Simplify auto-discovered package_dir

If the directory follows a src-layout-ish, try harder to
make `package_dir` in the form `{"": "src"}`.

This might be later important for PEP 660 (e.g. when composing pth
files or symlinking the toplevel packages).

2 files changed, 56 insertions, 3 deletions

diff --git a/setuptools/config/expand.py b/setuptools/config/expand.py
index ff9b2c9b..156d7473 100644
--- a/setuptools/config/expand.py
+++ b/setuptools/config/expand.py
@@ -312,8 +312,12 @@ def find_packages(
     where = kwargs.pop('where', ['.'])
     packages: List[str] = []
     fill_package_dir = {} if fill_package_dir is None else fill_package_dir
+    find = list(unique_everseen(always_iterable(where)))
 
-    for path in unique_everseen(always_iterable(where)):
+    if len(find) == 1 and all(not _same_path(find[0], x) for x in (".", root_dir)):
+        fill_package_dir.setdefault("", find[0])
+
+    for path in find:
         package_path = _nest_path(root_dir, path)
         pkgs = PackageFinder.find(package_path, **kwargs)
         packages.extend(pkgs)
@@ -326,8 +330,27 @@ def find_packages(
     return packages
 
 
+def _same_path(p1: _Path, p2: _Path) -> bool:
+    """Differs from os.path.samefile because it does not require paths to exist.
+    Purely string based (no comparison between i-nodes).
+    >>> _same_path("a/b", "./a/b")
+    True
+    >>> _same_path("a/b", "a/./b")
+    True
+    >>> _same_path("a/b", "././a/b")
+    True
+    >>> _same_path("a/b", "./a/b/c/..")
+    True
+    >>> _same_path("a/b", "../a/b/c")
+    False
+    >>> _same_path("a", "a/b")
+    False
+    """
+    return os.path.normpath(p1) == os.path.normpath(p2)
+
+
 def _nest_path(parent: _Path, path: _Path) -> str:
-    path = parent if path == "." else os.path.join(parent, path)
+    path = parent if path in {".", ""} else os.path.join(parent, path)
     return os.path.normpath(path)
 
 
diff --git a/setuptools/tests/config/test_expand.py b/setuptools/tests/config/test_expand.py
index 3a59edbb..15053c8f 100644
--- a/setuptools/tests/config/test_expand.py
+++ b/setuptools/tests/config/test_expand.py
@@ -130,7 +130,7 @@ def test_resolve_class(tmp_path, package_dir, file, module, return_value):
         ({}, {"pkg", "other", "dir1", "dir1.dir2"}),  # default value for `namespaces`
     ]
 )
-def test_find_packages(tmp_path, monkeypatch, args, pkgs):
+def test_find_packages(tmp_path, args, pkgs):
     files = {
         "pkg/__init__.py",
         "other/__init__.py",
@@ -153,3 +153,33 @@ def test_find_packages(tmp_path, monkeypatch, args, pkgs):
     ]
 
     assert set(expand.find_packages(where=where, **args)) == pkgs
+
+
+@pytest.mark.parametrize(
+    "files, where, expected_package_dir",
+    [
+        (["pkg1/__init__.py", "pkg1/other.py"], ["."], {}),
+        (["pkg1/__init__.py", "pkg2/__init__.py"], ["."], {}),
+        (["src/pkg1/__init__.py", "src/pkg1/other.py"], ["src"], {"": "src"}),
+        (["src/pkg1/__init__.py", "src/pkg2/__init__.py"], ["src"], {"": "src"}),
+        (
+            ["src1/pkg1/__init__.py", "src2/pkg2/__init__.py"],
+            ["src1", "src2"],
+            {"pkg1": "src1/pkg1", "pkg2": "src2/pkg2"},
+        ),
+        (
+            ["src/pkg1/__init__.py", "pkg2/__init__.py"],
+            ["src", "."],
+            {"pkg1": "src/pkg1"},
+        ),
+    ],
+)
+def test_fill_package_dir(tmp_path, files, where, expected_package_dir):
+    write_files({k: "" for k in files}, tmp_path)
+    pkg_dir = {}
+    kwargs = {"root_dir": tmp_path, "fill_package_dir": pkg_dir, "namespaces": False}
+    pkgs = expand.find_packages(where=where, **kwargs)
+    assert set(pkg_dir.items()) == set(expected_package_dir.items())
+    for pkg in pkgs:
+        pkg_path = find_package_path(pkg, pkg_dir, tmp_path)
+        assert os.path.exists(pkg_path)