needinit
This is a tiny python library to implement a fairly useful pattern of preventing method calls on an object which has not been initialized.
How does it work?
Any object which extends NeedsInit or NeedsInitMixin (and calls .require_init)
gets its methods temporarily replaced by a lambda which raises a NotInitialized
TypeError exception. Calling .initialize() on the object will replace the lambdas
with the originals, and can either be done externally before configuring the object
or from within a configuration/setup method.
In [1]: from needinit import NeedsInit
In [2]: class MyObject(NeedsInit):
...: def something(self):
...: return "hello, I'm ready for action"
...:
In [3]: o = MyObject()
In [4]: o.something()
... stack trace suppressed
NotInitialized: <__main__.MyObject object at 0x104382ed0>.something((), {}) - not callable, needs initializing.
In [5]: o.initialize()
In [6]: o.something()
Out[6]: "hello, I'm ready for action"Exempting methods
NeedsInit can take a list of exempt methods which is very useful to allow calling
non-constructor configuration/init methods -- the use case for which this library was
written, but other cases aren't hard to imagine, such as some status/inspection methods.
The __init__, require_init, and initialize methods are always exempt.
from needinit import NeedsInit
class MyAppExtension(NeedsInit):
def __init__(self, some_param, *args, **kwargs):
super(MyAppExtension, self).__init__(*args, exempt=['init_app'], **kwargs)
self.some_param = some_param
def init_app(self, app):
self.initialize() # all methods are now re-enabled
self.initialize_session(app.config['API_KEY'])
def initialize_session(self, api_key):
self.session = SessionMaker(api_key=api_key, environment=self.some_param)
