how to read and execute the scripts in file

Question

This is a simplified example, the python program run on a device and wait for a text file to understand how to execute the next step, the text file will be upload to the device frequently.

How to let the python program "translate" the text file (in this example, steps.txt only has one line "z = x + y") into the executable script?

import os
import time
x = 3
y = 9
s = None
while 1:
  time.sleep(1)
  try:
    with open("steps.txt", 'r') as f:
        s = f.readline()#s = "z = x + y"
        f.close()
        break
except:
    pass

os.system(s)#'z' is not recognized as an internal or external command,operable program or batch file.

score 4 · Answer 1 · edited Sep 12 '21 at 02:32

As you included in your post, the command prompt returned this error

'z' is not recognized as an internal or external command,operable program or batch file.

when you passed the string "z = x + y" into os.system(). That is because the command is not run in python. I believe what you were trying to do was

# s = "z = x + y"
os.system(f'python -c "{s}"')

which would of course return a NameError.

Instead, to execute the line of code in your python program, use the exec() method, like so:

# s = "z = x + y"
exec(s)

But be warned! Both exec() and eval() should really be avoided, as they pose serious security issues.

Note: The f.close() isn't necessary as you used the with statement.

score 2 · Accepted Answer · answered Sep 12 '21 at 01:40

2

Instead of os.system(s), you could use exec:

exec(s)

But note that this is really unsafe, more details here.

answered Sep 12 '21 at 01:40

U13-Forward

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thanks, if I have multiple lines in steps.txt, how should I use this? – adameye2020 Sep 12 '21 at 02:02
I noticed I have to use readlines(), and exec(s[0]), exec(s[1])....., is this same with what you mean? – adameye2020 Sep 12 '21 at 02:11
@adameye2020 Yeap! – U13-Forward Sep 12 '21 at 02:12
wondering whether there is a better way for exec line by line, I remember in Matlab I could exec the full scripts in one line – adameye2020 Sep 12 '21 at 02:13
@adameye2020 You could use `read` instead of `readlines`, than it would be 1 string, or use a for loop, like `for line is s: exec(line)` – U13-Forward Sep 12 '21 at 02:14

how to read and execute the scripts in file

2 Answers2