How to add a directory to System path variable in Python

Question

I am running python 3.9 on Windows.

print(sys.path)
currDir = os.getcwd()
currDir += "\node"

sys.path.append(currDir)
print(sys.path)

I see the 2nd print out of sys.path has my c:\working\node\

But my issue is when I run the command under c:\working\node\, e.g. npm install

p = subprocess.Popen(["npm.cmd", "install]),

I get error saying 'The system cannot find the file specified'

And after my script is run , I try 'echo %PATH%', I don't see c:\working\node\ in the %PATH% varaible too?

Can you please tell me how can I add a new directory to system path variable so that subprocess.Popen can see the new addition?

`sys.path` is just a list and you are appending, you not modifying anything. — Epsi95, Jul 20 '21 at 05:49
`sys.path` has **nothing to do with** the operating system's `PATH` environment variable. — Karl Knechtel, Mar 15 '23 at 06:33

score 1 · Accepted Answer · answered Jul 20 '21 at 05:51

sys.path is not the same as the PATH variable specifying where to search for binaries:

sys.path

A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.

You want to set the PATH variable in os.environ instead.

app_path = os.path.join(root_path, 'other', 'dir', 'to', 'app')
os.environ["PATH"] += os.pathsep + os.path.join(os.getcwd(), 'node')

score 1 · Answer 2 · answered Jul 20 '21 at 06:03

Despite the name, sys.path is NOT the system PATH. It is (from the documentation):

A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.

Additionally, any changes to the environment won't propagate to a new process you open w/in a python session unless you tell it explicitly to do so.

You can solve both of these with the env flag of Popen You can get the current environment from os.environ, then simply extend the PATH variable and pass this environment through to Popen via the env flag

Something like:

new_env = os.environ.copy()
new_env['PATH'] = os.pathsep.join((new_env['PATH'].split(os.pathsep) if 'PATH' in new_env else []) + [currDir])
p = subprocess.Popen(["npm.cmd", "install"], env=new_env)

Additionally, make sure you are properly escaping your paths on Windows if using backslash, see this answer for more details

currDir += "\\node"

How to add a directory to System path variable in Python

2 Answers2