subprocess

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 ‘rn’, 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.

Functions

call(*popenargs, **kwargs) Run command with arguments.
check_call(*popenargs, **kwargs) Run command with arguments.
check_output(*popenargs, **kwargs) Run command with arguments and return its output as a byte string.
list2cmdline(seq) Translate a sequence of arguments into a command line

Classes

Popen(args[, bufsize, executable, stdin, ...])

Exceptions

CalledProcessError(returncode, cmd[, output]) This exception is raised when a process run by check_call() or check_output() returns a non-zero exit status.