bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337)

When `allow_abbrev` was first added, disabling the abbreviation of
long options broke the grouping of short flags ([bpo-26967](https://bugs.python.org/issue26967)).  As a fix,
b1e4d1b603 (contained in v3.8) ignores `allow_abbrev=False` for a
given argument string if the string does _not_ start with "--"
(i.e. it doesn't look like a long option).

This fix, however, doesn't take into account that long options can
start with alternative characters specified via `prefix_chars`,
introducing a regression: `allow_abbrev=False` has no effect on long
options that start with an alternative prefix character.

The most minimal fix would be to replace the "starts with --" check
with a "starts with two prefix_chars characters".  But
`_get_option_tuples` already distinguishes between long and short
options, so let's instead piggyback off of that check by moving the
`allow_abbrev` condition into `_get_option_tuples`.





https://bugs.python.org/issue39546

Lib/argparse.py

@@ -2199,7 +2199,6 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
                 action = self._option_string_actions[option_string]
                 return action, option_string, explicit_arg
 
-        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)
@@ -2240,6 +2239,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         # split at the '='
         chars = self.prefix_chars
         if option_string[0] in chars and option_string[1] in chars:
+            if self.allow_abbrev:
                 if '=' in option_string:
                     option_prefix, explicit_arg = option_string.split('=', 1)
                 else:

Lib/test/test_argparse.py

@@ -810,6 +810,23 @@ class TestOptionalsDisallowLongAbbreviation(ParserTestCase):
     ]
 
 
+class TestOptionalsDisallowLongAbbreviationPrefixChars(ParserTestCase):
+    """Disallowing abbreviations works with alternative prefix characters"""
+
+    parser_signature = Sig(prefix_chars='+', allow_abbrev=False)
+    argument_signatures = [
+        Sig('++foo'),
+        Sig('++foodle', action='store_true'),
+        Sig('++foonly'),
+    ]
+    failures = ['+foon 3', '++foon 3', '++food', '++food ++foo 2']
+    successes = [
+        ('', NS(foo=None, foodle=False, foonly=None)),
+        ('++foo 3', NS(foo='3', foodle=False, foonly=None)),
+        ('++foonly 7 ++foodle ++foo 2', NS(foo='2', foodle=True, foonly='7')),
+    ]
+
+
 class TestDisallowLongAbbreviationAllowsShortGrouping(ParserTestCase):
     """Do not allow abbreviations of long options at all"""
 
@@ -828,6 +845,26 @@ class TestDisallowLongAbbreviationAllowsShortGrouping(ParserTestCase):
         ('-ccrcc', NS(r='cc', c=2)),
     ]
 
+
+class TestDisallowLongAbbreviationAllowsShortGroupingPrefix(ParserTestCase):
+    """Short option grouping works with custom prefix and allow_abbrev=False"""
+
+    parser_signature = Sig(prefix_chars='+', 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
 # ================

Misc/ACKS

@@ -1112,6 +1112,7 @@ Bruce Merry
 Alexis Métaireau
 Luke Mewburn
 Carl Meyer
+Kyle Meyer
 Mike Meyer
 Piotr Meyer
 Steven Miale

Misc/NEWS.d/next/Library/2020-02-03-15-12-51.bpo-39546._Kj0Pn.rst

@@ -0,0 +1,3 @@
+Fix a regression in :class:`~argparse.ArgumentParser` where
+``allow_abbrev=False`` was ignored for long options that used a prefix
+character other than "-".