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# -*- test-case-name: epsilon.test.test_descriptor -*-
"""
Provides an 'attribute' class for one-use descriptors.
"""
attribute = None
class _MetaAttribute(type):
def __new__(meta, name, bases, dict):
# for reals, yo.
for kw in ['get', 'set', 'delete']:
if kw in dict:
dict[kw] = staticmethod(dict[kw])
secretClass = type.__new__(meta, name, bases, dict)
if attribute is None:
return secretClass
return secretClass()
class attribute(object):
"""
Convenience class for providing one-shot descriptors, similar to
'property'. For example:
>>> from epsilon.descriptor import attribute
>>> class Dynamo(object):
... class dynamic(attribute):
... def get(self):
... self.dynCount += 1
... return self.dynCount
... def set(self, value):
... self.dynCount += value
... dynCount = 0
...
>>> d = Dynamo()
>>> d.dynamic
1
>>> d.dynamic
2
>>> d.dynamic = 6
>>> d.dynamic
9
>>> d.dynamic
10
>>> del d.dynamic
Traceback (most recent call last):
...
AttributeError: attribute cannot be removed
"""
__metaclass__ = _MetaAttribute
def __get__(self, oself, type):
"""
Private implementation of descriptor interface.
"""
if oself is None:
return self
return self.get(oself)
def __set__(self, oself, value):
"""
Private implementation of descriptor interface.
"""
return self.set(oself, value)
def __delete__(self, oself):
"""
Private implementation of descriptor interface.
"""
return self.delete(oself)
def set(self, value):
"""
Implement this method to provide attribute setting. Default behavior
is that attributes are not settable.
"""
raise AttributeError('read only attribute')
def get(self):
"""
Implement this method to provide attribute retrieval. Default behavior
is that unset attributes do not have any value.
"""
raise AttributeError('attribute has no value')
def delete(self):
"""
Implement this method to provide attribute deletion. Default behavior
is that attributes cannot be deleted.
"""
raise AttributeError('attribute cannot be removed')
def requiredAttribute(requiredAttributeName):
"""
Utility for defining attributes on base classes/mixins which require their
values to be supplied by their derived classes. C{None} is a common, but
almost never suitable default value for these kinds of attributes, as it
may cause operations in the derived class to fail silently in peculiar
ways. If a C{requiredAttribute} is accessed before having its value
changed, a C{AttributeError} will be raised with a helpful error message.
@param requiredAttributeName: The name of the required attribute.
@type requiredAttributeName: C{str}
Example:
>>> from epsilon.descriptor import requiredAttribute
...
>>> class FooTestMixin:
... expectedResult = requiredAttribute('expectedResult')
...
>>> class BrokenFooTestCase(TestCase, FooTestMixin):
... pass
...
>>> brokenFoo = BrokenFooTestCase()
>>> print brokenFoo.expectedResult
Traceback (most recent call last):
...
AttributeError: Required attribute 'expectedResult' has not been
changed from its default value on '<BrokenFooTestCase
instance>'.
...
>>> class WorkingFooTestCase(TestCase, FooTestMixin):
... expectedResult = 7
...
>>> workingFoo = WorkingFooTestCase()
>>> print workingFoo.expectedResult
... 7
>>>
"""
class RequiredAttribute(attribute):
def get(self):
if requiredAttributeName not in self.__dict__:
raise AttributeError(
('Required attribute %r has not been changed'
' from its default value on %r' % (
requiredAttributeName, self)))
return self.__dict__[requiredAttributeName]
def set(self, value):
self.__dict__[requiredAttributeName] = value
return RequiredAttribute
__all__ = ['attribute', 'requiredAttribute']
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