feat: Support python.exe fallback on Windows

src/virtualenv/discovery/windows/pep514.py

@@ -56,40 +56,60 @@ def process_company(hive_name, company, root_key, default_arch):
 
 
 def process_tag(hive_name, company, company_key, tag, default_arch):
+    LOGGER.info("Processing tag %s for company %s", tag, company)
     with winreg.OpenKeyEx(company_key, tag) as tag_key:
         version = load_version_data(hive_name, company, tag, tag_key)
         if version is not None:  # if failed to get version bail
             major, minor, _ = version
+            LOGGER.info("Found version %s.%s", major, minor)
             arch = load_arch_data(hive_name, company, tag, tag_key, default_arch)
             if arch is not None:
-                exe_data = load_exe(hive_name, company, company_key, tag)
+                LOGGER.info("Found arch %s", arch)
+                exe_data = load_exe(hive_name, company, company_key, tag, major)
                 if exe_data is not None:
                     exe, args = exe_data
+                    LOGGER.info("Found exe %s", exe)
                     threaded = load_threaded(hive_name, company, tag, tag_key)
                     return company, major, minor, arch, threaded, exe, args
-                return None
-            return None
-        return None
+    return None
 
 
-def load_exe(hive_name, company, company_key, tag):
+def load_exe(hive_name, company, company_key, tag, major):
     key_path = f"{hive_name}/{company}/{tag}"
+    LOGGER.info("Loading exe for %s", key_path)
     try:
-        with winreg.OpenKeyEx(company_key, rf"{tag}\InstallPath") as ip_key, ip_key:
+        with winreg.OpenKeyEx(company_key, rf"{tag}\InstallPath") as ip_key:
             exe = get_value(ip_key, "ExecutablePath")
+            LOGGER.info("ExecutablePath from registry: %s", exe)
             if exe is None:
-                ip = get_value(ip_key, None)
-                if ip is None:
-                    msg(key_path, "no ExecutablePath or default for it")
+                exe = _find_exe_in_install_path(ip_key, key_path, major)
+                LOGGER.info("Executable from install path: %s", exe)
 
-                else:
-                    exe = os.path.join(ip, "python.exe")
             if exe is not None and os.path.exists(exe):
                 args = get_value(ip_key, "ExecutableArguments")
+                LOGGER.info("Found executable %s with args %s", exe, args)
                 return exe, args
             msg(key_path, f"could not load exe with value {exe}")
     except OSError:
         msg(f"{key_path}/InstallPath", "missing")
+    LOGGER.info("Failed to load exe for %s", key_path)
+    return None
+
+
+def _find_exe_in_install_path(ip_key, key_path, major):
+    ip = get_value(ip_key, None)
+    if ip is None:
+        msg(key_path, "no ExecutablePath or default for it")
+        return None
+
+    names = ("python.exe",)
+    if major == 3:
+        names = ("python3.exe", "python.exe")
+
+    for name in names:
+        exe_path = os.path.join(ip, name)
+        if os.path.exists(exe_path):
+            return exe_path
     return None
 
 

tests/unit/discovery/windows/test_windows_pep514.py

@@ -80,7 +80,7 @@ def test_pep514():
         ),
         ("CompanyA", 3, 6, 64, False, "Z:\\CompanyA\\Python\\3.6\\python.exe", None),
         ("PythonCore", 2, 7, 64, False, "C:\\Python27\\python.exe", None),
-        ("PythonCore", 3, 7, 64, False, "C:\\Python37\\python.exe", None),
+        ("PythonCore", 3, 7, 64, False, "C:\\Python37\\python3.exe", None),
     ]
 
 
@@ -100,7 +100,7 @@ def test_pep514_run(capsys, caplog):
     ('PythonCore', 3, 10, 32, False, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python310-32\\python.exe', None)
     ('PythonCore', 3, 12, 64, False, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python312\\python.exe', None)
     ('PythonCore', 3, 13, 64, True, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python313\\python3.13t.exe', None)
-    ('PythonCore', 3, 7, 64, False, 'C:\\Python37\\python.exe', None)
+    ('PythonCore', 3, 7, 64, False, 'C:\\Python37\\python3.exe', None)
     ('PythonCore', 3, 8, 64, False, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python38\\python.exe', None)
     ('PythonCore', 3, 9, 64, False, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python39\\python.exe', None)
     ('PythonCore', 3, 9, 64, False, 'C:\\Users\\user\\AppData\\Local\\Programs\\Python\\Python39\\python.exe', None)
@@ -121,3 +121,54 @@ def test_pep514_run(capsys, caplog):
         f"{prefix}HKEY_CURRENT_USER/PythonCore/3.X error: invalid format 3.X",
     ]
     assert caplog.messages == expected_logs
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="no Windows registry")
+def test_pep514_python3_fallback(mocker, tmp_path):
+    from virtualenv.discovery.windows import pep514
+    from virtualenv.discovery.windows.pep514 import winreg
+
+    # Create a mock python3.exe, but no python.exe
+    python3_exe = tmp_path / "python3.exe"
+    python3_exe.touch()
+    mocker.patch("os.path.exists", side_effect=lambda p: str(p) == str(python3_exe))
+
+    # Mock winreg functions to simulate a single Python installation
+    mock_key = mocker.MagicMock()
+
+    def open_key_ex(key, sub_key, *args, **kwargs):
+        if sub_key == r"Software\Python":
+            return "Python"
+        if key == "Python" and sub_key == "PythonCore":
+            return "PythonCore"
+        if key == "PythonCore" and sub_key == "3.7-32":
+            return "3.7-32"
+        if key == "3.7-32" and sub_key == "InstallPath":
+            return mock_key
+        raise FileNotFoundError
+
+    mocker.patch.object(winreg, "OpenKeyEx", side_effect=open_key_ex)
+
+    def enum_key(key, at):
+        if key == "Python" and at == 0:
+            return "PythonCore"
+        if key == "PythonCore" and at == 0:
+            return "3.7-32"
+        raise StopIteration
+
+    mocker.patch.object(winreg, "EnumKey", side_effect=enum_key)
+
+    def get_value(key, name):
+        if name == "ExecutablePath":
+            raise FileNotFoundError
+        if name is None:
+            return str(tmp_path)
+        return "3.7"
+
+    mocker.patch.object(winreg, "QueryValueEx", side_effect=get_value)
+    mocker.patch.object(pep514, "load_arch_data", return_value=32)
+    mocker.patch.object(pep514, "load_threaded", return_value=False)
+
+    interpreters = list(pep514.discover_pythons())
+
+    assert interpreters == [("PythonCore", 3, 7, 32, False, str(python3_exe), None)]