feat: Support python.exe fallback on Windows

docs/changelog/2774.bugfix.rst

@@ -0,0 +1,2 @@
+Fall back to python.exe if python3.exe is not found on Windows.
+Contributed by :user:`esafak`.

src/virtualenv/discovery/windows/pep514.py

@@ -75,15 +75,11 @@ def process_tag(hive_name, company, company_key, tag, default_arch):
 def load_exe(hive_name, company, company_key, tag):
     key_path = f"{hive_name}/{company}/{tag}"
     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")
             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)
 
-                else:
-                    exe = os.path.join(ip, "python.exe")
             if exe is not None and os.path.exists(exe):
                 args = get_value(ip_key, "ExecutableArguments")
                 return exe, args
@@ -93,6 +89,19 @@ def load_exe(hive_name, company, company_key, tag):
     return None
 
 
+def _find_exe_in_install_path(ip_key, key_path):
+    ip = get_value(ip_key, None)
+    if ip is None:
+        msg(key_path, "no ExecutablePath or default for it")
+        return None
+
+    for name in ("python3.exe", "python.exe"):
+        exe_path = os.path.join(ip, name)
+        if os.path.exists(exe_path):
+            return exe_path
+    return None
+
+
 def load_arch_data(hive_name, company, tag, tag_key, default_arch):
     arch_str = get_value(tag_key, "SysArchitecture")
     if arch_str is not None:

tests/unit/discovery/windows/test_windows_pep514.py

@@ -121,3 +121,37 @@ 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.pep514 import discover_pythons, winreg  # noqa: PLC0415
+
+    # Create a mock python3.exe
+    python3_exe = tmp_path / "python3.exe"
+    python3_exe.touch()
+
+    # Mock the registry to return our test python distribution
+    def open_key_ex(key, sub_key, *args, **kwargs):
+        if sub_key == r"Software\Python\PythonCore\3.9-32":
+            return mocker.MagicMock()
+        if sub_key == r"Software\Python\PythonCore\3.9-32\InstallPath":
+            return mocker.MagicMock()
+        return winreg.OpenKeyEx(key, sub_key, *args, **kwargs)
+
+    def query_value_ex(key, value_name, *args, **kwargs):
+        if value_name == "ExecutablePath":
+            return None  # No executable path, forcing a fallback
+        if value_name is None:
+            return str(tmp_path)
+        return winreg.QueryValueEx(key, value_name, *args, **kwargs)
+
+    mocker.patch("winreg.OpenKeyEx", side_effect=open_key_ex)
+    mocker.patch("winreg.QueryValueEx", side_effect=query_value_ex)
+    mocker.patch("os.path.exists", return_value=True)
+
+    # Mock EnumKey to inject our test distribution
+    mocker.patch("winreg.EnumKey", return_value=["3.9-32"])
+
+    interpreters = list(discover_pythons())
+    assert any(str(python3_exe) in i for i in interpreters)