You could use the build in module winreg for this.

Then get the downloads folder from the registry which is

{374DE290-123F-4565-9164-39C4925E467B}

The full code would look like this:

import winreg
import os

reg_key = winreg.OpenKey(winreg.HKEY_CURRENT_USER, r"Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders")

downloads_path = winreg.QueryValueEx(reg_key, "{374DE290-123F-4565-9164-39C4925E467B}")[0]

winreg.CloseKey(reg_key)

print(downloads_path)

This will work on Windows 7 and later.

In your Python interpreter, type the following commands:

>>> import os, sys
>>> os.path.dirname(sys.executable)
'C:\\Python25'

Also, you can club all these and use a single line command. Open cmd and enter following command

python -c "import os, sys; print(os.path.dirname(sys.executable))"

from pathlib import Path
downloads_path = str(Path.home() / "Downloads")

For Windows 10/8/7:

1. Open System Properties (Right click Computer in the start menu, or use the keyboard shortcut Win+Pause)
2. Click Advanced system settings in the sidebar.
3. Click Environment Variables...
4. Select PATH in the System variables section
5. Click Edit

6. Add Python's path to the end of the list (the paths are separated by semicolons). For example:

   C:\Windows;C:\Windows\System32;C:\Python27
   

For Windows XP:

1. Open System Properties (Type it in the start menu, or use the keyboard shortcut Win+Pause)
2. Switch to the Advanced tab
3. Click Environment Variables...
4. Select PATH in the System variables section
5. Click Edit

6. Add Python's path to the end of the list (the paths are separated by semicolons). For example:

   C:\Windows;C:\Windows\System32;C:\Python27
   

7. Test on a new terminal window or if using an integrated terminal within a text editor, close and restart your editor or the changes won't be applied.