113 lines
4.1 KiB
Python
113 lines
4.1 KiB
Python
class SignalConnectMetaClass(type):
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"""
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Metaclass to connect the children of a base class to a Django signal.
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Classes must have a signal argument and a get_dispatch_uid classmethod.
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The signal argument must be the Django signal the class should be
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connected to. The get_dispatch_uid classmethod must return a unique
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value for each child class and None for base classes because they will
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not be connected to the signal.
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The classmethod get_all is added to every class using this metaclass.
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Calling this on a base class or on child classes will retrieve all
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connected children, one instance for each child class.
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These instances will have a check_permission method which returns True
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by default. You can override this method to return False on runtime if
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you want to filter some children.
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They will also have a get_default_weight method which returns the value
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of the default_weight attribute which is 0 by default. You can override
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the attribute or the method to sort the children.
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Don't forget to set up the __init__ method so that it is able to receive
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wildcard keyword arguments (see example below). This is necessary
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because of Django's signal API.
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Example:
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class Base(object, metaclass=SignalConnectMetaClass):
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signal = django.dispatch.Signal()
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def __init__(self, **kwargs):
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pass
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@classmethod
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def get_dispatch_uid(cls):
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if not cls.__name__ == 'Base':
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return cls.__name__
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class Child(Base):
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pass
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child = Base.get_all(request)[0]
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assert Child == type(child)
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"""
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def __new__(metaclass, class_name, class_parents, class_attributes):
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"""
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Creates the class and connects it to the signal if so. Adds all
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default attributes and methods.
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"""
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class_attributes['get_all'] = get_all
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new_class = super(SignalConnectMetaClass, metaclass).__new__(
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metaclass, class_name, class_parents, class_attributes)
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try:
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dispatch_uid = new_class.get_dispatch_uid()
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except AttributeError:
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raise NotImplementedError('Your class %s must have a get_dispatch_uid classmethod.' % class_name)
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if dispatch_uid is not None:
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try:
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signal = new_class.signal
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except AttributeError:
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raise NotImplementedError('Your class %s must have a signal argument, which must be a Django Signal instance.' % class_name)
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else:
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signal.connect(new_class, dispatch_uid=dispatch_uid)
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attributes = {'check_permission': check_permission,
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'get_default_weight': get_default_weight,
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'default_weight': 0}
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for name, attribute in attributes.items():
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if not hasattr(new_class, name):
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setattr(new_class, name, attribute)
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return new_class
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@classmethod # type: ignore
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def get_all(cls, request=None):
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"""
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Collects all objects of the class created by the SignalConnectMetaClass
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from all apps via signal. They are sorted using the get_default_weight
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method. Does not return objects where check_permission returns False.
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A django.http.HttpRequest object can optionally be given.
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This classmethod is added as get_all classmethod to every class using
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the SignalConnectMetaClass.
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"""
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kwargs = {'sender': cls}
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if request is not None:
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kwargs['request'] = request
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all_objects = [obj for __, obj in cls.signal.send(**kwargs) if obj.check_permission()]
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all_objects.sort(key=lambda obj: obj.get_default_weight())
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return all_objects
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def check_permission(self):
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"""
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Returns True by default. Override this to filter some children on runtime.
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This method is added to every instance of classes using the
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SignalConnectMetaClass.
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"""
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return True
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def get_default_weight(self):
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"""
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Returns the value of the default_weight attribute by default. Override
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this to sort some children on runtime.
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This method is added to every instance of classes using the
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SignalConnectMetaClass.
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"""
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return self.default_weight
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