用于检查系统调用结果的Pythonic样式

Question

我已经继承了一个基本上是Shell脚本的Python脚本，它使用OS.System（）的重复调用来运行一堆命令行实用程序，而无需检查结果。为了使脚本更加坚固，我正在考虑添加类似下面的类方法：

    def subprocess(self, cmd):
        proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
        while proc.poll() is None:
            if proc.wait() == 0:
                return True
            else:
                return False

    def check_package_installed(self, pkg):
       cmd = " ".join(["dnf list installed", pkg])
       rv = self.subprocess(cmd)
       if rv != True:
          raise Exception("Package {} not installed.".format(pkg)))

然后，在脚本的主要部分中，类似的是：

try:
    checker.check_package_installed("jdkasjdsa")
except Exception as e:
    print("Error: ",e)
    # recovery code here

这是一种惯用代码任务？

Answer 1

1. 使用异常有点宽。

   让我们假设您的pkg将是整数。然后，加入操作将使用typeerror失败。但是typeError是异常的子类，这意味着您将尝试在此处运行恢复代码，即使不同的异常提高

2. 看起来您为内部用法创建脚本，这不是一个大问题，但是通常将传递给命令的参数

3. 您的命令结果检查逻辑逻辑很尴尬而脆弱（最重要的问题）

您可以尝试以下代码：

import subprocess
import shlex


# this Exception class allows us to except exact Exceptions only
class PackageNotFoundException(OSError):
  pass


class UnexpectedException(OSError):
  pass


class Checker:

  # using static method we able to run this method without creating instance
  @staticmethod
  def check_package(pkg_name: str):
    # shlex.quote(pkg_name) -> escape command line parameter
    result = subprocess.Popen(
      "/usr/bin/env dnf list installed " + shlex.quote(pkg_name),
      shell=True,
      # hide output
      stdout=subprocess.DEVNULL,
      stderr=subprocess.PIPE
    )

    _, err = result.communicate()
    err = err.decode('utf8').rstrip('\r\n')
    exit_code = result.wait()

    # check exact error (dnf returns 1 if package not found)
    if exit_code == 1 and 'No matching Packages' in err:
      raise PackageNotFoundException(f"Package not found {pkg_name}")
    # if command status code is not 0
    if exit_code != 0:
      raise UnexpectedException(f"Error [{err}] with status code {exit_code} happens when trying to check {pkg_name}")

package = "package name with spaces"

try:
  Checker.check_package(package)
# handle package not found error
except PackageNotFoundException as e:
  print(f"Trying to install package {package}")
  # recovery code here
# re-raise error if unknown (unexpected) error
except Exception as e:
  raise e

Answer 2

我的答案与 @juanpa.arrivillaga评论相似。您可以使用以下示例使用subprocess.run命令，而不是使用popen。 subprocess.run将等待命令完成，并将返回完整的程序实例。如果您不关心输出，则可以使用Pass Check = true，并且如果命令返回错误代码！= 0，则可以提高子过程。

import subprocess
class CheckProcess:

    def execute_cmd(self, cmd):
        result = False
        try:
            output = subprocess.run(cmd, shell=True, check=True)
            result = True
        except subprocess.CalledProcessError as exc:
            print('Exception hit: %s', exc)
        else:
            return result

    def check_package_installed(self, pkg):
       cmd = " ".join(["dnf list installed", pkg])
       result = self.execute_cmd(cmd)
       if result != True:
          raise Exception("Package {} not installed.".format(pkg))

c = CheckProcess()
c.check_package_installed('cchfhfjgj')

输出

Exception hit: %s Command 'dnf list installed cchfhfjgj' returned non-zero exit status 127.

---------------------------------------------------------------------------

Exception                                 Traceback (most recent call last)

<ipython-input-2-029eeeb6eef7> in <module>()
     19 
     20 c = CheckProcess()
---> 21 c.check_package_installed('cchfhfjgj')

<ipython-input-2-029eeeb6eef7> in check_package_installed(self, pkg)
     16        rv = self.subprocess(cmd)
     17        if rv != True:
---> 18           raise Exception("Package {} not installed.".format(pkg))
     19 
     20 c = CheckProcess()

Exception: Package cchfhfjgj not installed.

请阅读您在系统上使用的Python版本的subprocess.run文档。