如何在 Python 中将目录添加到系统路径变量

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\\ , eg 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?

Answer 1

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')

Answer 2

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"