Handling the 'Path' Windows registry value correctly

A recent recipe posted to the Python Cookbook site shows how to manipulate environment variables in Windows by modifying the registry. I've been using something similar in my code, mainly for adding directories on the fly to the 'Path' environment variable by modifying the \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment\Path registry value.

Here are two simple functions for getting and setting a registry value of type string (REG_SZ):

def get_registry_value(reg, key, value_name):
k = _winreg.OpenKey(reg, key)
value = _winreg.QueryValueEx(k, value_name)[0]
_winreg.CloseKey(k)
return value

def set_registry_value(reg, key, value_name, value):
k = _winreg.OpenKey(reg, key, 0, _winreg.KEY_WRITE)
value_type = _winreg.REG_SZ
_winreg.SetValueEx(k, value_name, 0, value_type, value)
_winreg.CloseKey(k)

To add a directory dir to the Path registry value, you would do:

reg = _winreg.ConnectRegistry(None, _winreg.HKEY_LOCAL_MACHINE)
key = r"SYSTEM\CurrentControlSet\Control\Session Manager\Environment"
path = get_registry_value(reg, key, "Path")
path += ";" + dir
set_registry_value(reg, key, "Path", path)

However, there was a problem with this approach. If you look at a typical Path registry value (using for example regedit), you'll see that it contains directories such as %SystemRoot% and %SystemRoot%\system32, where %SystemRoot% is a variable which has the Windows system directory as its value. When I modified the Path value, I mistakenly set its type to REG_SZ, and in consequence the command prompt interpreter did not replace the %SystemRoot% variable with its value. The unfortunate side effect of all this was that typical Windows binaries residing in the system32 directory, such as ping, ipconfig, etc., were not found anymore by the command prompt.

As a workaround, I replaced %SystemRoot% with os.environ['SYSTEMROOT'] in the path variable, like this:

 dirs = path.split(';')
 try:
     systemroot = os.environ['SYSTEMROOT']
 except KeyError:
     pass
 else:
     dirs = [re.sub('%SystemRoot%', systemroot, dir)
             for dir in dirs]
 path = ';'.join(dirs)

A much better approach, suggested by Ori Berger in a comment to the recipe I mentioned, is to set the value type for the Path registry value to REG_EXPAND_SZ, and not to REG_SZ. This makes variables like %SystemRoot% be automatically expanded by the command prompt interpreter. So the modified version of my set_registry_value function is now:

def set_registry_value(reg, key, value_name, value, value_type=_winreg.REG_SZ):
 k = _winreg.OpenKey(reg, key, 0, _winreg.KEY_WRITE)
 _winreg.SetValueEx(k, value_name, 0, value_type, value)
 _winreg.CloseKey(k)

For modifying the 'Path' value, you would now do:

reg = _winreg.ConnectRegistry(None, _winreg.HKEY_LOCAL_MACHINE)
key = r"SYSTEM\CurrentControlSet\Control\Session Manager\Environment"
path = get_registry_value(reg, key, "Path")
path += ";" + dir
set_registry_value(reg, key, "Path", path, _winreg.REG_EXPAND_SZ)

Of course, the whole registry value manipulation code can be neatly packaged in a class, like here.