A few simple discriptor classes to compute and cache attribute value on demand and to define attribute as alias to other.
Python, 72 lines
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'''Computes attribute value and caches it in instance.
Example:
class MyClass(object):
def myMethod(self):
# ...
myMethod = CachedAttribute(myMethod)
Use "del inst.myMethod" to clear cache.'''
def __init__(self, method, name=None):
self.method = method
self.name = name or method.__name__
def __get__(self, inst, cls):
if inst is None:
return self
result = self.method(inst)
setattr(inst, self.name, result)
return result
class CachedClassAttribute(object):
'''Computes attribute value and caches it in class.
Example:
class MyClass(object):
def myMethod(cls):
# ...
myMethod = CachedClassAttribute(myMethod)
Use "del MyClass.myMethod" to clear cache.'''
def __init__(self, method, name=None):
self.method = method
self.name = name or method.__name__
def __get__(self, inst, cls):
result = self.method(cls)
setattr(cls, self.name, result)
return result
class ReadAliasAttribute(object):
'''If not explcitly assigned this attribute is an alias for other.
Example:
class Document(object):
title='?'
shortTitle=ReadAliasAttribute('title')'''
def __init__(self, name):
self.name = name
def __get__(self, inst, cls):
if inst is None:
return self
return getattr(inst, self.name)
class AliasAttribute(ReadAliasAttribute):
'''This attribute is an alias for other.
Example:
class Document(object):
newAttrName=somevalue
deprecatedAttrName=AliasAttribute('newAttrName')'''
def __set__(self, inst, value):
setattr(inst, self.name, value)
def __delete__(self, inst):
delattr(inst, self.name)
|
First two decriptor classes are usful for lazy attribute evaluation. Note, that after it's evaluated there is no overhead since cached value is stored in instance/class __dict__. ReadAliasAttribute is intended for cases when one attribute should bound to other by default. And AliasAttribute can be used when you find a better name for an attribute, but want keep compatibility.
Tags: oop
