Calling a subprocess in Python 3.4 and handling errors if a command is invalid?

Question

I am trying to execute an external process in Python 3.4. When my "command" is wrong the program just crashes. How can I handle the errors gracefully and recover?

    # Call a system process
    try:
        pipe = subprocess.Popen(command,
                                stdout=subprocess.PIPE)
    except (OSError,
            subprocess.CalledProcessError) as error:
        print("Failed to execute command")
        print(command)
        print("Error: " + error.output)

    while True:
        output = pipe.stdout.readline()
        if not output:
            print("Finished...")
            break

        output = output.decode()
        output = output.strip("\r")
        output = output.strip("\n")
        print(output)

When I call an invalid command. I get a crash like:

C:\SDKs\Python34\python.exe C:\Users\user\Documents\GitHub\PyQtApps\QtDeploy\src\main.py
Traceback (most recent call last):
Executing:  windeployqt.exe  C:\Users\user\Documents\GitHub\SpaCentralSoftBin\GUIController.exe
  File "C:\Users\user\Documents\GitHub\PyQtApps\QtDeploy\src\forms\maindialog.py", line 81, in execute_windeployqt
Failed to execute command
    stdout=subprocess.PIPE)
windeployqt.exe  C:\Users\user\Documents\GitHub\SpaCentralSoftBin\GUIController.exe
  File "C:\SDKs\Python34\lib\subprocess.py", line 858, in __init__
    restore_signals, start_new_session)
  File "C:\SDKs\Python34\lib\subprocess.py", line 1111, in _execute_child
    startupinfo)
FileNotFoundError: [WinError 2] The system cannot find the file specified

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\user\Documents\GitHub\PyQtApps\QtDeploy\src\forms\maindialog.py", line 159, in on_buttonBox_clicked
    self.execute_windeployqt()
  File "C:\Users\user\Documents\GitHub\PyQtApps\QtDeploy\src\forms\maindialog.py", line 86, in execute_windeployqt
    print("Error: " + error.output)
AttributeError: 'FileNotFoundError' object has no attribute 'output'

Answer 1

The larger problem is printing an error message does not "handle" an error in any meaningful sense of "handle" (although many people think it does).

Log an error message if you want and then exit your program. There are few instances where you can handle an error, for example if a configuration file can't be read, see if you can read a global configuration file or KeyboardInterrupt can be passed up to a main loop that is reading commands.

In your example, if you didn't fault in the except block, the output = pipe.stdout... would yield:

NameError: name 'pipe' is not defined

and if you caught that, there'd still be nothing meaningful to do to recover the state your code expects to be in. You'll also move the traceback you do get to a place where the fault isn't.

A good idiom for your situation is

except SomeError:
    log what you want
    raise

where an empty raise re-raises the last exception which will propagate up, forming a full stacktrace which will give you a better idea of where the fault is. If you don't do this, you are discarding useful information.