The Win32 API function linked by eryksun is the correct way to determine whether to include the current directory in the search - the doc specifically refers to matching cmd.exe's behaviour (searching ".;%PATH%" vs. just "%PATH%").

The PATHEXT behaviour should be to look for the name as provided first, followed be appending each extension. So looking for "python" will look for ["python", *("python" + p for p in getenv('PATHEXT').split(';'))] in that order.