Python - Proper way to call nested functions?

Question

Lets say you have the following code

def set_args():
    #Set Arguments
    parser = argparse.ArgumentParser()
    parser.add_argument("-test", help = 'test')
    return parser

def run_code():
    def fileCommand():
        print("the file type is\n")
        os.system("file " + filename).read().strip()

def main():
    parser = set_args()
    args = parser.parse_args()

what is best way to call that fileCommand() function from def main():?

I have the following code which of course doesn't work:

def main():
    parser = set_args()
    args = parser.parse_args()
    
    #If -test, 
    if args.test:
        filename=args.test
        fileCommand()

So if you were to run python test.py -test test.txt it should run the file command on it to get the file type

I know if I keep messing around I'll get it one way or another, but I typically start to over complicated things so its harder down the line later. So what is the proper way to call nested functions?

Thanks in advance!

You can't; the function is only bound to a local variable inside run_code, and goes away once run_code returns and the variable goes out of scope. (Never mind the fact that you never call run_code, meaning the function is never defined at all.) Why are you defining it inside run_code in the first place? — chepner
– chepner, Commented Jun 18, 2021 at 13:23
The proper way would be to call the nested function from the function itself: fileCommand() after its definition, inrun_code()'s body. Or even not to nest it if you don't need to. — Whole Brain
– Whole Brain, Commented Jun 18, 2021 at 13:23
Also, os.system() doesn't return the output of the command; it returns None. Just drop the read and strip functions, as file will just write to the standard output it inherits from your script. — chepner
– chepner, Commented Jun 18, 2021 at 13:26
yeah sorry I originally had s = os.popen for the file command that worked, but some of the commands I have require os.system so I switched it to get a broader answer to incorporate the os.system without even running it first. — tjw
– tjw, Commented Jun 18, 2021 at 14:04

Leo · Accepted Answer · 2021-06-18 13:32:59Z

1

Python inner functions (or nested functions) have many use cases but in non of them, accessing through the nested function from outside is an option. A nested function is created to have access to the parent function variables and calling it from outside of the parent function is in conflict with principle.

You can call the function from the parent function or make it a decorator:

Calling the nested function from the parent would be:

def run_code(filename):
    def fileCommand():
        print("the file type is\n")
        os.system("file " + filename).read().strip()
    fileCommand()

If you describe more about your use case and the reason why you want to run the code like this, I can give more details about how you can implement your code.

answered Jun 18, 2021 at 13:32

Leo

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1 Comment

tjw Over a year ago

Yeah I think the issue with that is there are going to be multiple commands (file, binwalk, strings, etc) and based on the file type (exe, pdf, doc) it runs certain commands based off that extension. So for exes It would run binwalk, but not for txt files, but the file command would run for both if that makes sense. I got the code to work outside the run_code to where each were its own functions, guessing i might have to revert back to that

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