse flag grouping with allow_abbrev=False

The `allow_abbrev` option for ArgumentParser is documented and
intended to disable support for unique prefixes of --options,
which may sometimes be ambiguous due to deferred parsing.

However, the initial implementation also broke parsing of
grouped short flags, such as `-ab` meaning `-a -b`.
Checking for a leading `--` fixes this.
---
 Doc/library/argparse.rst                      |  4 ++++
 Lib/argparse.py                               |  2 +-
 Lib/test/test_argparse.py                     | 19 +++++++++++++++++++
 Misc/ACKS                                     |  1 +
 .../2019-06-23-12-46-10.bpo-26967.xEuem1.rst  |  3 +++
 5 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-06-23-12-46-10.bpo-26967.xEuem1.rst

diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index b77a38ccd48577..ef2fd42783c877 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -182,6 +182,10 @@ ArgumentParser objects
    .. versionchanged:: 3.5
       *allow_abbrev* parameter was added.
 
+   .. versionchanged:: 3.8
+      In previous versions, *allow_abbrev* also disabled grouping of short
+      flags such as ``-vv`` to mean ``-v -v``.
+
 The following sections describe how each of these are used.
 
 
diff --git a/Lib/argparse.py b/Lib/argparse.py
index 4f3aea928bf6f4..e45b67b67704a9 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2130,7 +2130,7 @@ def _parse_optional(self, arg_string):
                 action = self._option_string_actions[option_string]
                 return action, option_string, explicit_arg
 
-        if self.allow_abbrev:
+        if self.allow_abbrev or not arg_string.startswith('--'):
             # search through all possible prefixes of the option string
             # and all actions in the parser for possible interpretations
             option_tuples = self._get_option_tuples(arg_string)
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index bcf2cc9b26a319..765fdb99f9c8cb 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -786,6 +786,25 @@ class TestOptionalsDisallowLongAbbreviation(ParserTestCase):
         ('--foonly 7 --foodle --foo 2', NS(foo='2', foodle=True, foonly='7')),
     ]
 
+
+class TestDisallowLongAbbreviationAllowsShortGrouping(ParserTestCase):
+    """Do not allow abbreviations of long options at all"""
+
+    parser_signature = Sig(allow_abbrev=False)
+    argument_signatures = [
+        Sig('-r'),
+        Sig('-c', action='count'),
+    ]
+    failures = ['-r', '-c -r']
+    successes = [
+        ('', NS(r=None, c=None)),
+        ('-ra', NS(r='a', c=None)),
+        ('-rcc', NS(r='cc', c=None)),
+        ('-cc', NS(r=None, c=2)),
+        ('-cc -ra', NS(r='a', c=2)),
+        ('-ccrcc', NS(r='cc', c=2)),
+    ]
+
 # ================
 # Positional tests
 # ================
diff --git a/Misc/ACKS b/Misc/ACKS
index 05a3e61f694149..611fcad4a23a7f 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -641,6 +641,7 @@ Travis B. Hartwell
 Shane Harvey
 Larry Hastings
 Tim Hatch
+Zac Hatfield-Dodds
 Shane Hathaway
 Michael Haubenwallner
 Janko Hauser
diff --git a/Misc/NEWS.d/next/Library/2019-06-23-12-46-10.bpo-26967.xEuem1.rst b/Misc/NEWS.d/next/Library/2019-06-23-12-46-10.bpo-26967.xEuem1.rst
new file mode 100644
index 00000000000000..c5852f6142150d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-06-23-12-46-10.bpo-26967.xEuem1.rst
@@ -0,0 +1,3 @@
+An :class:`~argparse.ArgumentParser` with ``allow_abbrev=False`` no longer
+disables grouping of short flags, such as ``-vv``, but only disables
+abbreviation of long flags as documented. Patch by Zac Hatfield-Dodds.