python - 在 Python 3.4 中调用子进程并在命令无效时处理错误？

Question

我正在尝试在 Python 3.4 中执行外部进程。当我的“命令”错误时，程序就会崩溃。如何优雅地处理错误并恢复？

    # 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)

当我调用无效命令时。我崩溃了:

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

更大的问题是打印一条错误消息并没有“处理”任何有意义的“处理”意义上的错误(尽管许多人认为它确实如此)。

如果需要，记录一条错误消息，然后退出您的程序。在少数情况下您可以处理错误，例如，如果无法读取配置文件，请查看是否可以读取全局配置文件，或者是否可以将 KeyboardInterrupt 传递给正在读取命令的主循环。

在您的示例中，如果您没有在 except block 中出错，则 output = pipe.stdout... 将产生:

NameError: name 'pipe' is not defined

如果您发现了这一点，仍然没有任何有意义的事情可以恢复您的代码预期的状态。您还将移动您确实到达的回溯到没有错误的地方。

适合您情况的成语是

except SomeError:
    log what you want
    raise

空的 raise 会重新引发最后一个异常，该异常会向上传播，形成完整的堆栈跟踪，让您更好地了解错误所在。如果不这样做，就会丢弃有用的信息。