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# -*- test-case-name: epsilon.test.test_descriptor -*-
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Provides an 'attribute' class for one-use descriptors.
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class _MetaAttribute(type):
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def __new__(meta, name, bases, dict):
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for kw in ['get', 'set', 'delete']:
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dict[kw] = staticmethod(dict[kw])
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secretClass = type.__new__(meta, name, bases, dict)
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class attribute(object):
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Convenience class for providing one-shot descriptors, similar to
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'property'. For example:
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>>> from epsilon.descriptor import attribute
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>>> class Dynamo(object):
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... class dynamic(attribute):
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... self.dynCount += 1
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... return self.dynCount
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... def set(self, value):
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... self.dynCount += value
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Traceback (most recent call last):
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AttributeError: attribute cannot be removed
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__metaclass__ = _MetaAttribute
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def __get__(self, oself, type):
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Private implementation of descriptor interface.
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return self.get(oself)
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def __set__(self, oself, value):
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Private implementation of descriptor interface.
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return self.set(oself, value)
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def __delete__(self, oself):
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Private implementation of descriptor interface.
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return self.delete(oself)
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Implement this method to provide attribute setting. Default behavior
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is that attributes are not settable.
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raise AttributeError('read only attribute')
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Implement this method to provide attribute retrieval. Default behavior
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is that unset attributes do not have any value.
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raise AttributeError('attribute has no value')
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Implement this method to provide attribute deletion. Default behavior
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is that attributes cannot be deleted.
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raise AttributeError('attribute cannot be removed')
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def requiredAttribute(requiredAttributeName):
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Utility for defining attributes on base classes/mixins which require their
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values to be supplied by their derived classes. C{None} is a common, but
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almost never suitable default value for these kinds of attributes, as it
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may cause operations in the derived class to fail silently in peculiar
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ways. If a C{requiredAttribute} is accessed before having its value
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changed, a C{AttributeError} will be raised with a helpful error message.
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@param requiredAttributeName: The name of the required attribute.
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@type requiredAttributeName: C{str}
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>>> from epsilon.descriptor import requiredAttribute
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>>> class FooTestMixin:
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... expectedResult = requiredAttribute('expectedResult')
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>>> class BrokenFooTestCase(TestCase, FooTestMixin):
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>>> brokenFoo = BrokenFooTestCase()
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>>> print brokenFoo.expectedResult
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Traceback (most recent call last):
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AttributeError: Required attribute 'expectedResult' has not been
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changed from its default value on '<BrokenFooTestCase
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>>> class WorkingFooTestCase(TestCase, FooTestMixin):
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... expectedResult = 7
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>>> workingFoo = WorkingFooTestCase()
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>>> print workingFoo.expectedResult
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class RequiredAttribute(attribute):
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if requiredAttributeName not in self.__dict__:
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raise AttributeError(
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('Required attribute %r has not been changed'
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' from its default value on %r' % (
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requiredAttributeName, self)))
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return self.__dict__[requiredAttributeName]
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def set(self, value):
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self.__dict__[requiredAttributeName] = value
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return RequiredAttribute
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__all__ = ['attribute', 'requiredAttribute']
added
removed
Epsilon/epsilon/descriptor.py
