How to add to and remove from system's environment variable "PATH"?

Here's something that does what you want which is similar to code in the jaraco.windows project. And like it, only uses built-in Python modules—so doesn't require first downloading and installing the pywin32 extensions. Plus it's Python 2.6+ and 3.x compatible and supports Unicode environment variables and values (directory paths in this case).

Note that Windows administrator rights are required to change the permanent system-level environment variables.

import ctypes
from ctypes.wintypes import HWND, UINT, WPARAM, LPARAM, LPVOID
LRESULT = LPARAM  # synonymous
import os
import sys
try:
    import winreg
    unicode = str
except ImportError:
    import _winreg as winreg  # Python 2.x

class Environment(object):
    path = r'SYSTEM\CurrentControlSet\Control\Session Manager\Environment'
    hklm = winreg.ConnectRegistry(None, winreg.HKEY_LOCAL_MACHINE)
    key = winreg.OpenKey(hklm, path, 0, winreg.KEY_READ | winreg.KEY_WRITE)
    SendMessage = ctypes.windll.user32.SendMessageW
    SendMessage.argtypes = HWND, UINT, WPARAM, LPVOID
    SendMessage.restype = LRESULT
    HWND_BROADCAST = 0xFFFF
    WM_SETTINGCHANGE = 0x1A
    NO_DEFAULT_PROVIDED = object()

    def get(self, name, default=NO_DEFAULT_PROVIDED):
        try:
            value = winreg.QueryValueEx(self.key, name)[0]
        except WindowsError:
            if default is self.NO_DEFAULT_PROVIDED:
                raise ValueError("No such registry key", name)
            value = default
        return value

    def set(self, name, value):
        if value:
            winreg.SetValueEx(self.key, name, 0, winreg.REG_EXPAND_SZ, value)
        else:
            winreg.DeleteValue(self.key, name)
        self.notify()

    def notify(self):
        self.SendMessage(self.HWND_BROADCAST, self.WM_SETTINGCHANGE, 0, u'Environment')

Environment = Environment()  # singletion - create instance

PATH_VAR = 'PATH'

def append_path_envvar(addpath):
    def canonical(path):
        path = unicode(path.upper().rstrip(os.sep))
        return winreg.ExpandEnvironmentStrings(path)  # Requires Python 2.6+
    canpath = canonical(addpath)
    curpath = Environment.get(PATH_VAR, '')
    if not any(canpath == subpath
                for subpath in canonical(curpath).split(os.pathsep)):
        Environment.set(PATH_VAR, os.pathsep.join((curpath, addpath)))

def remove_envvar_path(folder):
    """ Remove *all* paths in PATH_VAR that contain the folder path. """
    curpath = Environment.get(PATH_VAR, '')
    folder = folder.upper()
    keepers = [subpath for subpath in curpath.split(os.pathsep)
                if folder not in subpath.upper()]
    Environment.set(PATH_VAR, os.pathsep.join(keepers))

Sample usage:

print(Environment.get('path'))
append_path_envvar(r'C:\path\to\myprogram\dist')
append_path_envvar(r'D:\another\path\to\myprogram\dist')
print(Environment.get('path'))
remove_envvar_path(r'\myprogram')  # remove *both* added paths
print(Environment.get('path'))

There is a python package pathtub (I am the author), which can do the job. You can read the code and docs on GitHub.

Installing

pip install pathtub

Example usage

Reading Path

from pathtub import get_path

# Getting path (can be 'process' (default), 'user' or 'machine')
user_path = get_path('user')

print(user_path)
# C:\Program Files\Java\jdk-13.0.1\bin;C:\Programs;C:\Programs\apache-maven-3.6.2\bin;C:\Programs\cloc;C:\Programs\fciv;C:\Python\Python37;C:\Python\Python37\lib\site-packages\pywin32_system32;C:\Python\Python37\Scripts;C:\texlive\2018\bin\win32;C:\Users\USER\AppData\Local\Microsoft\WindowsApps;C:\Users\USER\AppData\Local\Programs\Microsoft VS Code\bin;C:\Users\USER\AppData\Roaming\npm

Adding to path

from pathtub import add_to_path
added = add_to_path(r'C:\Add this folder\to user path', mode='user')

print(added)
#True

# Adding duplicate entries is prevented
added = add_to_path(r'C:\Add this folder\to user path', mode='user')

print(added)
#False

Removing from Path

removed = remove_from_path(r'C:\Add this folder\to user path', mode='user')

print(removed)
#True

# Removing non-existing folder just returns False
removed = remove_from_path(r'C:\Add this folder\to user path', mode='user')

print(removed)
#False

Requirements

Current implementation (v.1.1.2) uses Powershell for the permanent Path modifications. I have only tested this with newer (Python 3.7 and 3.8), but I guess it should work also on some older versions.

I am inferring from the paths in your question that your are interested in doing this on the Windows platform.

The documentation describes the process:

To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParam set to the string "Environment". This allows applications, such as the shell, to pick up your updates.

You should check os.environ. It's a dictionary that can be manipulated directly or via os.putenv:

Set the environment variable named varname to the string value. Such changes to the environment affect subprocesses started with os.system(), popen() or fork() and execv().

Hence:

>>> import os
>>> os.environ["PATH"] =  path_old + ":/tmp/hallo" 
>>> os.environ["PATH"] 
'/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/vendor_perl:/usr/bin/core_perl:/tmp/hallo'

[update]

according to this answer you can make them persistent via windows registry