python execute system commands (windows)

Question

So I have this uber script which constantly checks the system path for a program (openvpn). When you install openvpn it adds itself to the system path. I run my script in the console and, while it runs and checks, I install openvpn. In that console my script will never find openvpn in sys path. If I open a new console and run the same script it finds it.

Any idea how I can make my script a little less dumb?

import os
import time
import subprocess
def cmd( command ):
 return subprocess.check_output( command, shell = True ) 
def program_in_path( program ):
 path = cmd( "path" ).split(";")
 for p in path:
 if "openvpn" in p.lower():
 return True
 return False
if __name__ == '__main__':
 while True:
 print program_in_path("openvpn")
 time.sleep( 2 )

I presume it's from the shell = True thing but how else would I find it if not with path or WHERE openvpn /Q ? Running with no sehll I get WindowsError: [Error 2] The system cannot find the file specified

Here's slightly the same program done in ruby which works 100%:

loop do
 puts system( "WHERE openvpn /Q" )
 sleep( 5 )
end

Unfortunately my project is too deep into python to switch languages now. Too bad.

• Blender: This checks to see if the PATH has a folder that contains openvpn in its name. Is that what you're looking for?
• Romeo Mihalcea: I know what it does. I could also use WHERE openvpn /Q with the same results. See my updated question.
• Romeo Mihalcea: Seems like bugs.python.org/issue463506 .. I'm screwed
• Blender: That bug report is almost 13 years old. Why don't you just run openvpn without shell=True until it works?
• Romeo Mihalcea: Yes, going to college soon. Still, alive and active though. They probably considered it's not a python bug after all.

Answer 1

It's actually because when your program starts, it has an environment configured. Part of that environment is the system path. When you start a subshell, it inherits the environment of the parent process.

I'm not a Windows programmer, and I don't have a Windows machine available to test on right now. But according to that bug report, if you import nt in your script and reload(nt) in your while True loop that it will pull down a fresh copy of the environment from the system. I don't know whether that's true or not. It might be worth a try.

For what it's worth, you can see the same behavior from the cmd window by, for instance, opening a command window, adding a program folder to the System Path, and then trying to run an exe from that program folder in your existing cmd window. It won't work -- but open a new cmd window, and it will.

The bug report you cite is about a different problem. That problem outlined there is that from within Python, if you load in one of the system DLLs and use a particular function Windows provides for manipulating your environment, Python does not reflect the change. However, if you make a change to os.environ, Python recognizes that change. The conclusion from the community was that the particular function that the reporter was using, was not the correct function to use to get the results he expected.

• Romeo Mihalcea: Not working with reload either. Tried that since I found the bug report. I know it points to a slightly different thing but they do mention that on reload(os) the env is not refreshed. I'm out of solutions right now unfortunately.
• Geoff Gerrietts: For what it's worth, you'd get similar behavior if you were running under Linux or Mac OS. When you launch your program, the OS copies the parent process's environment into the child process. From that point forward, there's no connection between the two. In a Unix-like environment, you might spawn a sub-shell, which would read in the .bashrc or .bash_profile files that configure the environment. I think getting your answer out of the registry is the Windows equivalent to that.

Answer 2

Perhaps this approach works for you, getting the PATH variable straight from the registry (since you're on Windows).

For instance you could do something like this:

import winreg
def PathFromReg():
 loc = r'SYSTEM\CurrentControlSet\Control\Session Manager\Environment'
 reg = winreg.ConnectRegistry(None, winreg.HKEY_LOCAL_MACHINE)
 key = winreg.OpenKey(reg, loc)
 n_val = winreg.QueryInfoKey(key)[1]
 for i in range(n_val):
 val = winreg.EnumValue(key, i)
 if val[0] == 'Path':
 return val[1]
path = PathFromReg() 
print('openvpn' in path.lower())

I think you only need to assign the key once and then query the values inside the loop.

Note: In Python 2 the module is called _winreg.