How to catch exception from a system() command in python

Question

I have a system() command and I want to catch the exception it may generate. The code that I have is:

def test():

    filename = "test.txt"
    try:
        cmd = "cp /Users/user1/Desktop/Test_Folder/"+filename+" /Users/user1/Desktop/"
        output = system(cmd)
    except:
        print 'In the except'
        traceback.print_exc()
        sys.exit(1)

if __name__ == '__main__':

    test()

When I execute the above code and say the file that I want to copy is not present then the error is not caught and the code does not enter the except section. How can I catch such errors generated by system() commands?

Note: The above system() command is just an example. There are multiple such system() commands and each of them vary from one another

pastebin.com/PAPD8WnC, if you wanted to use system then you would check the return code i.e output — Padraic Cunningham
– Padraic Cunningham, Commented Jul 2, 2015 at 23:14
@PadraicCunningham if I want to use system() then in that case is there any use of putting it in a try: except:? — user2966197
– user2966197, Commented Jul 2, 2015 at 23:19
no it won't error, you will either just get the return code or for example system("foo") -> sh: 1: foo: not found The subprocess module would be the preferred way to do it — Padraic Cunningham
– Padraic Cunningham, Commented Jul 2, 2015 at 23:21
@PadraicCunningham but say if there is some syntax error in the command like in above post the filename variable is undefined then that error wont get caught if I dont put the try: except: right? If I will put try then will the syntax errors get caught? — user2966197
– user2966197, Commented Jul 2, 2015 at 23:23
That would be a NameError, you should catch the specific exceptions you expect could happen and log the output, you should not catch all exceptions blindly — Padraic Cunningham
– Padraic Cunningham, Commented Jul 2, 2015 at 23:25

Michael Aaron Safyan · Accepted Answer · 2015-07-02 23:09:00Z

3

The system() command doesn't throw an exception on failure; it will simply return the exit status code of the application. If you want an exception thrown on failure, use subprocess.check_call, instead. (And, in general, using the subprocess module is superior in that it gives you greater control over the invocation as well as the ability to redirect the subprocess's standard input/output).

Note, though, that if most of the operations you are doing are simple filesystem operations like copying files from one location to another, that there are Python functions that do the equivalent. For example, shutil provides the ability to copy files from one location to another. Where there are Python functions to do the task, it is generally better to use those rather than invoke a sub process to do it (especially since the Python-provided methods may be able to do it more efficiently without forking a process, and the Python versions will also be more robust to cross-platform considerations).

answered Jul 2, 2015 at 23:09

Michael Aaron Safyan

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3 Comments

user2966197 Over a year ago

No, most of the operations are not filesystem operations. A bunch of them are non filesystem commands like hdfs, impala shell etc.

user2966197 Over a year ago

so the only use of putting a try: except: in case of system() commands will be to catch NameError right?

Michael Aaron Safyan Over a year ago

You're right... technically, system() could throw an exception for a programmer error (like misspelling it or passing in something other than a string), but in terms of the exceptions you are thinking about, it doesn't throw them. And you generally should not be catching the kind of programmer error exception like NameError.

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