3 from abc import ABC, abstractmethod
13 logger = logging.getLogger(__name__)
16 class Persistent(ABC):
18 A base class of an object with a load/save method. Classes that are
19 decorated with @persistent_autoloaded_singleton should subclass this
20 and implement their save() and load() methods.
25 def save(self) -> bool:
27 Save this thing somewhere that you'll remember when someone calls
28 load() later on in a way that makes sense to your code.
37 Load this thing from somewhere and give back an instance which
38 will become the global singleton and which will may (see
39 below) be save()d at program exit time.
41 Oh, in case this is handy, here's how to write a factory
42 method that doesn't call the c'tor in python:
45 def load_from_somewhere(cls, somewhere):
46 # Note: __new__ does not call __init__.
47 obj = cls.__new__(cls)
49 # Don't forget to call any polymorphic base class initializers
50 super(MyClass, obj).__init__()
52 # Load the piece(s) of obj that you want to from somewhere.
53 obj._state = load_from_somewhere(somewhere)
60 def was_file_written_today(filename: str) -> bool:
61 """Returns True if filename was written today."""
63 if not file_utils.does_file_exist(filename):
66 mtime = file_utils.get_file_mtime_as_datetime(filename)
67 now = datetime.datetime.now()
69 mtime.month == now.month
70 and mtime.day == now.day
71 and mtime.year == now.year
75 def was_file_written_within_n_seconds(
79 """Returns True if filename was written within the pas limit_seconds
83 if not file_utils.does_file_exist(filename):
86 mtime = file_utils.get_file_mtime_as_datetime(filename)
87 now = datetime.datetime.now()
88 return (now - mtime).total_seconds() <= limit_seconds
91 class PersistAtShutdown(enum.Enum):
93 An enum to describe the conditions under which state is persisted
94 to disk. See details below.
103 class persistent_autoloaded_singleton(object):
104 """A decorator that can be applied to a Persistent subclass (i.e. a
105 class with a save() and load() method. It will intercept attempts
106 to instantiate the class via it's c'tor and, instead, invoke the
107 class' load() method to give it a chance to read state from
108 somewhere persistent.
110 If load() fails (returns None), the c'tor is invoked with the
111 original args as a fallback.
113 Based upon the value of the optional argument persist_at_shutdown,
114 (NEVER, IF_NOT_LOADED, ALWAYS), the save() method of the class will
115 be invoked just before program shutdown to give the class a chance
116 to save its state somewhere.
118 The implementations of save() and load() and where the class
119 persists its state are details left to the Persistent
127 persist_at_shutdown: PersistAtShutdown = PersistAtShutdown.IF_NOT_LOADED,
129 self.persist_at_shutdown = persist_at_shutdown
132 def __call__(self, cls: Persistent):
133 @functools.wraps(cls)
134 def _load(*args, **kwargs):
136 # If class has already been loaded, act like a singleton
137 # and return a reference to the one and only instance in
139 if self.instance is not None:
141 f'Returning already instantiated singleton instance of {cls.__name__}.'
145 # Otherwise, try to load it from persisted state.
148 f'Attempting to load {cls.__name__} from persisted state.'
150 self.instance = cls.load()
151 if not self.instance:
152 msg = 'Loading from cache failed.'
155 f'Attempting to instantiate {cls.__name__} directly.'
157 self.instance = cls(*args, **kwargs)
160 f'Class {cls.__name__} was loaded from persisted state successfully.'
164 assert self.instance is not None
166 if self.persist_at_shutdown is PersistAtShutdown.ALWAYS or (
168 and self.persist_at_shutdown is PersistAtShutdown.IF_NOT_LOADED
171 'Scheduling a deferred called to save at process shutdown time.'
173 atexit.register(self.instance.save)