1.2. Popen 对象

Popen类的实例有下列方法:

1. Popen.poll()

检查子进程是否已经结束，设置并返回返回码值。

2. Popen.wait()

等待子进程结束，设置并返回返回码值。
WARNING: 当使用 stdout=PIPE 或 stderr=PIPE 并且子进程生成了足够多的输出信息到管道，以至于管道阻塞，将会造成死锁。
         使用 communicate()可以避免这种情况。

3. Popen.communicate(input=None)

和子进程进行交互: 发送数据到stdin, 从stdout和 stderr读取数据，直到遇到 end-of-file。等待子进程结束。
可选的 input 参数是一个传送给子进程的字符串，或 None。
communicate() 返回一个元组(stdoutdata, stderrdata).
NOTE: 如果想发送数据到子进程的 stdin, 需要设置 stdin=PIPE并创建Popen对象。
      相似的，要想获得返回结果元组中的非空值，需要设置 stdout=PIPE或 stderr=PIPE.
      实际上，读取的数据是缓冲在内存中，因此，如果数据太大或无限时不要使用这个上方法。

4. Popen.send_signal(signal)

发送一个信号 signal到子进程。
NOTE: 在Windows中， SIGTERM是terminate()的别名。

5. Popen.terminate()

停止子程序，在Posix操作系统中，这个方法发送的是SIGTERM到子程序。

6. Popen.kill()

杀死子进程，在Posix操作系统中，这个上函数发送SIGKILL给子进程。

下面的属性也是同样有效的：
WARNING: 使用communicate()而不是 .stdin.write, .stdout.read 或 stderr.read,可以避免因管道阻塞而造成的死锁。
1. Popen.stdin
如果参数值为 PIPE, 那么这个属性值是一个文件对象，用来提供子进程的输入。
否则，它的值 为 None.
2. Popen.stdout
如果参数值为 PIPE, 那么这个属性值是一个文件对象，它提供了子进程的输出。
否则，值为 None
3. Popen.stderr
If the stderr argument was PIPE, this attribute is a file object that provides error output from the child process. 
Otherwise, it is None.
4. Popen.pid
子进程的进程ID
Note that if you set the shell argument to True, this is the process ID of the spawned shell.
5. Popen.returncode
子进程的返回码，由poll()和wait()设置(并间接由 communicate()设置)。
非零值表示子进程还没有被结束。
负值 -N 表示子进程被信号N结束。

1.3. Windows Popen 使用

The STARTUPINFO class and following constants are only available on Windows.
1. class subprocess.STARTUPINFO
   Partial support of the Windows STARTUPINFO structure is used for Popen creation.
2. dwFlags
A bit field that determines whether certain STARTUPINFO attributes are used when the process creates a window.
   si = subprocess.STARTUPINFO()
   si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
3. hStdInput
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard input handle for the process. If 
STARTF_USESTDHANDLES is not specified, the default for standard input is the keyboard buffer.
4. hStdOutput
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard output handle for the process. Otherwise, 
this attribute is ignored and the default for standard output is the console window’s buffer.
5. hStdError
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard error handle for the process. Otherwise, 
this attribute is ignored and the default for standard error is the console window’s buffer.
6. wShowWindow
If dwFlags specifies STARTF_USESHOWWINDOW, this attribute can be any of the values that can be specified in the 
nCmdShow parameter for the ShowWindow function, except for SW_SHOWDEFAULT. Otherwise, this attribute is ignored.
7. SW_HIDE is provided for this attribute. It is used when Popen is called with shell=True.

1.3.1. 常量

The subprocess module exposes the following constants.
subprocess.STD_INPUT_HANDLE
The standard input device. Initially, this is the console input buffer, CONIN$.
subprocess.STD_OUTPUT_HANDLE
The standard output device. Initially, this is the active console screen buffer, CONOUT$.
subprocess.STD_ERROR_HANDLE
The standard error device. Initially, this is the active console screen buffer, CONOUT$.
subprocess.SW_HIDE
Hides the window. Another window will be activated.
subprocess.STARTF_USESTDHANDLES
Specifies that the STARTUPINFO.hStdInput, STARTUPINFO.hStdOutput, and STARTUPINFO.hStdError attributes contain 
additional information.
subprocess.STARTF_USESHOWWINDOW
Specifies that the STARTUPINFO.wShowWindow attribute contains additional information.
subprocess.CREATE_NEW_CONSOLE
The new process has a new console, instead of inheriting its parent’s console (the default).
This flag is always set when Popen is created with shell=True.
subprocess.CREATE_NEW_PROCESS_GROUP
A Popen creationflags parameter to specify that a new process group will be created. 
This flag is necessary for using os.kill() on the subprocess
This flag is ignored if CREATE_NEW_CONSOLE is specified.

1.4. 使用 subprocess模块代替旧函数 

In this section, “a becomes b” means that b can be used as a replacement for a.
Note All “a” functions in this section fail (more or less) silently if the executed program cannot be found;
the “b” replacements raise OSError instead.
In addition, the replacements using check_output() will fail with a CalledProcessError if the requested operation 
produces a non-zero return code. The output is still available as the output attribute of the raised exception.
In the following examples, we assume that the relevant functions have already been imported from the subprocess 
module.

1.4.1. Replacing /bin/sh shell backquote

output=`mycmd myarg`
# becomes
output = check_output(["mycmd", "myarg"])

1.4.2. Replacing shell pipeline

output=`dmesg | grep hda`
# becomes
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]
The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.
Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:
output=`dmesg | grep hda`
# becomes
output=check_output("dmesg | grep hda", shell=True)

1.4.3. Replacing os.system()

status = os.system("mycmd" + " myarg")
# becomes
status = subprocess.call("mycmd" + " myarg", shell=True)
Notes:
Calling the program through the shell is usually not required.
A more realistic example would look like this:
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 as e:
    print >>sys.stderr, "Execution failed:", e

1.4.4. Replacing the os.spawn family

P_NOWAIT example:
pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid
P_WAIT example:
retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])
Vector example:
os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])
Environment example:
os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"}

1.4.5. Replacing os.popen(), os.popen2(), os.popen3()

pipe = os.popen("cmd", ‘r‘, bufsize)
==>
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout
pipe = os.popen("cmd", ‘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)
On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as the command to execute, in which case arguments 


will be passed directly to the program without shell intervention. This usage can be replaced as follows:


(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
                                        bufsize)
==>
p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
(child_stdin, child_stdout) = (p.stdin, p.stdout)
Return code handling translates as follows:


pipe = os.popen("cmd", ‘w‘)
...
rc = pipe.close()
if rc is not None and rc >> 8:
    print "There were some errors"
==>
process = Popen("cmd", ‘w‘, shell=True, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
    print "There were some errors"

1.4.6. Replacing functions from the popen2 module

(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)
On Unix, popen2 also accepts a sequence as the command to execute, in which case arguments will be passed directly to 
the program without shell intervention. This usage can be replaced as follows:

(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 and popen2.Popen4 basically work as subprocess.Popen, except that:
Popen raises an exception if the execution fails.
the capturestderr argument is replaced with the stderr argument.
stdin=PIPE and stdout=PIPE must be specified.
popen2 closes all file descriptors by default, but you have to specify close_fds=True with Popen.

1.5. NOTES

1.5.1. Converting an argument sequence to a string on Windows

On Windows, an args sequence is converted to a string that can be parsed using the following rules (which correspond 
to the rules used by the MS C runtime):

Arguments are delimited by white space, which is either a space or a tab.
A string surrounded by double quotation marks is interpreted as a single argument, regardless of white space 
contained within. A quoted string can be embedded in an argument.
A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
If backslashes immediately precede a double quotation mark, every pair of backslashes is interpreted as a literal 
backslash. If the number of backslashes is odd, the last backslash escapes the next double quotation mark as 
described in rule 3.

python子进程模块subprocess详解与应用实例 之二