Interface for modifying Windows environment variables from Python

Question

How can I persistently modify the Windows environment variables from a Python script? (it's the setup.py script)

I'm looking for a standard function or module to use for this. I'm already familiar with the registry way of doing it, but any comments regarding that are also welcome.

Answer 1

The registry way is if you want to modify it permanently for everything, which I guess is what you want here since it's in setup.py.

Temporarily for just your process, then os.environ is the trick.

Answer 2

In the os module, there is getenv and putenv functions. However, it seems that the putenv is not working correctly and that you must use the windows registry instead

Look at this discussion

Answer 3

It may be just as easy to use the external Windows setx command:

C:\>set NEWVAR
Environment variable NEWVAR not defined

C:\>python
Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.system('setx NEWVAR newvalue')
0
>>> os.getenv('NEWVAR')
>>> ^Z


C:\>set NEWVAR
Environment variable NEWVAR not defined

Now open a new Command Prompt:

C:\>set NEWVAR
NEWVAR=newvalue

As you can see, setx neither sets the variable for the current session, nor for the parent process (the first Command Prompt). But it does set the variable persistently in the registry for future processes.

I don't think there is a way of changing the parent process's environment at all (and if there is, I'd love to hear it!).

Answer 4

Using setx has few drawbacks, especially if you're trying to append to environment variables (eg. setx PATH %Path%;C:\mypath) This will repeatedly append to the path every time you run it, which can be a problem. Worse, it doesn't distinguish between the machine path (stored in HKEY_LOCAL_MACHINE), and the user path, (stored in HKEY_CURRENT_USER). The environment variable you see at a command prompt is made up of a concatenation of these two values. Hence, before calling setx:

user PATH == u
machine PATH == m
%PATH% == m;u

> setx PATH %PATH%;new

Calling setx sets the USER path by default, hence now:
user PATH == m;u;new
machine PATH == m
%PATH% == m;m;u;new

The system path is unavoidably duplicated in the %PATH% environment variable every time you call setx to append to PATH. These changes are permanent, never reset by reboots, and so accumulate through the life of the machine.

Trying to compensate for this in DOS is beyond my ability. So I turned to Python. The solution I have come up with today, to set environment variables by tweaking the registry, including appending to PATH without introducing duplicates, is as follows:

from os import system, environ
import win32con
from win32gui import SendMessage
from _winreg import (
    CloseKey, OpenKey, QueryValueEx, SetValueEx,
    HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE,
    KEY_ALL_ACCESS, KEY_READ, REG_EXPAND_SZ, REG_SZ
)

def env_keys(user=True):
    if user:
        root = HKEY_CURRENT_USER
        subkey = 'Environment'
    else:
        root = HKEY_LOCAL_MACHINE
        subkey = r'SYSTEM\CurrentControlSet\Control\Session Manager\Environment'
    return root, subkey


def get_env(name, user=True):
    root, subkey = env_keys(user)
    key = OpenKey(root, subkey, 0, KEY_READ)
    try:
        value, _ = QueryValueEx(key, name)
    except WindowsError:
        return ''
    return value


def set_env(name, value):
    key = OpenKey(HKEY_CURRENT_USER, 'Environment', 0, KEY_ALL_ACCESS)
    SetValueEx(key, name, 0, REG_EXPAND_SZ, value)
    CloseKey(key)
    SendMessage(
        win32con.HWND_BROADCAST, win32con.WM_SETTINGCHANGE, 0, 'Environment')


def remove(paths, value):
    while value in paths:
        paths.remove(value)


def unique(paths):
    unique = []
    for value in paths:
        if value not in unique:
            unique.append(value)
    return unique


def prepend_env(name, values):
    for value in values:
        paths = get_env(name).split(';')
        remove(paths, '')
        paths = unique(paths)
        remove(paths, value)
        paths.insert(0, value)
        set_env(name, ';'.join(paths))


def prepend_env_pathext(values):
    prepend_env('PathExt_User', values)
    pathext = ';'.join([
        get_env('PathExt_User'),
        get_env('PathExt', user=False)
    ])
    set_env('PathExt', pathext)



set_env('Home', '%HomeDrive%%HomePath%')
set_env('Docs', '%HomeDrive%%HomePath%\docs')
set_env('Prompt', '$P$_$G$S')

prepend_env('Path', [
    r'%SystemDrive%\cygwin\bin', # Add cygwin binaries to path
    r'%HomeDrive%%HomePath%\bin', # shortcuts and 'pass-through' bat files
    r'%HomeDrive%%HomePath%\docs\bin\mswin', # copies of standalone executables
])

# allow running of these filetypes without having to type the extension
prepend_env_pathext(['.lnk', '.exe.lnk', '.py'])

It does not affect the current process or the parent shell, but it will affect all cmd windows opened after it is run, without needing a reboot, and can safely be edited and re-run many times without introducing any duplicates.