Daemonization API

Simple daemonization may be performed by directly calling the daemonize() funtion. In general, it should be the first thing called by your code (except perhaps import statements, global declarations, and so forth). If you need to do anything more complicated, use the Daemonizer context manager.

daemonize(pid_file, *args, chdir=None, stdin_goto=None, stdout_goto=None, stderr_goto=None, umask=0o027, shielded_fds=None, fd_fallback_limit=1024, success_timeout=30, strip_cmd_args=False)

New in version 0.1.

The function used to actually perform daemonization. It may be called directly, but is intended to be used within the Daemonizer context manager.

Warning

When used directly, all code prior to daemonize() will be repeated by the daemonized process on Windows systems. It is best to limit all pre-daemonize code to import statements, global declarations, etc. If you want to run specialized setup code, use the Daemonizer context manager.

Note

All *args must be pickleable on Windows systems.

Parameters:
  • pid_file (str) – The path to use for the PID file.
  • *args – All variables to preserve across the daemonization boundary. On Windows, only these values (which will be returned by daemonize) are guaranteed to be persistent.
  • chdir (str) – The path to use for the working directory after daemonizing. Defaults to the current directory, which can result in “directory busy” errors on both Unix and Windows systems. This argument is keyword-only.
  • stdin_goto (str) – A filepath to redirect stdin into. A value of None defaults to os.devnull. This argument is keyword-only.
  • stdout_goto (str) – A filepath to redirect stdout into. A value of None defaults to os.devnull. This argument is keyword-only.
  • stderr_goto (str) – A filepath to redirect stderr into. A value of None defaults to os.devnull. This argument is keyword-only.
  • umask (int) – The file creation mask to apply to the daemonized process. Unused on Windows. This argument is keyword-only.
  • shielded_fds – An iterable of integer file descriptors to shield from closure. Unused on Windows. This argument is keyword-only.
  • fd_ballback_limit (int) – If the file descriptor resource hard limit and soft limit are both infinite, this fallback integer will be one greater than the highest file descriptor closed. Unused on Windows. This argument is keyword-only.
  • success_timeout – A numeric limit, in seconds, for how long the parent process should wait for acknowledgment of successful startup by the daughter process. Unused on Unix. This argument is keyword-only.
  • strip_cmd_args (bool) – If the current script was started from a prompt using arguments, as in python script.py --arg1 --arg2, this value determines whether or not those arguments should be stripped when re-invoking the script. In this example, calling daemonize with strip_cmd_args=True would be re-invoke the script as python script.py. Unused on Unix. This argument is keyword-only.
Returns:

*args

 
>>> from daemoniker import daemonize
>>> daemonize('pid.pid')
class Daemonizer

New in version 0.1.

A context manager for more advanced daemonization. Entering the context assigns a tuple of (boolean is_setup, callable daemonizer) to the with Daemonizer() as target: target.

from daemoniker import Daemonizer

with Daemonizer() as (is_setup, daemonizer):
    if is_setup:
        # This code is run before daemonization.
        do_things_here()

    # We need to explicitly pass resources to the daemon; other variables
    # may not be correct
    is_parent, my_arg1, my_arg2 = daemonizer(
        'path/to/pid/file.pid',
        my_arg1,
        my_arg2,
        ...,
        **daemonize_kwargs
    )

    # This allows us to run parent-only post-daemonization code
    if is_parent:
        run_some_parent_code_here()

# We are now daemonized and the parent has exited.
code_continues_here(my_arg1, my_arg2)

When used in this manner, the Daemonizer context manager will return a boolean is_setup and a wrapped daemonize() function.

Note

Do not include an else clause after is_setup. It will not be run on Unix:

from daemoniker import Daemonizer

with Daemonizer() as (is_setup, daemonizer):
    if is_setup:
        # This code is run before daemonization.
        do_things_here()
    else:
        # This code will never run on Unix systems.
        do_no_things_here()

    ...

Note

To prevent resource contention with the daemonized child, the parent process must be terminated via os._exit when exiting the context. You must perform any cleanup inside the if is_parent: block.

__enter__()

Entering the context will return a tuple of:

with Daemonizer() as (is_setup, daemonizer):

is_setup is a bool that will be True when code is running in the parent (pre-daemonization) process.

daemonizer wraps daemonize(), prepending a bool is_parent to its return value. To prevent accidental manipulation of already-passed variables from the parent process, it also replaces them with None in the parent caller. It is otherwise identical to daemonize(). For example:

from daemoniker import Daemonizer

with Daemonizer() as (is_setup, daemonizer):
    if is_setup:
        my_arg1 = 'foo'
        my_arg2 = 'bar'

    is_parent, my_arg1, my_arg2 = daemonizer(
        'path/to/pid/file.pid',
        my_arg1,
        my_arg2
    )

    # This code will only be run in the parent process
    if is_parent:
        # These will return True
        my_arg1 == None
        my_arg2 == None

    # This code will only be run in the daemonized child process
    else:
        # These will return True
        my_arg1 == 'foo'
        my_arg2 == 'bar'

# The parent has now exited. All following code will only be run in
# the daemonized child process.
program_continues_here(my_arg1, my_arg2)
__exit__()

Exiting the context will do nothing in the child. In the parent, leaving the context will initiate a forced termination via os._exit to prevent resource contention with the daemonized child.