pythonGH-124321: Fix argparse negative number parsing to capture -.5(p…

…ythonGH-124322) Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
savannahostrowski · Sep 23, 2024 · dc48312 · dc48312
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commit dc48312
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diff --git a/Lib/argparse.py b/Lib/argparse.py
@@ -1360,7 +1360,7 @@ def __init__(self,
         self._defaults = {}
 
         # determines whether an "option" looks like a negative number
-        self._negative_number_matcher = _re.compile(r'^-\d[\d_]*(\.\d[\d_]*)?$')
+        self._negative_number_matcher = _re.compile(r'^-(?:\d+(?:_\d+)*(?:\.\d+(?:_\d+)*)?|\.\d+(?:_\d+)*)$')
 
         # whether or not there are any optionals that look like negative
         # numbers -- uses a list so it can be shared and edited

diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
@@ -2111,6 +2111,8 @@ class TestNegativeNumber(ParserTestCase):
         ('--int -1_000_000 --float -1_000_000.0', NS(int=-1000000, float=-1000000.0)),
         ('--float -1_000.0', NS(int=None, float=-1000.0)),
         ('--float -1_000_000.0_0', NS(int=None, float=-1000000.0)),
+        ('--float -.5', NS(int=None, float=-0.5)),
+        ('--float -.5_000', NS(int=None, float=-0.5)),
     ]
 
 class TestInvalidAction(TestCase):