Stop best_effort_version duplicating part of a v-prefixed version

newsfragments/5338.bugfix.rst

@@ -0,0 +1,2 @@
+Stopped duplicating part of the version when sanitizing an unparseable version
+that starts with a ``v`` prefix.

setuptools/_normalization.py

@@ -81,6 +81,8 @@ def best_effort_version(version: str) -> str:
     '0.dev0+sanitized'
     >>> best_effort_version("42.+?1")
     '42.dev0+sanitized.1'
+    >>> best_effort_version("v42.+?1")  # a `v` prefix changes nothing
+    '42.dev0+sanitized.1'
     """
     try:
         return safe_version(version)
@@ -89,7 +91,10 @@ def best_effort_version(version: str) -> str:
         match = _PEP440_FALLBACK.search(v)
         if match:
             safe = match["safe"]
-            rest = v[len(safe) :]
+            # `safe` may be shorter than the match, because `_PEP440_FALLBACK`
+            # also consumes an optional `v` prefix. Slice from the end of the
+            # whole match, so the prefix is not left out of `rest`.
+            rest = v[match.end() :]
         else:
             safe = "0"
             rest = version

setuptools/tests/test_normalization.py

@@ -0,0 +1,43 @@
+"""Tests for :mod:`setuptools._normalization`.
+
+The functions here are pure string transformations, so they are checked
+directly, without building anything.
+"""
+
+import pytest
+
+from setuptools import _normalization
+
+
+@pytest.mark.parametrize(
+    ('version', 'expected'),
+    [
+        # A `v` prefix is legal PEP 440 (and explicitly accepted by
+        # ``_PEP440_FALLBACK``), so it must not change the sanitized result.
+        ('0.23-', '0.23.dev0+sanitized'),
+        ('v0.23-', '0.23.dev0+sanitized'),
+        ('V0.23-', '0.23.dev0+sanitized'),
+        ('1.2-foo', '1.2.dev0+sanitized.foo'),
+        ('v1.2-foo', '1.2.dev0+sanitized.foo'),
+        ('1!2.0-foo', '1!2.0.dev0+sanitized.foo'),
+        ('v1!2.0-foo', '1!2.0.dev0+sanitized.foo'),
+        # Nothing after the numeric prefix: no leftover in the local segment.
+        ('v1.2', '1.2'),
+        ('v1.2.', '1.2.dev0+sanitized'),
+    ],
+)
+def test_best_effort_version_v_prefix_is_not_duplicated(version, expected):
+    """The ``v`` prefix is consumed by the fallback regex, so it must not be
+    miscounted when splitting the version into its numeric and remaining parts.
+    """
+    assert _normalization.best_effort_version(version) == expected
+
+
+@pytest.mark.parametrize('prefix', ['', 'v', 'V'])
+def test_best_effort_version_ignores_v_prefix(prefix):
+    """``best_effort_version`` is reached exactly when ``safe_version`` fails,
+    and both accept a ``v`` prefix, so adding one cannot change the result.
+    """
+    assert _normalization.best_effort_version(f'{prefix}1.2-foo') == (
+        _normalization.best_effort_version('1.2-foo')
+    )