Develop Reference

Opening a command window and passing content to it using Python

I am currently working with a software package that allows you to create Python scripts and execute them from inside that package. The results of any script are saved back into the program. When the script executes, it does not show a command prompt window.
Is there an easy way to open a command prompt window from inside the script and pass over information for display, such as a dataframe header, a string or a list of values?
I have found from earlier SO posts that I can use:
import os
os.system('cmd /k "Some random text"')
This works as expected, but when I use the following code:
x = str(2 * 2)
output= f'cmd /k "{x}"'
os.system(output)
The number 4 is passed to the command window, but the following message appears:
'4' is not recognized as an internal or external command, operable program or batch file.

The answer is in the question.
'4' is not recognized as an internal or external command, operable program or batch file.
Open cmd and type anything it will give error unless we type something which is recognized by cmd. e.g a help command.
if there is something we want to type in cmd and let it get processed/printed on console we use a command
echo
in your program only the echo command was missing, which will let your output get printed on cmd.
Last but not the least, always remember the ZEN of Python

Use subprocess instead
The `subprocess` has some more benefits compared to `Os`:
The subprocess module provides a consistent interface to creating and working with additional processes.
It offers a higher-level interface than some of the other available modules, and is intended to replace functions such as os.system(), os.spawn*(), os.popen*(), popen2.*() and commands.*().
Reference
If you want to write something like to print 4 in another cmd tab, do like this:
import subprocess
var = '4'
subprocess.Popen(['start','cmd','/k','echo',var], shell = True, stdin = subprocess.PIPE, stdout = subprocess.PIPE, text = True)
Result:
It opens another cmd tab and passes a command such as echo var.

How do I embed my shell scanning-script into a Python script?

Iv'e been using the following shell command to read the image off a scanner named scanner_name and save it in a file named file_name
scanimage -d <scanner_name> --resolution=300 --format=tiff --mode=Color 2>&1 > <file_name>
This has worked fine for my purposes.
I'm now trying to embed this in a python script. What I need is to save the scanned image, as before, into a file and also capture any std output (say error messages) to a string
I've tried
scan_result = os.system('scanimage -d {} --resolution=300 --format=tiff --mode=Color 2>&1 > {} '.format(scanner, file_name))
But when I run this in a loop (with different scanners), there is an unreasonably long lag between scans and the images aren't saved until the next scan starts (the file is created as an empty file and is not filled until the next scanning command). All this with scan_result=0, i.e. indicating no error
The subprocess method run() has been suggested to me, and I have tried
with open(file_name, 'w') as scanfile:
input_params = '-d {} --resolution=300 --format=tiff --mode=Color 2>&1 > {} '.format(scanner, file_name)
scan_result = subprocess.run(["scanimage", input_params], stdout=scanfile, shell=True)
but this saved the image in some kind of an unreadable file format
Any ideas as to what may be going wrong? Or what else I can try that will allow me to both save the file and check the success status?

subprocess.run() is definitely preferred over os.system() but neither of them as such provides support for running multiple jobs in parallel. You will need to use something like Python's multiprocessing library to run several tasks in parallel (or painfully reimplement it yourself on top of the basic subprocess.Popen() API).
You also have a basic misunderstanding about how to run subprocess.run(). You can pass in either a string and shell=True or a list of tokens and shell=False (or no shell keyword at all; False is the default).
with_shell = subprocess.run(
"scanimage -d {} --resolution=300 --format=tiff --mode=Color 2>&1 > {} ".format(
scanner, file_name), shell=True)
with open(file_name) as write_handle:
no_shell = subprocess.run([
"scanimage", "-d", scanner, "--resolution=300", "--format=tiff",
"--mode=Color"], stdout=write_handle)
You'll notice that the latter does not support redirection (because that's a shell feature) but this is reasonably easy to implement in Python. (I took out the redirection of standard error -- you really want error messages to remain on stderr!)
If you have a larger working Python program this should not be awfully hard to integrate with a multiprocessing.Pool(). If this is a small isolated program, I would suggest you peel off the Python layer entirely and go with something like xargs or GNU parallel to run a capped number of parallel subprocesses.

I suspect the issue is you're opening the output file, and then running the subprocess.run() within it. This isn't necessary. The end result is, you're opening the file via Python, then having the command open the file again via the OS, and then closing the file via Python.
JUST run the subprocess, and let the scanimage 2>&1> filename command create the file (just as it would if you ran the scanimage at the command line directly.)
I think subprocess.check_output() is now the preferred method of capturing the output.
I.e.
from subprocess import check_output
# Command must be a list, with all parameters as separate list items
command = ['scanimage',
'-d{}'.format(scanner),
'--resolution=300',
'--format=tiff',
'--mode=Color',
'2>&1>{}'.format(file_name)]
scan_result = check_output(command)
print(scan_result)
However, (with both run and check_output) that shell=True is a big security risk ... especially if the input_params come into the Python script externally. People can pass in unwanted commands, and have them run in the shell with the permissions of the script.
Sometimes, the shell=True is necessary for the OS command to run properly, in which case the best recommendation is to use an actual Python module to interface with the scanner - versus having Python pass an OS command to the OS.

Execute batch file in different directory

I have a a file structure like the following (Windows):
D:\
dir_1\
batch_1.bat
dir_1a\
batch_2.bat
dir_2\
main.py
For the sake of this question, batch_1.bat simply calls batch_2.bat, and looks like:
cd dir_1a
start batch_2.bat %*
Opening batch_1.bat from a command prompt indeed opens batch_2.bat as it's supposed to, and from there on, everything is golden.
Now I want my Python file, D:\dir_2\main.py, to spawn a new process which starts batch_1.bat, which in turn should start batch_2.bat. So I figured the following Python code should work:
import subprocess
subprocess.Popen(['cd "D:/dir_1"', "start batch_1.bat"], shell=True)
This results in "The system cannot find the path specified" being printed to my Python console. (No error is raised, of course.) This is due to the first command. I get the same result even if I cut it down to:
subprocess.Popen(['cd "D:/"'], shell=True)
I also tried starting the batch file directly, like so:
subprocess.Popen("start D:/dir_1/batch_1.bat", shell=True)
For reasons that I don't entirely get, this seems to just open a windows command prompt, in dir_2.
If I forego the start part of this command, then my Python process is going to end up waiting for batch_1 to finish, which I don't want. But it does get a little further:
subprocess.Popen("D:/dir_1/batch_1.bat", shell=True)
This results in batch_1.bat successfully executing... in dir_2, the directory of the Python script, rather than the directory of batch_1.bat, which results in it not being able to find dir_1a\ and hence, batch_2.bat is not executed at all.
I am left highly confused. What am I doing wrong, and what should I be doing instead?

Your question is answered here: Python specify popen working directory via argument
In a nutshell, just pass an optional cwd argument to Popen:
subprocess.Popen(["batch_1.bat"], shell=True, cwd=r'd:\<your path>\dir1')

Python subprocess command execution got stuck

I have an issue where a unix command executed with the python subprocess module is stuck:
(The full code is here:
https://github.com/discoproject/disco/blob/master/lib/disco/worker/classic/func.py)
The unix command is a simple in-place sort.
The way the process is created:
env = os.environ.copy()
env['LC_ALL'] = 'C'
cmd, shell = sort_cmd(filename, sort_buffer_size)
subprocess.check_call(cmd, env=env, shell=shell)
where the sort_cmd is:
def sort_cmd(filename, sort_buffer_size):
return (r"sort -z -t$'\xff' -k 1,1 -T . -S {0} -o {1} {1}"
.format(sort_buffer_size, filename), True)
The input file (which is also the output file) of the sort command is empty. The file was not empty before calling this command (it is printed).
The question is, if this is a python issue, how could the file be empty. (One hypothesis is this python 2.7 bug: http://bugs.python.org/issue19809).
Issuing strace on the sort process showed that it was stuck on a futex. Unfortunately, I haven't been able to reproduce this problem and I do not have the input file. When the sort process was killed manually, it returned (with an error of course).
I am using gnu coreutils 8.10

This cannot be a python issue, as it all happens from within the subshell and Python has no notion that it is a filename whatsoever.
In fact, the sort command outputs its output file for writing and empties it. If it is the input file at the same time, you are lost.
A solution could be to output everything into a temporary file and rename that afterwards.

Why are os.system and subprocess.call spawning so many processes?

import os
import subprocess
import sys
import re
## fname_ext=sys.argv[1]
fname_ext=r"C:\mine\.cs\test.cs"
exe=os.path.splitext(fname_ext)[0]+".exe" # Executable
fdir=os.path.split(fname_ext)[0]
fcontent=open(fname_ext).read()
p_using=re.compile("\s*using\s+((\w+[.]*)+)")
p_namespace=re.compile("\s*namespace\s+(\w+)")
usings=p_using.findall(fcontent)
usings=[x[0] for x in usings]
references=[]
for i in os.listdir(fdir):
path=fdir+"\\"+i
try:
if os.path.isdir(path) or (not path.endswith('cs')):continue
with open(path) as fp:
content=fp.read()
namespaces=p_namespace.findall(content)
for n in namespaces:
if n in usings and 'System' not in n:
references+=[path]
except:
pass
command="csc /nologo "+" ".join(references)+" "+fname_ext
## command=" ".join(references)
#~ ---------------------------------------------------------
# Build:
option=1
if option==0:
# using os.system
print ">>",command
if os.system(command)==0:
os.system(exe)
else:
#~ Using subprocess module
## print type(references)
command=['csc']
## print command,references
command.extend(["/nologo","/out:"+exe])
command.extend(references)
command.append(fname_ext)
## print command
if subprocess.call(command,shell=True)==0:
## print "running %s"%exe
subprocess.call([exe],shell=True)
else:
pass
## print "Failed to run"
#~ ---------------------------------------------------------
I have this code above that is supposed to run a Csharp program from SciTE. It searches
every .cs file in the directory and finds the file with the namespace that the current
file has included. The command to run the file in SciTE is:
command.go.*.cs=python C:\mine\.py\csc.py $(FilePath)
command.go.subsystem.*.cs=0
That program logic part is okay.
The issue is that when hit F5 with sample Csharp code like this:
using System;
using System.Collections;
using MyNamespace;
class Test{
public static void Main(String[] args){
MyObject inst=new MyObject();
MyObject.self_destruct(inst);
}
}
it runs ok. But when I uncomment the second fname_ext and comment the first one
and run the csc.py file, a window opens and keeps running, printing command(this happens
using the os.system option). When you use the subprocess.call option, the same thing
happens but this time only when shell=True. It ran for only 15 seconds and there were 800+
cmd.exe and python.exe processes.I had to wait almost 5 minutes after killing cmd.exe
for the mouse to start responding and 2 minutes more for desktop peek to work.
When shell=False, it runs ok, the same way as when you hit the F5 key from the file.
What is happening here?
What is shell=True doing that makes it behave that way?

The problem is that your sys.argv looks something like this:
['python', r'C:\mine\.py\csc.py', 'whatever.cs']
So, with the fname_ext line uncommented, you set fname_ext to r'C:\mine\.py\csc.py'. Which means your script ends up just running itself—which again runs itself, etc., as fast as possible until your system chokes.
The reason it doesn't happen with shell=False is that you can't actually exec a Python script. Ultimately you end up calling CreateProcess with your script, which tries to interpret it as a .exe file, fails, and returns an error. But with shell=True, you pass your script to cmd.exe to run as a program, and it does the same thing an interactive prompt or Explorer would do: finds the right mapping to execute .py files and uses it. (And os.system does effectively the same thing as shell=True, but with a couple extra layers tossed in for good measure.)

Okay, I'll take a stab at this. If I understand the situation, this script is called csc.py and you want to call the csc c# compiler. When you run csc /nologo (etc...) through cmd.exe, it starts looking for something called 'csc' with a known extension. It finds csc.py in the current directory and since .py is a registered extension, that's what gets executed.
The solution is to rename your python file or call out 'csc.exe' explicitly.