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Resource management classes and functions.

Project description

Resource management classes and functions.

Latest release 20210731: RunState: tune the sanity checks around whether the state is "running".

Class ClosedError(builtins.Exception,builtins.BaseException)

Exception for operations invalid when something is closed.

Class MultiOpen(MultiOpenMixin)

Context manager class that manages a single open/close object using a MultiOpenMixin.

Method MultiOpen.__init__(self, openable, finalise_later=False)

Initialise: save the openable and call the MultiOpenMixin initialiser.

Method MultiOpen.shutdown(self)

Close the associated openable object.

Method MultiOpen.startup(self)

Open the associated openable object.

Class MultiOpenMixin

A multithread safe mixin to count open and close calls, and to call .startup on the first .open and to call .shutdown on the last .close.

If used as a context manager this mixin calls open()/close() from __enter__() and __exit__().

Recommended subclass implementations do as little as possible during __init__, and do almost all setup during startup so that the class may perform multiple startup/shutdown iterations.

Classes using this mixin need to either:

  • either define a context manager method .startup_shutdown which does the startup actions before yeilding and then does the shutdown actions
  • or define separate .startup and .shutdown methods.

Example:

class DatabaseThing(MultiOpenMixin):
    @contextmanager
    def startup_shutdown(self):
        self._db = open_the_database()
        yield
        self._db.close()
...
with DatabaseThing(...) as db_thing:
    ... use db_thing ...

Why not a plain context manager? Because in multithreaded code one wants to keep the instance "open" while any thread is still using it. This mixin lets threads use an instance in overlapping fashion:

db_thing = DatabaseThing(...)
with db_thing:
    ... kick off threads with access to the db ...
...
thread 1:
with db_thing:
   ... use db_thing ...
thread 2:
with db_thing:
   ... use db_thing ...

TODO:

  • subopens: if true (default false) then .open will return a proxy object with its own .closed attribute set by the proxy's .close.

Method MultiOpenMixin.close(self, enforce_final_close=False, caller_frame=None, unopened_ok=False)

Decrement the open count. If the count goes to zero, call self.shutdown() and return its value.

Parameters:

  • enforce_final_close: if true, the caller expects this to be the final close for the object and a RuntimeError is raised if this is not actually the case.
  • caller_frame: used for debugging; the caller may specify this if necessary, otherwise it is computed from cs.py.stack.caller when needed. Presently the caller of the final close is recorded to help debugging extra close calls.
  • unopened_ok: if true, it is not an error if this is not open. This is intended for closing callbacks which might get called even if the original open never happened. (I'm looking at you, cs.resources.RunState.)

Property MultiOpenMixin.closed

Whether this object has been closed. Note: False if never opened.

Method MultiOpenMixin.finalise(self)

Finalise the object, releasing all callers of .join(). Normally this is called automatically after .shutdown unless finalise_later was set to true during initialisation.

Method MultiOpenMixin.is_opened(func)

Decorator to wrap MultiOpenMixin proxy object methods which should raise if the object is not yet open.

Method MultiOpenMixin.join(self)

Join this object.

Wait for the internal _finalise Condition (if still not None). Normally this is notified at the end of the shutdown procedure unless the object's finalise_later parameter was true.

Method MultiOpenMixin.open(self, caller_frame=None)

Increment the open count. On the first .open call self.startup().

Method MultiOpenMixin.startup_shutdown(self)

Default context manager form of startup/shutdown - just calls them.

Method MultiOpenMixin.tcm_get_state(self)

Support method for TrackedClassMixin.

Function not_closed(func)

Decorator to wrap methods of objects with a .closed property which should raise when self.closed.

Class Pool

A generic pool of objects on the premise that reuse is cheaper than recreation.

All the pool objects must be suitable for use, so the new_object callable will typically be a closure. For example, here is the init for a per-thread AWS Bucket using a distinct Session:

def __init__(self, bucket_name):
    Pool.__init__(self, lambda: boto3.session.Session().resource('s3').Bucket(bucket_name)

Method Pool.__init__(self, new_object, max_size=None, lock=None)

Initialise the Pool with creator new_object and maximum size max_size.

Parameters:

  • new_object is a callable which returns a new object for the Pool.
  • max_size: The maximum size of the pool of available objects saved for reuse. If omitted or None, defaults to 4. If 0, no upper limit is applied.
  • lock: optional shared Lock; if omitted or None a new Lock is allocated

Method Pool.instance(self)

Context manager returning an object for use, which is returned to the pool afterwards.

Class RunState

A class to track a running task whose cancellation may be requested.

Its purpose is twofold, to provide easily queriable state around tasks which can start and stop, and to provide control methods to pronounce that a task has started (.start), should stop (.cancel) and has stopped (.stop).

A RunState can be used as a context manager, with the enter and exit methods calling .start and .stop respectively. Note that if the suite raises an exception then the exit method also calls .cancel before the call to .stop.

Monitor or daemon processes can poll the RunState to see when they should terminate, and may also manage the overall state easily using a context manager. Example:

def monitor(self):
    with self.runstate:
        while not self.runstate.cancelled:
            ... main loop body here ...

A RunState has three main methods:

  • .start(): set .running and clear .cancelled
  • .cancel(): set .cancelled
  • .stop(): clear .running

A RunState has the following properties:

  • cancelled: true if .cancel has been called.
  • running: true if the task is running. Further, assigning a true value to it also sets .start_time to now. Assigning a false value to it also sets .stop_time to now.
  • start_time: the time .running was last set to true.
  • stop_time: the time .running was last set to false.
  • run_time: max(0,.stop_time-.start_time)
  • stopped: true if the task is not running.
  • stopping: true if the task is running but has been cancelled.
  • notify_start: a set of callables called with the RunState instance to be called whenever .running becomes true.
  • notify_end: a set of callables called with the RunState instance to be called whenever .running becomes false.
  • notify_cancel: a set of callables called with the RunState instance to be called whenever .cancel is called.

Method RunState.__bool__(self)

Return true if the task is running.

Method RunState.__nonzero__(self)

Return true if the task is running.

Method RunState.cancel(self)

Set the cancelled flag; the associated process should notice and stop.

Method RunState.end(self)

Stop: adjust state, set stop_time to now. Sets sets .running to False.

Property RunState.run_time

Property returning most recent run time (stop_time-start_time). If still running, use now as the stop time. If not started, return 0.0.

Property RunState.running

Property expressing whether the task is running.

Method RunState.start(self, *a, **kw)

Start: adjust state, set start_time to now. Sets .cancelled to False and sets .running to True.

Method RunState.stop(self)

Stop: adjust state, set stop_time to now. Sets sets .running to False.

Property RunState.stopped

Was the process stopped? Running is false and cancelled is true.

Property RunState.stopping

Is the process stopping? Running is true and cancelled is true.

Class RunStateMixin

Mixin to provide convenient access to a RunState.

Provides: .runstate, .cancelled, .running, .stopping, .stopped.

Method RunStateMixin.__init__(self, runstate=None)

Initialise the RunStateMixin; sets the .runstate attribute.

Parameters:

  • runstate: optional RunState instance or name. If a str, a new RunState with that name is allocated.

Method RunStateMixin.cancel(self)

Call .runstate.cancel().

Property RunStateMixin.cancelled

Test .runstate.cancelled.

Property RunStateMixin.running

Test .runstate.running.

Property RunStateMixin.stopped

Test .runstate.stopped.

Property RunStateMixin.stopping

Test .runstate.stopping.

Release Log

Release 20210731: RunState: tune the sanity checks around whether the state is "running".

Release 20210420: MultiOpenMixin: run startup/shutdown entirely via the new default method @contextmanager(startup_shutdown), paving the way for subclasses to just define their own startup_shutdown context manager methods instead of distinct startup/shutdown methods.

Release 20201025: MultiOpenMixin.__mo_getstate: dereference self.dict because using AttributeError was pulling a state object from another instance, utterly weird.

Release 20200718: MultiOpenMixin: as a hack to avoid having an init, move state into an on demand object accesses by a private method.

Release 20200521: Sweeping removal of cs.obj.O, universally supplanted by types.SimpleNamespace.

Release 20190812:

  • MultiOpenMixin: no longer subclass cs.obj.O.
  • MultiOpenMixin: remove lock param support, the mixin has its own lock.
  • MultiOpen: drop lock param support, no longer used by MultiOpenMixin.
  • MultiOpenMixin: do finalise inside the lock for the same reason as shutdown (competition with open/startup).
  • MultiOpenMixin.close: new unopened_ok=False parameter intended for callback closes which might fire even if the initial open does not occur.

Release 20190617: RunState.exit: if an exception was raised call .canel() before calling .stop().

Release 20190103:

  • Bugfixes for context managers.
  • MultiOpenMixin fixes and changes.
  • RunState improvements.

Release 20171024:

  • bugfix MultiOpenMixin finalise logic and other small logic fixes and checs
  • new class RunState for tracking or controlling a running task

Release 20160828: Use "install_requires" instead of "requires" in DISTINFO.

Release 20160827:

  • BREAKING CHANGE: rename NestingOpenCloseMixin to MultiOpenMixin.
  • New Pool class for generic object reuse.
  • Assorted minor improvements.

Release 20150115: First PyPI release.

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