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"subprocess" --- Subprocess management
**************************************
New in version 2.4.
The "subprocess" module allows you to spawn new processes, connect to
their input/output/error pipes, and obtain their return codes. This
module intends to replace several older modules and functions:
os.system
os.spawn*
os.popen*
popen2.*
commands.*
Information about how this module can be used to replace the older
functions can be found in the subprocess-replacements section.
See also: POSIX users (Linux, BSD, etc.) are strongly encouraged to
install and use the much more recent subprocess32 module instead of
the version included with python 2.7. It is a drop in replacement
with better behavior in many situations.
**PEP 324** -- PEP proposing the subprocess module
Using the "subprocess" Module
=============================
The recommended way to launch subprocesses is to use the following
convenience functions. For more advanced use cases when these do not
meet your needs, use the underlying "Popen" interface.
subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
Run the command described by *args*. Wait for command to complete,
then return the "returncode" attribute.
The arguments shown above are merely the most common ones,
described below in Frequently Used Arguments (hence the slightly
odd notation in the abbreviated signature). The full function
signature is the same as that of the "Popen" constructor - this
functions passes all supplied arguments directly through to that
interface.
Examples:
>>> subprocess.call(["ls", "-l"])
0
>>> subprocess.call("exit 1", shell=True)
1
Warning: Using "shell=True" can be a security hazard. See the
warning under Frequently Used Arguments for details.
Note: Do not use "stdout=PIPE" or "stderr=PIPE" with this
function as that can deadlock based on the child process output
volume. Use "Popen" with the "communicate()" method when you need
pipes.
subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
Run command with arguments. Wait for command to complete. If the
return code was zero then return, otherwise raise
"CalledProcessError". The "CalledProcessError" object will have the
return code in the "returncode" attribute.
The arguments shown above are merely the most common ones,
described below in Frequently Used Arguments (hence the slightly
odd notation in the abbreviated signature). The full function
signature is the same as that of the "Popen" constructor - this
functions passes all supplied arguments directly through to that
interface.
Examples:
>>> subprocess.check_call(["ls", "-l"])
0
>>> subprocess.check_call("exit 1", shell=True)
Traceback (most recent call last):
...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
New in version 2.5.
Warning: Using "shell=True" can be a security hazard. See the
warning under Frequently Used Arguments for details.
Note: Do not use "stdout=PIPE" or "stderr=PIPE" with this
function as that can deadlock based on the child process output
volume. Use "Popen" with the "communicate()" method when you need
pipes.
subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)
Run command with arguments and return its output as a byte string.
If the return code was non-zero it raises a "CalledProcessError".
The "CalledProcessError" object will have the return code in the
"returncode" attribute and any output in the "output" attribute.
The arguments shown above are merely the most common ones,
described below in Frequently Used Arguments (hence the slightly
odd notation in the abbreviated signature). The full function
signature is largely the same as that of the "Popen" constructor,
except that *stdout* is not permitted as it is used internally. All
other supplied arguments are passed directly through to the "Popen"
constructor.
Examples:
>>> subprocess.check_output(["echo", "Hello World!"])
'Hello World!\n'
>>> subprocess.check_output("exit 1", shell=True)
Traceback (most recent call last):
...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
To also capture standard error in the result, use
"stderr=subprocess.STDOUT":
>>> subprocess.check_output(
... "ls non_existent_file; exit 0",
... stderr=subprocess.STDOUT,
... shell=True)
'ls: non_existent_file: No such file or directory\n'
New in version 2.7.
Warning: Using "shell=True" can be a security hazard. See the
warning under Frequently Used Arguments for details.
Note: Do not use "stderr=PIPE" with this function as that can
deadlock based on the child process error volume. Use "Popen"
with the "communicate()" method when you need a stderr pipe.
subprocess.PIPE
Special value that can be used as the *stdin*, *stdout* or *stderr*
argument to "Popen" and indicates that a pipe to the standard
stream should be opened.
subprocess.STDOUT
Special value that can be used as the *stderr* argument to "Popen"
and indicates that standard error should go into the same handle as
standard output.
exception subprocess.CalledProcessError
Exception raised when a process run by "check_call()" or
"check_output()" returns a non-zero exit status.
returncode
Exit status of the child process.
cmd
Command that was used to spawn the child process.
output
Output of the child process if this exception is raised by
"check_output()". Otherwise, "None".
Frequently Used Arguments
-------------------------
To support a wide variety of use cases, the "Popen" constructor (and
the convenience functions) accept a large number of optional
arguments. For most typical use cases, many of these arguments can be
safely left at their default values. The arguments that are most
commonly needed are:
*args* is required for all calls and should be a string, or a
sequence of program arguments. Providing a sequence of arguments is
generally preferred, as it allows the module to take care of any
required escaping and quoting of arguments (e.g. to permit spaces
in file names). If passing a single string, either *shell* must be
"True" (see below) or else the string must simply name the program
to be executed without specifying any arguments.
*stdin*, *stdout* and *stderr* specify the executed program's
standard input, standard output and standard error file handles,
respectively. Valid values are "PIPE", an existing file descriptor
(a positive integer), an existing file object, and "None". "PIPE"
indicates that a new pipe to the child should be created. With the
default settings of "None", no redirection will occur; the child's
file handles will be inherited from the parent. Additionally,
*stderr* can be "STDOUT", which indicates that the stderr data from
the child process should be captured into the same file handle as
for stdout.
When *stdout* or *stderr* are pipes and *universal_newlines* is
"True" then all line endings will be converted to "'\n'" as
described for the *universal newlines* "'U'" mode argument to
"open()".
If *shell* is "True", the specified command will be executed
through the shell. This can be useful if you are using Python
primarily for the enhanced control flow it offers over most system
shells and still want convenient access to other shell features
such as shell pipes, filename wildcards, environment variable
expansion, and expansion of "~" to a user's home directory.
However, note that Python itself offers implementations of many
shell-like features (in particular, "glob", "fnmatch", "os.walk()",
"os.path.expandvars()", "os.path.expanduser()", and "shutil").
Warning: Executing shell commands that incorporate unsanitized
input from an untrusted source makes a program vulnerable to
shell injection, a serious security flaw which can result in
arbitrary command execution. For this reason, the use of
"shell=True" is **strongly discouraged** in cases where the
command string is constructed from external input:
>>> from subprocess import call
>>> filename = input("What file would you like to display?\n")
What file would you like to display?
non_existent; rm -rf / #
>>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...
"shell=False" disables all shell based features, but does not
suffer from this vulnerability; see the Note in the "Popen"
constructor documentation for helpful hints in getting
"shell=False" to work.When using "shell=True", "pipes.quote()"
can be used to properly escape whitespace and shell
metacharacters in strings that are going to be used to construct
shell commands.
These options, along with all of the other options, are described in
more detail in the "Popen" constructor documentation.
Popen Constructor
-----------------
The underlying process creation and management in this module is
handled by the "Popen" class. It offers a lot of flexibility so that
developers are able to handle the less common cases not covered by the
convenience functions.
class subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
Execute a child program in a new process. On Unix, the class uses
"os.execvp()"-like behavior to execute the child program. On
Windows, the class uses the Windows "CreateProcess()" function.
The arguments to "Popen" are as follows.
*args* should be a sequence of program arguments or else a single
string. By default, the program to execute is the first item in
*args* if *args* is a sequence. If *args* is a string, the
interpretation is platform-dependent and described below. See the
*shell* and *executable* arguments for additional differences from
the default behavior. Unless otherwise stated, it is recommended
to pass *args* as a sequence.
On Unix, if *args* is a string, the string is interpreted as the
name or path of the program to execute. However, this can only be
done if not passing arguments to the program.
Note: "shlex.split()" can be useful when determining the correct
tokenization for *args*, especially in complex cases:
>>> import shlex, subprocess
>>> command_line = raw_input()
/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
>>> args = shlex.split(command_line)
>>> print args
['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
>>> p = subprocess.Popen(args) # Success!
Note in particular that options (such as *-input*) and arguments
(such as *eggs.txt*) that are separated by whitespace in the
shell go in separate list elements, while arguments that need
quoting or backslash escaping when used in the shell (such as
filenames containing spaces or the *echo* command shown above)
are single list elements.
On Windows, if *args* is a sequence, it will be converted to a
string in a manner described in Converting an argument sequence to
a string on Windows. This is because the underlying
"CreateProcess()" operates on strings.
The *shell* argument (which defaults to *False*) specifies whether
to use the shell as the program to execute. If *shell* is *True*,
it is recommended to pass *args* as a string rather than as a
sequence.
On Unix with "shell=True", the shell defaults to "/bin/sh". If
*args* is a string, the string specifies the command to execute
through the shell. This means that the string must be formatted
exactly as it would be when typed at the shell prompt. This
includes, for example, quoting or backslash escaping filenames with
spaces in them. If *args* is a sequence, the first item specifies
the command string, and any additional items will be treated as
additional arguments to the shell itself. That is to say, "Popen"
does the equivalent of:
Popen(['/bin/sh', '-c', args[0], args[1], ...])
On Windows with "shell=True", the "COMSPEC" environment variable
specifies the default shell. The only time you need to specify
"shell=True" on Windows is when the command you wish to execute is
built into the shell (e.g. **dir** or **copy**). You do not need
"shell=True" to run a batch file or console-based executable.
Warning: Passing "shell=True" can be a security hazard if
combined with untrusted input. See the warning under Frequently
Used Arguments for details.
*bufsize*, if given, has the same meaning as the corresponding
argument to the built-in open() function: "0" means unbuffered, "1"
means line buffered, any other positive value means use a buffer of
(approximately) that size. A negative *bufsize* means to use the
system default, which usually means fully buffered. The default
value for *bufsize* is "0" (unbuffered).
Note: If you experience performance issues, it is recommended
that you try to enable buffering by setting *bufsize* to either
-1 or a large enough positive value (such as 4096).
The *executable* argument specifies a replacement program to
execute. It is very seldom needed. When "shell=False",
*executable* replaces the program to execute specified by *args*.
However, the original *args* is still passed to the program. Most
programs treat the program specified by *args* as the command name,
which can then be different from the program actually executed. On
Unix, the *args* name becomes the display name for the executable
in utilities such as **ps**. If "shell=True", on Unix the
*executable* argument specifies a replacement shell for the default
"/bin/sh".
*stdin*, *stdout* and *stderr* specify the executed program's
standard input, standard output and standard error file handles,
respectively. Valid values are "PIPE", an existing file descriptor
(a positive integer), an existing file object, and "None". "PIPE"
indicates that a new pipe to the child should be created. With the
default settings of "None", no redirection will occur; the child's
file handles will be inherited from the parent. Additionally,
*stderr* can be "STDOUT", which indicates that the stderr data from
the child process should be captured into the same file handle as
for stdout.
If *preexec_fn* is set to a callable object, this object will be
called in the child process just before the child is executed.
(Unix only)
If *close_fds* is true, all file descriptors except "0", "1" and
"2" will be closed before the child process is executed. (Unix
only). Or, on Windows, if *close_fds* is true then no handles will
be inherited by the child process. Note that on Windows, you
cannot set *close_fds* to true and also redirect the standard
handles by setting *stdin*, *stdout* or *stderr*.
If *cwd* is not "None", the child's current directory will be
changed to *cwd* before it is executed. Note that this directory
is not considered when searching the executable, so you can't
specify the program's path relative to *cwd*.
If *env* is not "None", it must be a mapping that defines the
environment variables for the new process; these are used instead
of inheriting the current process' environment, which is the
default behavior.
Note: If specified, *env* must provide any variables required for
the program to execute. On Windows, in order to run a side-by-
side assembly the specified *env* **must** include a valid
"SystemRoot".
If *universal_newlines* is "True", the file objects *stdout* and
*stderr* are opened as text files in *universal newlines* mode.
Lines may be terminated by any of "'\n'", the Unix end-of-line
convention, "'\r'", the old Macintosh convention or "'\r\n'", the
Windows convention. All of these external representations are seen
as "'\n'" by the Python program.
Note: This feature is only available if Python is built with
universal newline support (the default). Also, the newlines
attribute of the file objects "stdout", "stdin" and "stderr" are
not updated by the communicate() method.
If given, *startupinfo* will be a "STARTUPINFO" object, which is
passed to the underlying "CreateProcess" function. *creationflags*,
if given, can be "CREATE_NEW_CONSOLE" or
"CREATE_NEW_PROCESS_GROUP". (Windows only)
Exceptions
----------
Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent. Additionally,
the exception object will have one extra attribute called
"child_traceback", which is a string containing traceback information
from the child's point of view.
The most common exception raised is "OSError". This occurs, for
example, when trying to execute a non-existent file. Applications
should prepare for "OSError" exceptions.
A "ValueError" will be raised if "Popen" is called with invalid
arguments.
"check_call()" and "check_output()" will raise "CalledProcessError" if
the called process returns a non-zero return code.
Security
--------
Unlike some other popen functions, this implementation will never call
a system shell implicitly. This means that all characters, including
shell metacharacters, can safely be passed to child processes.
Obviously, if the shell is invoked explicitly, then it is the
application's responsibility to ensure that all whitespace and
metacharacters are quoted appropriately.
Popen Objects
=============
Instances of the "Popen" class have the following methods:
Popen.poll()
Check if child process has terminated. Set and return "returncode"
attribute.
Popen.wait()
Wait for child process to terminate. Set and return "returncode"
attribute.
Warning: This will deadlock when using "stdout=PIPE" and/or
"stderr=PIPE" and the child process generates enough output to a
pipe such that it blocks waiting for the OS pipe buffer to accept
more data. Use "communicate()" to avoid that.
Popen.communicate(input=None)
Interact with process: Send data to stdin. Read data from stdout
and stderr, until end-of-file is reached. Wait for process to
terminate. The optional *input* argument should be a string to be
sent to the child process, or "None", if no data should be sent to
the child.
"communicate()" returns a tuple "(stdoutdata, stderrdata)".
Note that if you want to send data to the process's stdin, you need
to create the Popen object with "stdin=PIPE". Similarly, to get
anything other than "None" in the result tuple, you need to give
"stdout=PIPE" and/or "stderr=PIPE" too.
Note: The data read is buffered in memory, so do not use this
method if the data size is large or unlimited.
Popen.send_signal(signal)
Sends the signal *signal* to the child.
Note: On Windows, SIGTERM is an alias for "terminate()".
CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes
started with a *creationflags* parameter which includes
*CREATE_NEW_PROCESS_GROUP*.
New in version 2.6.
Popen.terminate()
Stop the child. On Posix OSs the method sends SIGTERM to the child.
On Windows the Win32 API function "TerminateProcess()" is called to
stop the child.
New in version 2.6.
Popen.kill()
Kills the child. On Posix OSs the function sends SIGKILL to the
child. On Windows "kill()" is an alias for "terminate()".
New in version 2.6.
The following attributes are also available:
Warning: Use "communicate()" rather than ".stdin.write",
".stdout.read" or ".stderr.read" to avoid deadlocks due to any of
the other OS pipe buffers filling up and blocking the child process.
Popen.stdin
If the *stdin* argument was "PIPE", this attribute is a file object
that provides input to the child process. Otherwise, it is "None".
Popen.stdout
If the *stdout* argument was "PIPE", this attribute is a file
object that provides output from the child process. Otherwise, it
is "None".
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".
Popen.pid
The process ID of the child process.
Note that if you set the *shell* argument to "True", this is the
process ID of the spawned shell.
Popen.returncode
The child return code, set by "poll()" and "wait()" (and indirectly
by "communicate()"). A "None" value indicates that the process
hasn't terminated yet.
A negative value "-N" indicates that the child was terminated by
signal "N" (Unix only).
Windows Popen Helpers
=====================
The "STARTUPINFO" class and following constants are only available on
Windows.
class subprocess.STARTUPINFO
Partial support of the Windows STARTUPINFO structure is used for
"Popen" creation.
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
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.
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.
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.
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.
"SW_HIDE" is provided for this attribute. It is used when
"Popen" is called with "shell=True".
Constants
---------
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.
Replacing Older Functions with the "subprocess" Module
======================================================
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.
Replacing /bin/sh shell backquote
---------------------------------
output=`mycmd myarg`
# becomes
output = check_output(["mycmd", "myarg"])
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)
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
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"})
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", shell=True, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
print "There were some errors"
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".
Notes
=====
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):
1. Arguments are delimited by white space, which is either a space
or a tab.
2. 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.
3. A double quotation mark preceded by a backslash is interpreted
as a literal double quotation mark.
4. Backslashes are interpreted literally, unless they immediately
precede a double quotation mark.
5. 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.