Windows subprocess.Popen a batch file without shel

问题:

I have a function that runs lessc (installed with npm install -g less):

>>> import subprocess
>>> subprocess.Popen(['lessc'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\subprocess.py", line 679, in __init__
    errread, errwrite)
  File "C:\Python27\lib\subprocess.py", line 896, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified

Unfortunately, it doesn't work unless I add shell=True:

>>> subprocess.Popen(['lessc'], shell=True)
<subprocess.Popen object at 0x01F619D0>

What can I do to make lessc run without using shell=True?

回答1:

Change the file to lessc.bat, or create .bat file that calls lessc. That way the file will be recognized by Windows as a batch file and will be executed properly.

You may also need to set cwd in addition to this depending on where the .bat file is.

回答2:

From both https://docs.python.org/3/library/subprocess.html#subprocess.Popen and https://docs.python.org/2/library/subprocess.html#subprocess.Popen:

You do not need shell=True to run a batch file or console-based executable.

as already cited by @JBernardo.

So, lets try:

where lessc actually tells

C:\Users\myname\AppData\Roaming\npm\lessc
C:\Users\myname\AppData\Roaming\npm\lessc.cmd

That means, the file to execute is lessc.cmd, not some .bat file. And indeed:

>>> import subprocess
>>> subprocess.Popen([r'C:\Users\myname\AppData\Roaming\npm\lessc.cmd'])
<subprocess.Popen object at 0x035BA070>
>>> lessc: no input files

usage: lessc [option option=parameter ...] <source> [destination]

So, this does work if you specify the full path. I assume there was a typo involved when you had this experience. May be you wrote .bat instead of .cmd?

---

If you don't want to patch the full path of lessc into your script, you can bake yourself a where:

import plaform
import os

def where(file_name):
    # inspired by http://nedbatchelder.com/code/utilities/wh.py
    # see also: http://stackoverflow.com/questions/11210104/
    path_sep = ":" if platform.system() == "Linux" else ";"
    path_ext = [''] if platform.system() == "Linux" or '.' in file_name else os.environ["PATHEXT"].split(path_sep)
    for d in os.environ["PATH"].split(path_sep):
        for e in path_ext:
            file_path = os.path.join(d, file_name + e)
            if os.path.exists(file_path):
                return file_path
    raise Exception(file_name + " not found")

Then you can write:

import subprocess
subprocess.Popen([where('lessc')])