PEP 324 — subprocess 新的进程模块（subprocess – New process module）

用 subprocess 模块替换旧函数（Replacing older functions with the subprocess module）

本节中的“a ==> b”表示可以将 b 换用 a。

注意：如果找不到被执行的程序，本节中的所有函数（或多或少）都会静默地失败；本模块将触发 OSError 异常。

以下示例假设 subprocess 模块是用 from subprocess import * 语句导入的。

替换反引号包裹的 /bin/sh shell 命令

    output=`mycmd myarg`
    ==>
    output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]

替换 shell 管道

    output=`dmesg | grep hda`
    ==>
    p1 = Popen(["dmesg"], stdout=PIPE)
    p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
    output = p2.communicate()[0]

替换 os.system()

    sts = os.system("mycmd" + " myarg")
    ==>
    p = Popen("mycmd" + " myarg", shell=True)
    sts = os.waitpid(p.pid, 0)

注意：

• 通常没有必要通过 shell 调用程序。
• 查看 returncode 属性要比查看退出状态更为容易。

现实中的代码实例可能会如下所示：

    try:
        retcode = call("mycmd" + " myarg", shell=True)
        if retcode < 0:
            print >>sys.stderr, "Child was terminated by signal", -retcode
        else:
            print >>sys.stderr, "Child returned", retcode
    except OSError, e:
        print >>sys.stderr, "Execution failed:", e

替换os.spawn*

P_NOWAIT 示例：

    pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
    ==>
    pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT 示例：

    retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
    ==>
    retcode = call(["/bin/mycmd", "myarg"])

Vector 示例：

    os.spawnvp(os.P_NOWAIT, path, args)
    ==>
    Popen([path] + args[1:])

环境变量示例：

    os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
    ==>
    Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})

替换 os.popen*

    pipe = os.popen(cmd, mode='r', bufsize)
    ==>
    pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout

    pipe = os.popen(cmd, mode='w', bufsize)
    ==>
    pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin


    (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
    ==>
    p = Popen(cmd, shell=True, bufsize=bufsize,
              stdin=PIPE, stdout=PIPE, close_fds=True)
    (child_stdin, child_stdout) = (p.stdin, p.stdout)


    (child_stdin,
     child_stdout,
     child_stderr) = os.popen3(cmd, mode, bufsize)
    ==>
    p = Popen(cmd, shell=True, bufsize=bufsize,
              stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
    (child_stdin,
     child_stdout,
     child_stderr) = (p.stdin, p.stdout, p.stderr)


    (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
    ==>
    p = Popen(cmd, shell=True, bufsize=bufsize,
              stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
    (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)

替换popen2.*

注意：如果 popen2 函数的 cmd 参数是个字符串，则命令将通过 /bin/sh 执行。如果是个列表，则直接执行命令。

    (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
    ==>
    p = Popen(["somestring"], shell=True, bufsize=bufsize
              stdin=PIPE, stdout=PIPE, close_fds=True)
    (child_stdout, child_stdin) = (p.stdout, p.stdin)


    (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
    ==>
    p = Popen(["mycmd", "myarg"], bufsize=bufsize,
              stdin=PIPE, stdout=PIPE, close_fds=True)
    (child_stdout, child_stdin) = (p.stdout, p.stdin)

popen2.Popen3和popen3.Popen4的工作方式基本和subprocess.Popen一样，除了：

• 如果执行失败，subprocess.Popen将触发异常。
• capturestderr参数将用 stderr 替换。
• stdin=PIPE和stdout=PIPE必须指定。
• popen2默认会关闭所有文件描述符，而 subprocess.Popen 则必须指定close_fds=True才行。