| subprocess | index subprocess module subprocess2 module |
subprocess - Subprocesses with accessible I/O streams
This module allows you to spawn 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 the subprocess module can be used to replace these
modules and functions can be found below.
Using the subprocess module
===========================
This module defines one class called Popen:
class 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):
Arguments are:
args should be a string, or a sequence of program arguments. The
program to execute is normally the first item in the args sequence or
string, but can be explicitly set by using the executable argument.
On UNIX, with shell=False (default): In this case, the Popen class
uses os.execvp() to execute the child program. args should normally
be a sequence. A string will be treated as a sequence with the string
as the only item (the program to execute).
On UNIX, with shell=True: If args is a string, it specifies the
command string to execute through the shell. If args is a sequence,
the first item specifies the command string, and any additional items
will be treated as additional shell arguments.
On Windows: the Popen class uses CreateProcess() to execute the child
program, which operates on strings. If args is a sequence, it will be
converted to a string using the list2cmdline method. Please note that
not all MS Windows applications interpret the command line the same
way: The list2cmdline is designed for applications using the same
rules as the MS C runtime.
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).
stdin, stdout and stderr specify the executed programs' 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 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 applications 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.
If close_fds is true, all file descriptors except 0, 1 and 2 will be
closed before the child process is executed.
if shell is true, the specified command will be executed through the
shell.
If cwd is not None, the current directory will be changed to cwd
before the child is executed.
If env is not None, it defines the environment variables for the new
process.
If universal_newlines is true, the file objects stdout and stderr are
opened as a text files, but lines may be terminated by any of '\n',
the Unix end-of-line convention, '\r', the 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.
The startupinfo and creationflags, if given, will be passed to the
underlying CreateProcess() function. They can specify things such as
appearance of the main window and priority for the new process.
(Windows only)
This module also defines some shortcut functions:
call(*popenargs, **kwargs):
Run command with arguments. Wait for command to complete, then
return the returncode attribute.
The arguments are the same as for the Popen constructor. Example:
retcode = call(["ls", "-l"])
check_call(*popenargs, **kwargs):
Run command with arguments. Wait for command to complete. If the
exit code was zero then return, otherwise raise
CalledProcessError. The CalledProcessError object will have the
return code in the returncode attribute.
The arguments are the same as for the Popen constructor. Example:
check_call(["ls", "-l"])
check_output(*popenargs, **kwargs):
Run command with arguments and return its output as a byte string.
If the exit code was non-zero it raises a CalledProcessError. The
CalledProcessError object will have the return code in the returncode
attribute and output in the output attribute.
The arguments are the same as for the Popen constructor. Example:
output = check_output(["ls", "-l", "/dev/null"])
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 OSErrors.
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
/bin/sh implicitly. This means that all characters, including shell
metacharacters, can safely be passed to child processes.
Popen objects
=============
Instances of the Popen class have the following methods:
poll()
Check if child process has terminated. Returns returncode
attribute.
wait()
Wait for child process to terminate. Returns returncode attribute.
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 (stdout, stderr).
Note: The data read is buffered in memory, so do not use this
method if the data size is large or unlimited.
The following attributes are also available:
stdin
If the stdin argument is PIPE, this attribute is a file object
that provides input to the child process. Otherwise, it is None.
stdout
If the stdout argument is PIPE, this attribute is a file object
that provides output from the child process. Otherwise, it is
None.
stderr
If the stderr argument is PIPE, this attribute is file object that
provides error output from the child process. Otherwise, it is
None.
pid
The process ID of the child process.
returncode
The child return code. 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).
Replacing older functions with the subprocess module
====================================================
In this section, "a ==> b" means that b can be used as a replacement
for a.
Note: All functions in this section fail (more or less) silently if
the executed program cannot be found; this module raises an OSError
exception.
In the following examples, we assume that the subprocess module is
imported with "from subprocess import *".
Replacing /bin/sh shell backquote
---------------------------------
output=`mycmd myarg`
==>
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
Replacing shell pipe line
-------------------------
output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]
Replacing os.system()
---------------------
sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
pid, sts = os.waitpid(p.pid, 0)
Note:
* Calling the program through the shell is usually not required.
* It's easier to look at the returncode attribute than the
exitstatus.
A more real-world 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, e:
print >>sys.stderr, "Execution failed:", e
Replacing os.spawn*
-------------------
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*
-------------------
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)
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 % 256:
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"
Replacing popen2.*
------------------
(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)
The popen2.Popen3 and popen2.Popen4 basically works as subprocess.Popen,
except that:
* subprocess.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 filedescriptors by default, but you have to specify
close_fds=True with subprocess.Popen.
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| PIPE = -1 STDOUT = -2 __all__ = ['Popen', 'PIPE', 'STDOUT', 'call', 'check_call', 'check_output', 'CalledProcessError'] | ||