Python try block does not catch os.system exceptions

Question

I have this python code:

import os
try:
  os.system('wrongcommand')
except:
  print("command does not work")

The code prints:

wrongcommand: command not found

Instead of command does not work. Does anyone know why it's not printing my error message?

Solution 1

If you want to have an exception thrown when the command doesn't exist, you should use subprocess:

import subprocess
try:
    subprocess.run(['wrongcommand'], check = True)
except subprocess.CalledProcessError:
    print ('wrongcommand does not exist')

Come to think of it, you should probably use subprocess instead of os.system anyway ...

Solution 2

Because os.system() indicates a failure through the exit code of the method

• return value == 0 -> everything ok
• return value != 0 -> some error

The exit code of the called command is directly passed back to Python.

There is documentation telling you that os.system() would raise an exeption in case of a failure. os.system() just calls the underlaying system() call of the OS and returns its return value.

Please read the os.system() documentation carefully.

Solution 3

Although subprocess might be your best friend. os.system is still useful in somewhere, especially to the programmer play C/C++ mode.

Hence, the code will be below.

import os

try:
  os_cmd = 'wrongcommand'
  if os.system(os_cmd) != 0:
      raise Exception('wrongcommand does not exist')
except:
  print("command does not work")

Solution 4

There are two problems in your code snippet. First of all, never just do try: ... except:, always be specific about which exception you want to handle. Otherwise, your program simply swallows any kind of error, also those that you do not expect. In most cases, this will lead to unexpected behavior at some other point during runtime.

Furthermore, os.system() calls should most of the time be replaced by their counterparts from the subprocess module.

To see what goes wrong, leave out the try/except block and actually look at the traceback/exception. As others have pointed out, you will notice that there is no exception in your case which is why your custom string is not printed.

Bottom line: think about which specific exceptions can occur in your code block. Think hard about which of them you expect to happen for certain reasons and handle those appropriately. Do not handle those that you do not expect.

Solution 5

wrongcommand: command not found is the output of the shell os.system is using to invoke the command. os.system did not throw an exception

EDIT: edited by copy-and-pasting part of mgilson's comment

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Comments

[Cinder]

thanks, but how do you catch it in python? is it even possible?

[Dr. Jan-Philip Gehrcke]

Yes, but not using os.system(). This is one reason why you should use the subprocess module, as outlined in @mgilson's answer.

[mgilson]

More specifically, wrongcommand: command not found is the output of the shell that os.system is using to invoke the command. (a different shell with a different PATH might find the command).

[Shiva Krishna Chippa]

My command is for running a .jar file. When I run the correct command with square braces the above snippet still showing the exception. When I run the wrong command without square braces the above snippet not showing the exception. How to handle this?

[mgilson]

What does your command look like? I suspect that you want something like: subprocess.call(["java", "-jar", "yourjar.jar"])

[Shiva Krishna Chippa]

sikuli_jar = "C:\\sikulix.jar" sikuli_script = "C:\\Sikuli.sikuli" try: command = "java -jar {} -r {}".format(sikuli_jar, sikuli_script) subprocess.call([command]) except OSError: raise ("\"{}\" \"{}\" one of these is missing.".format(sikuli_jar, sikuli_script)) With the above I was facing problem.

[Shiva Krishna Chippa]

But fixed with below code. try: command = "java -jar {} -r {}".format(sikuli_jar, sikuli_script) subprocess.check_call(command) except (OSError, subprocess.SubprocessError, subprocess.CalledProcessError): raise ("\"{}\" \"{}\" one of these is missing.".format(sikuli_jar, sikuli_script))

[Anu]

@ShivakrishnaChippa, can you add your answer as an EDIT: to your question! It would be easier to read instead of what's in the comment!

[Eric Nelson]

With SSH keys in place, when invoked with subprocess.call(), rsync asks for a password, but does not ask when invoked with os.system(). Did I miss something?