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

1 files changed, 28 insertions, 28 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 5d3ce2a..9c710ce 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2199,24 +2199,23 @@ 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)
-
-            # if multiple actions match, the option string was ambiguous
-            if len(option_tuples) > 1:
-                options = ', '.join([option_string
-                    for action, option_string, explicit_arg in option_tuples])
-                args = {'option': arg_string, 'matches': options}
-                msg = _('ambiguous option: %(option)s could match %(matches)s')
-                self.error(msg % args)
-
-            # if exactly one action matched, this segmentation is good,
-            # so return the parsed action
-            elif len(option_tuples) == 1:
-                option_tuple, = option_tuples
-                return option_tuple
+        # 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)
+
+        # if multiple actions match, the option string was ambiguous
+        if len(option_tuples) > 1:
+            options = ', '.join([option_string
+                for action, option_string, explicit_arg in option_tuples])
+            args = {'option': arg_string, 'matches': options}
+            msg = _('ambiguous option: %(option)s could match %(matches)s')
+            self.error(msg % args)
+
+        # if exactly one action matched, this segmentation is good,
+        # so return the parsed action
+        elif len(option_tuples) == 1:
+            option_tuple, = option_tuples
+            return option_tuple
 
         # if it was not found as an option, but it looks like a negative
         # number, it was meant to be positional
@@ -2240,16 +2239,17 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         # split at the '='
         chars = self.prefix_chars
         if option_string[0] in chars and option_string[1] in chars:
-            if '=' in option_string:
-                option_prefix, explicit_arg = option_string.split('=', 1)
-            else:
-                option_prefix = option_string
-                explicit_arg = None
-            for option_string in self._option_string_actions:
-                if option_string.startswith(option_prefix):
-                    action = self._option_string_actions[option_string]
-                    tup = action, option_string, explicit_arg
-                    result.append(tup)
+            if self.allow_abbrev:
+                if '=' in option_string:
+                    option_prefix, explicit_arg = option_string.split('=', 1)
+                else:
+                    option_prefix = option_string
+                    explicit_arg = None
+                for option_string in self._option_string_actions:
+                    if option_string.startswith(option_prefix):
+                        action = self._option_string_actions[option_string]
+                        tup = action, option_string, explicit_arg
+                        result.append(tup)
 
         # single character options can be concatenated with their arguments
         # but multiple character options always have to have their argument