1.2. Popen 物件
Popen類的例項有下列方法:
1. Popen.poll()
檢查子程序是否已經結束，設定並返回返回碼值。
2. Popen.wait()
等待子程序結束，設定並返回返回碼值。
WARNING: 當使用 stdout=PIPE 或 stderr=PIPE 並且子程序生成了足夠多的輸出資訊到管道，以至於管道阻塞，將會造成死鎖。
使用 communicate()可以避免這種情況。
3. Popen.communicate(input=None)
和子程序進行互動: 傳送資料到stdin, 從stdout和 stderr讀取資料，直到遇到 end-of-file。等待子程序結束。
可選的 input 引數是一個傳送給子程序的字串，或 None。
communicate() 返回一個元組(stdoutdata, stderrdata).
NOTE: 如果想傳送資料到子程序的 stdin, 需要設定 stdin=PIPE並建立Popen物件。
相似的，要想獲得返回結果元組中的非空值，需要設定 stdout=PIPE或 stderr=PIPE.
實際上，讀取的資料是緩衝在記憶體中，因此，如果資料太大或無限時不要使用這個上方法。
4. Popen.send_signal(signal)
傳送一個訊號 signal到子程序。
NOTE: 在Windows中， SIGTERM是terminate()的別名。
5. Popen.terminate()
停止子程式，在Posix作業系統中，這個方法傳送的是SIGTERM到子程式。
6. Popen.kill()
殺死子程序，在Posix作業系統中，這個上函式傳送SIGKILL給子程序。

下面的屬性也是同樣有效的：
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鎖。
1. Popen.stdin
如果引數值為 PIPE, 那麼這個屬性值是一個檔案物件，用來提供子程序的輸入。
否則，它的值 為 None.
2. Popen.stdout
如果引數值為 PIPE, 那麼這個屬性值是一個檔案物件，它提供了子程序的輸出。
否則，值為 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
子程序的程序ID
Note that if you set the shell argument to True, this is the process ID of the spawned shell.
5. Popen.returncode
子程序的返回碼，由poll()和wait()設定(並間接由 communicate()設定)。
非零值表示子程序還沒有被結束。
負值 -N 表示子程序被訊號N結束。
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.STDOUTPUTHANDLEThestandardoutputdevice.Initially,thisistheactiveconsolescreenbuffer,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模組代替舊函式
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.