Source code for cement.core.handler

Cement core handler module.


import re
from abc import ABC
from ..core import exc, meta
from ..utils.misc import minimal_logger

LOG = minimal_logger(__name__)

[docs] class Handler(ABC, meta.MetaMixin): """Base handler class that all Cement Handlers should subclass from."""
[docs] class Meta: """ Handler meta-data (can also be passed as keyword arguments to the parent class). """ label = NotImplemented """The string identifier of this handler.""" interface = NotImplemented """The interface that this class implements.""" config_section = None """ A config section to merge config_defaults with. Note: Though ``App.Meta.config_section`` defaults to ``None``, Cement will set this to the value of ``<interface_label>.<handler_label>`` if no section is set by the user/developer. """ config_defaults = None """ A config dictionary that is merged into the applications config in the ``[<config_section>]`` block. These are defaults and do not override any existing defaults under that section. """ overridable = False """ Whether or not handler can be overridden by ``App.Meta.handler_override_options``. Will be listed as an available choice to override the specific handler (i.e. ``App.Meta.output_handler``, etc). """
def __init__(self, **kw): super(Handler, self).__init__(**kw) try: assert self._meta.label, \ "%s.Meta.label undefined." % self.__class__.__name__ assert self._meta.interface, \ "%s.Meta.interface undefined." % self.__class__.__name__ except AssertionError as e: raise exc.FrameworkError(e.args[0]) = None
[docs] def _setup(self, app): """ Called during application initialization and must ``setup`` the handler object making it ready for the framework or the application to make further calls to it. Args: app (instance): The application object. """ = app if self._meta.config_section is None: self._meta.config_section = "%s.%s" % \ (self._meta.interface, self._meta.label) if self._meta.config_defaults is not None: LOG.debug("merging config defaults from '%s' " % self + "into section '%s'" % self._meta.config_section) dict_obj = dict() dict_obj[self._meta.config_section] = self._meta.config_defaults, override=False) self._validate()
[docs] def _validate(self): """ Perform any validation to ensure proper data, meta-data, etc. """ pass # pragma: nocover
[docs] class HandlerManager(object): """ Manages the handler system to define, get, resolve, etc handlers with the Cement Framework. """ def __init__(self, app): = app self.__handlers__ = {}
[docs] def get(self, interface, handler_label, fallback=None, **kwargs): """ Get a handler object. Args: interface (str): The interface of the handler (i.e. ``output``) handler_label (str): The label of the handler (i.e. ``json``) fallback (Handler): A fallback value to return if handler_label doesn't exist. Keyword Args: setup (bool): Whether or not to call ``setup()`` on the handler before returning. This will not be called on the ``fallback`` if no the handler given does not exist. Returns: Handler: An uninstantiated handler object Raises: cement.core.exc.InterfaceError: If the ``interface`` does not exist, or if the handler itself does not exist. Example: .. code-block:: python _handler = app.handler.get('output', 'json') output = _handler() output._setup(app) output.render(dict(foo='bar')) """ setup = kwargs.get('setup', False) if interface not in raise exc.InterfaceError("Interface '%s' does not exist!" % interface) if handler_label in self.__handlers__[interface]: if setup is True: han = self.__handlers__[interface][handler_label] return self.setup(han) else: return self.__handlers__[interface][handler_label] elif fallback is not None: return fallback else: raise exc.InterfaceError("handlers['%s']['%s'] does not exist!" % (interface, handler_label))
[docs] def list(self, interface): """ Return a list of handlers for a given ``interface``. Args: interface (str): The interface of the handler (i.e. ``output``) Returns: list: Handler labels (str) that match ``interface``. Raises: cement.core.exc.InterfaceError: If the ``interface`` does not exist. Example: .. code-block:: python app.handler.list('log') """ if not raise exc.InterfaceError("Interface '%s' does not exist!" % interface) res = [] for label in self.__handlers__[interface]: res.append(self.__handlers__[interface][label]) return res
[docs] def register(self, handler_class, force=False): """ Register a handler class to an interface. If the same object is already registered then no exception is raised, however if a different object attempts to be registered to the same name a ``InterfaceError`` is raised. Args: handler_class (Handler): The uninstantiated handler class to register. Keyword Arguments: force (bool): Whether to allow replacement if an existing handler of the same ``label`` is already registered. Raises: cement.core.exc.InterfaceError: If the ``handler_class`` does not implement :class:`Handler`, or if ``handler_class`` does not properly sub-class it's interface. cement.core.exc.InterfaceError: If the ``handler_class.Meta.interface`` does not exist Usage: .. code-block:: python class MyDatabaseHandler(object): class Meta: interface = IDatabase label = 'mysql' def connect(self): # ... app.handler.register(MyDatabaseHandler) """ # for checks if not issubclass(handler_class, Handler): raise exc.InterfaceError("Class %s " % handler_class + "does not implement Handler") obj = handler_class() # translate dashes to underscores handler_class.Meta.label = re.sub('-', '_', obj._meta.label) obj._meta.label = re.sub('-', '_', obj._meta.label) interface = obj._meta.interface LOG.debug("registering handler '%s' into handlers['%s']['%s']" % (handler_class, interface, obj._meta.label)) if interface not in raise exc.InterfaceError("Handler interface '%s' doesn't exist." % interface) elif interface not in self.__handlers__.keys(): self.__handlers__[interface] = {} if obj._meta.label in self.__handlers__[interface] and \ self.__handlers__[interface][obj._meta.label] != handler_class: if force is True: LOG.debug( "handlers['%s']['%s'] already exists" % (interface, obj._meta.label) + ", but `force==True`" ) else: raise exc.InterfaceError( "handlers['%s']['%s'] already exists" % (interface, obj._meta.label) ) interface_class = if not issubclass(handler_class, interface_class): raise exc.InterfaceError("Handler %s " % handler_class.__name__ + "does not sub-class %s" % interface_class.__name__) self.__handlers__[interface][obj._meta.label] = handler_class
[docs] def registered(self, interface, handler_label): """ Check if a handler is registered. Args: interface (str): The interface of the handler (interface label) handler_label (str): The label of the handler Returns: bool: ``True`` if the handler is registered, ``False`` otherwise Example: .. code-block:: python app.handler.registered('log', 'colorlog') """ if interface in if interface in self.__handlers__.keys() and \ handler_label in self.__handlers__[interface]: return True return False
[docs] def setup(self, handler_class): """ Setup a handler class so that it can be used. Args: handler_class (class): An uninstantiated handler class. Returns: None Example: .. code-block:: python for controller in app.handler.list('controller'): ch = app.handler.setup(controller) """ h = handler_class() h._setup( return h
[docs] def resolve(self, interface, handler_def, **kwargs): """ Resolves the actual handler, as it can be either a string identifying the handler to load from ``self.__handlers__``, or it can be an instantiated or non-instantiated handler class. Args: interface (str): The interface of the handler (ex: ``output``) handler_def(str,instance,Handler): The loose references of the handler, by label, instantiated object, or non-instantiated class. Keyword args: raise_error (bool): Whether or not to raise an exception if unable to resolve the handler. meta_defaults (dict): Optional meta-data dictionary used as defaults to pass when instantiating uninstantiated handlers. Use ``App.Meta.meta_defaults`` by default. setup (bool): Whether or not to call ``.setup()`` before return. Default: ``False`` Returns: instance: The instantiated handler object. Example: .. code-block:: python # via label (str) log = app.handler.resolve('log', 'colorlog') # via uninstantiated handler class log = app.handler.resolve('log', ColorLogHanddler) # via instantiated handler instance log = app.handler.resolve('log', ColorLogHandler()) """ raise_error = kwargs.get('raise_error', True) meta_defaults = kwargs.get('meta_defaults', None) if meta_defaults is None: meta_defaults = {} if type(handler_def) is str: _meta_label = "%s.%s" % (interface, handler_def) meta_defaults =, {}) elif hasattr(handler_def, 'Meta'): _meta_label = "%s.%s" % (interface, handler_def.Meta.label) meta_defaults =, {}) setup = kwargs.get('setup', False) han = None if type(handler_def) is str: han = self.get(interface, handler_def)(**meta_defaults) elif hasattr(handler_def, '_meta'): if not self.registered(interface, handler_def._meta.label): self.register(handler_def.__class__) han = handler_def elif hasattr(handler_def, 'Meta'): han = handler_def(**meta_defaults) if not self.registered(interface, han._meta.label): self.register(handler_def) msg = "Unable to resolve handler '%s' of interface '%s'" % \ (handler_def, interface) if han is not None: if setup is True: han._setup( return han elif han is None and raise_error: raise exc.FrameworkError(msg) elif han is None: LOG.debug(msg) return None