53 lines
1.8 KiB
Python
53 lines
1.8 KiB
Python
"""
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Convenience routines for creating non-trivial Field subclasses, as well as
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backwards compatibility utilities.
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Add SubfieldBase as the __metaclass__ for your Field subclass, implement
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to_python() and the other necessary methods and everything will work seamlessly.
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"""
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class SubfieldBase(type):
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"""
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A metaclass for custom Field subclasses. This ensures the model's attribute
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has the descriptor protocol attached to it.
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"""
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def __new__(cls, name, bases, attrs):
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new_class = super(SubfieldBase, cls).__new__(cls, name, bases, attrs)
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new_class.contribute_to_class = make_contrib(
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new_class, attrs.get('contribute_to_class')
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)
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return new_class
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class Creator(object):
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"""
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A placeholder class that provides a way to set the attribute on the model.
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"""
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def __init__(self, field):
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self.field = field
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def __get__(self, obj, type=None):
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if obj is None:
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raise AttributeError('Can only be accessed via an instance.')
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return obj.__dict__[self.field.name]
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def __set__(self, obj, value):
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obj.__dict__[self.field.name] = self.field.to_python(value)
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def make_contrib(superclass, func=None):
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"""
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Returns a suitable contribute_to_class() method for the Field subclass.
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If 'func' is passed in, it is the existing contribute_to_class() method on
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the subclass and it is called before anything else. It is assumed in this
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case that the existing contribute_to_class() calls all the necessary
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superclass methods.
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"""
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def contribute_to_class(self, cls, name):
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if func:
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func(self, cls, name)
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else:
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super(superclass, self).contribute_to_class(cls, name)
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setattr(cls, self.name, Creator(self))
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return contribute_to_class
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