docs,tests: Clarify how py_wheel.strip_path_prefixes works; add test case

python/private/py_wheel.bzl

@@ -217,7 +217,15 @@ _other_attrs = {
     ),
     "strip_path_prefixes": attr.string_list(
         default = [],
-        doc = "path prefixes to strip from files added to the generated package",
+        doc = """\
+Path prefixes to strip from files added to the generated package.
+Prefixes are checked **in order** and only the **first match** will be used.
+
+For example:
++ `["foo", "foo/bar/baz"]` will strip `"foo/bar/baz/file.py"` to `"bar/baz/file.py"`
++ `["foo/bar/baz", "foo"]` will strip `"foo/bar/baz/file.py"` to `"file.py"` and
+  `"foo/file2.py"` to `"file2.py"`
+""",
     ),
     "summary": attr.string(
         doc = "A one-line summary of what the distribution does",

tests/tools/BUILD.bazel

@@ -0,0 +1,23 @@
+# Copyright 2025 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+load("//python:py_test.bzl", "py_test")
+
+licenses(["notice"])
+
+py_test(
+    name = "wheelmaker_test",
+    size = "small",
+    srcs = ["wheelmaker_test.py"],
+    deps = ["//tools:wheelmaker"],
+)

tests/tools/wheelmaker_test.py

@@ -0,0 +1,38 @@
+import unittest
+
+import tools.wheelmaker as wheelmaker
+
+
+class ArcNameFromTest(unittest.TestCase):
+    def test_arcname_from(self) -> None:
+        # (name, distribution_prefix, strip_path_prefixes, want) tuples
+        checks = [
+            ("a/b/c/file.py", "", [], "a/b/c/file.py"),
+            ("a/b/c/file.py", "", ["a"], "/b/c/file.py"),
+            ("a/b/c/file.py", "", ["a/b/"], "c/file.py"),
+            # only first found is used and it's not cumulative.
+            ("a/b/c/file.py", "", ["a/", "b/"], "b/c/file.py"),
+            # Examples from docs
+            ("foo/bar/baz/file.py", "", ["foo", "foo/bar/baz"], "/bar/baz/file.py"),
+            ("foo/bar/baz/file.py", "", ["foo/bar/baz", "foo"], "/file.py"),
+            ("foo/file2.py", "", ["foo/bar/baz", "foo"], "/file2.py"),
+            # Files under the distribution prefix (eg mylib-1.0.0-dist-info)
+            # are unmodified
+            ("mylib-0.0.1-dist-info/WHEEL", "mylib", [], "mylib-0.0.1-dist-info/WHEEL"),
+            ("mylib/a/b/c/WHEEL", "mylib", ["mylib"], "mylib/a/b/c/WHEEL"),
+        ]
+        for name, prefix, strip, want in checks:
+            with self.subTest(
+                name=name,
+                distribution_prefix=prefix,
+                strip_path_prefixes=strip,
+                want=want,
+            ):
+                got = wheelmaker.arcname_from(
+                    name=name, distribution_prefix=prefix, strip_path_prefixes=strip
+                )
+                self.assertEqual(got, want)
+
+
+if __name__ == "__main__":
+    unittest.main()

tools/wheelmaker.py

@@ -24,6 +24,7 @@
 import stat
 import sys
 import zipfile
+from collections.abc import Iterable
 from pathlib import Path
 
 _ZIP_EPOCH = (1980, 1, 1, 0, 0, 0)
@@ -98,6 +99,30 @@ def normalize_pep440(version):
         return str(packaging.version.Version(f"0+{sanitized}"))
 
 
+def arcname_from(
+    name: str, distribution_prefix: str, strip_path_prefixes: Sequence[str] = ()
+) -> str:
+    """Return the within-archive name for a given file path name.
+
+    Prefixes to strip are checked in order and only the first match will be used.
+
+    Args:
+        name: The file path eg 'mylib/a/b/c/file.py'
+        distribution_prefix: The
+        strip_path_prefixes: Remove these prefixes from names.
+    """
+    # Always use unix path separators.
+    normalized_arcname = name.replace(os.path.sep, "/")
+    # Don't manipulate names filenames in the .distinfo or .data directories.
+    if distribution_prefix and normalized_arcname.startswith(distribution_prefix):
+        return normalized_arcname
+    for prefix in strip_path_prefixes:
+        if normalized_arcname.startswith(prefix):
+            return normalized_arcname[len(prefix) :]
+
+    return normalized_arcname
+
+
 class _WhlFile(zipfile.ZipFile):
     def __init__(
         self,
@@ -126,18 +151,6 @@ def data_path(self, basename):
     def add_file(self, package_filename, real_filename):
         """Add given file to the distribution."""
 
-        def arcname_from(name):
-            # Always use unix path separators.
-            normalized_arcname = name.replace(os.path.sep, "/")
-            # Don't manipulate names filenames in the .distinfo or .data directories.
-            if normalized_arcname.startswith(self._distribution_prefix):
-                return normalized_arcname
-            for prefix in self._strip_path_prefixes:
-                if normalized_arcname.startswith(prefix):
-                    return normalized_arcname[len(prefix) :]
-
-            return normalized_arcname
-
         if os.path.isdir(real_filename):
             directory_contents = os.listdir(real_filename)
             for file_ in directory_contents:
@@ -147,7 +160,11 @@ def arcname_from(name):
                 )
             return
 
-        arcname = arcname_from(package_filename)
+        arcname = arcname_from(
+            package_filename,
+            distribution_prefix=self._distribution_prefix,
+            strip_path_prefixes=self._strip_path_prefixes,
+        )
         zinfo = self._zipinfo(arcname)
 
         # Write file to the zip archive while computing the hash and length
@@ -569,7 +586,9 @@ def get_new_requirement_line(reqs_text, extra):
                 else:
                     return f"Requires-Dist: {req.name}{req_extra_deps}{req.specifier}; {req.marker}"
             else:
-                return f"Requires-Dist: {req.name}{req_extra_deps}{req.specifier}; {extra}".strip(" ;")
+                return f"Requires-Dist: {req.name}{req_extra_deps}{req.specifier}; {extra}".strip(
+                    " ;"
+                )
 
         for meta_line in metadata.splitlines():
             if not meta_line.startswith("Requires-Dist: "):