#!/usr/bin/env python3
+# © Copyright 2021-2022, Scott Gasch
+
+"""A :class:`Persistent` is just a class with a load and save method. This
+module defines the :class:`Persistent` base and a decorator that can be used to
+create a persistent singleton that autoloads and autosaves."""
+
import atexit
import datetime
import enum
class Persistent(ABC):
"""
A base class of an object with a load/save method. Classes that are
- decorated with @persistent_autoloaded_singleton should subclass this
- and implement their save() and load() methods.
-
+ decorated with :code:`@persistent_autoloaded_singleton` should subclass
+ this and implement their :meth:`save` and :meth:`load` methods.
"""
@abstractmethod
def save(self) -> bool:
"""
Save this thing somewhere that you'll remember when someone calls
- load() later on in a way that makes sense to your code.
-
+ :meth:`load` later on in a way that makes sense to your code.
"""
pass
@classmethod
@abstractmethod
def load(cls) -> Any:
- """
- Load this thing from somewhere and give back an instance which
- will become the global singleton and which will may (see
- below) be save()d at program exit time.
+ """Load this thing from somewhere and give back an instance which
+ will become the global singleton and which may (see
+ below) be saved (via :meth:`save`) at program exit time.
- Oh, in case this is handy, here's how to write a factory
- method that doesn't call the c'tor in python:
+ Oh, in case this is handy, here's a reminder how to write a
+ factory method that doesn't call the c'tor in python::
@classmethod
def load_from_somewhere(cls, somewhere):
# Load the piece(s) of obj that you want to from somewhere.
obj._state = load_from_somewhere(somewhere)
return obj
-
"""
pass
def was_file_written_today(filename: str) -> bool:
- """Returns True if filename was written today."""
+ """Convenience wrapper around :meth:`was_file_written_within_n_seconds`.
+
+ Args:
+ filename: filename to check
+
+ Returns:
+ True if filename was written today.
+
+ >>> import os
+ >>> filename = f'/tmp/testing_persistent_py_{os.getpid()}'
+ >>> os.system(f'touch {filename}')
+ 0
+ >>> was_file_written_today(filename)
+ True
+ >>> os.system(f'touch -d 1974-04-15T01:02:03.99 {filename}')
+ 0
+ >>> was_file_written_today(filename)
+ False
+ >>> os.system(f'/bin/rm -f {filename}')
+ 0
+ >>> was_file_written_today(filename)
+ False
+ """
if not file_utils.does_file_exist(filename):
return False
mtime = file_utils.get_file_mtime_as_datetime(filename)
- assert mtime
+ assert mtime is not None
now = datetime.datetime.now()
return mtime.month == now.month and mtime.day == now.day and mtime.year == now.year
filename: str,
limit_seconds: int,
) -> bool:
- """Returns True if filename was written within the pas limit_seconds
- seconds.
-
+ """Helper for determining persisted state staleness.
+
+ Args:
+ filename: the filename to check
+ limit_seconds: how fresh, in seconds, it must be
+
+ Returns:
+ True if filename was written within the past limit_seconds
+ or False otherwise (or on error).
+
+ >>> import os
+ >>> filename = f'/tmp/testing_persistent_py_{os.getpid()}'
+ >>> os.system(f'touch {filename}')
+ 0
+ >>> was_file_written_within_n_seconds(filename, 60)
+ True
+ >>> import time
+ >>> time.sleep(2.0)
+ >>> was_file_written_within_n_seconds(filename, 2)
+ False
+ >>> os.system(f'/bin/rm -f {filename}')
+ 0
+ >>> was_file_written_within_n_seconds(filename, 60)
+ False
"""
+
if not file_utils.does_file_exist(filename):
return False
mtime = file_utils.get_file_mtime_as_datetime(filename)
- assert mtime
+ assert mtime is not None
now = datetime.datetime.now()
return (now - mtime).total_seconds() <= limit_seconds
class PersistAtShutdown(enum.Enum):
"""
An enum to describe the conditions under which state is persisted
- to disk. See details below.
-
+ to disk. This is passed as an argument to the decorator below and
+ is used to indicate when to call :meth:`save` on a :class:`Persistent`
+ subclass.
+
+ * NEVER: never call :meth:`save`
+ * IF_NOT_LOADED: call :meth:`save` as long as we did not successfully
+ :meth:`load` its state.
+ * ALWAYS: always call :meth:`save`
"""
NEVER = (0,)
class persistent_autoloaded_singleton(object):
- """A decorator that can be applied to a Persistent subclass (i.e. a
- class with a save() and load() method. It will intercept attempts
- to instantiate the class via it's c'tor and, instead, invoke the
- class' load() method to give it a chance to read state from
- somewhere persistent.
-
- If load() fails (returns None), the c'tor is invoked with the
+ """A decorator that can be applied to a :class:`Persistent` subclass
+ (i.e. a class with :meth:`save` and :meth:`load` methods. The
+ decorator will intercept attempts to instantiate the class via
+ it's c'tor and, instead, invoke the class' :meth:`load` to give it a
+ chance to read state from somewhere persistent (disk, db,
+ whatever). Subsequent calls to construt instances of the wrapped
+ class will return a single, global instance (i.e. the wrapped
+ class is a singleton).
+
+ If :meth:`load` fails (returns None), the c'tor is invoked with the
original args as a fallback.
- Based upon the value of the optional argument persist_at_shutdown,
- (NEVER, IF_NOT_LOADED, ALWAYS), the save() method of the class will
- be invoked just before program shutdown to give the class a chance
- to save its state somewhere.
+ Based upon the value of the optional argument
+ :code:`persist_at_shutdown` argument, (NEVER, IF_NOT_LOADED,
+ ALWAYS), the :meth:`save` method of the class will be invoked just
+ before program shutdown to give the class a chance to save its
+ state somewhere.
- The implementations of save() and load() and where the class
- persists its state are details left to the Persistent
- implementation.
+ .. note::
+ The implementations of :meth:`save` and :meth:`load` and where the
+ class persists its state are details left to the :class:`Persistent`
+ implementation. Essentially this decorator just handles the
+ plumbing of calling your save/load and appropriate times and
+ creates a transparent global singleton whose state can be
+ persisted between runs.
"""
# memory.
if self.instance is not None:
logger.debug(
- f'Returning already instantiated singleton instance of {cls.__name__}.'
+ 'Returning already instantiated singleton instance of %s.', cls.__name__
)
return self.instance
# Otherwise, try to load it from persisted state.
was_loaded = False
- logger.debug(f'Attempting to load {cls.__name__} from persisted state.')
+ logger.debug('Attempting to load %s from persisted state.', cls.__name__)
self.instance = cls.load()
if not self.instance:
msg = 'Loading from cache failed.'
logger.warning(msg)
- logger.debug(f'Attempting to instantiate {cls.__name__} directly.')
+ logger.debug('Attempting to instantiate %s directly.', cls.__name__)
self.instance = cls(*args, **kwargs)
else:
- logger.debug(
- f'Class {cls.__name__} was loaded from persisted state successfully.'
- )
+ logger.debug('Class %s was loaded from persisted state successfully.', cls.__name__)
was_loaded = True
assert self.instance is not None
if self.persist_at_shutdown is PersistAtShutdown.ALWAYS or (
- not was_loaded
- and self.persist_at_shutdown is PersistAtShutdown.IF_NOT_LOADED
+ not was_loaded and self.persist_at_shutdown is PersistAtShutdown.IF_NOT_LOADED
):
- logger.debug(
- 'Scheduling a deferred called to save at process shutdown time.'
- )
+ logger.debug('Scheduling a deferred called to save at process shutdown time.')
atexit.register(self.instance.save)
return self.instance
return _load
+
+
+if __name__ == '__main__':
+ import doctest
+
+ doctest.testmod()