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# Copyright 2009 Canonical Ltd. This software is licensed under the
# GNU Affero General Public License version 3 (see the file LICENSE).
"""Cached properties for situations where a property is computed once and
then returned each time it is asked for.
The clear_cachedproperties function can be used to wipe the cache of properties
from an instance.
"""
__metaclass__ = type
__all__ = [
'cache_property',
'cachedproperty',
'clear_cachedproperties',
'clear_property',
]
from zope.security.proxy import removeSecurityProxy
from canonical.lazr.utils import safe_hasattr
# XXX: JonathanLange 2010-01-11 bug=505731: Move this to lp.services.
def cachedproperty(attrname_or_fn):
"""A decorator for methods that makes them properties with their return
value cached.
The value is cached on the instance, using the attribute name provided.
If you don't provide a name, the mangled name of the property is used.
cachedproperty is not threadsafe - it should not be used on objects which
are shared across threads / external locking should be used on those
objects.
>>> class CachedPropertyTest(object):
...
... @cachedproperty('_foo_cache')
... def foo(self):
... print 'foo computed'
... return 23
...
... @cachedproperty
... def bar(self):
... print 'bar computed'
... return 69
>>> cpt = CachedPropertyTest()
>>> getattr(cpt, '_foo_cache', None) is None
True
>>> cpt.foo
foo computed
23
>>> cpt.foo
23
>>> cpt._foo_cache
23
>>> cpt.bar
bar computed
69
>>> cpt._bar_cached_value
69
Cached properties are listed on instances.
>>> sorted(cpt._cached_properties)
['_bar_cached_value', '_foo_cache']
"""
if isinstance(attrname_or_fn, basestring):
attrname = attrname_or_fn
return CachedPropertyForAttr(attrname)
else:
fn = attrname_or_fn
attrname = '_%s_cached_value' % fn.__name__
return CachedProperty(attrname, fn)
def cache_property(instance, attrname, value):
"""Cache value on instance as attrname.
instance._cached_properties is updated with attrname.
>>> class CachedPropertyTest(object):
...
... @cachedproperty('_foo_cache')
... def foo(self):
... return 23
...
>>> instance = CachedPropertyTest()
>>> cache_property(instance, '_foo_cache', 42)
>>> instance.foo
42
>>> instance._cached_properties
['_foo_cache']
Caching a new value does not duplicate the cache keys.
>>> cache_property(instance, '_foo_cache', 84)
>>> instance._cached_properties
['_foo_cache']
And does update the cached value.
>>> instance.foo
84
"""
naked_instance = removeSecurityProxy(instance)
clear_property(naked_instance, attrname)
setattr(naked_instance, attrname, value)
cached_properties = getattr(naked_instance, '_cached_properties', [])
cached_properties.append(attrname)
naked_instance._cached_properties = cached_properties
def clear_property(instance, attrname):
"""Remove a cached attribute from instance.
The attribute name is removed from instance._cached_properties.
If the property is not cached, nothing happens.
:seealso clear_cachedproperties: For clearing all cached items at once.
>>> class CachedPropertyTest(object):
...
... @cachedproperty('_foo_cache')
... def foo(self):
... return 23
...
>>> instance = CachedPropertyTest()
>>> instance.foo
23
>>> clear_property(instance, '_foo_cache')
>>> instance._cached_properties
[]
>>> is_cached(instance, '_foo_cache')
False
>>> clear_property(instance, '_foo_cache')
"""
naked_instance = removeSecurityProxy(instance)
if not is_cached(naked_instance, attrname):
return
delattr(naked_instance, attrname)
naked_instance._cached_properties.remove(attrname)
def clear_cachedproperties(instance):
"""Clear cached properties from an object.
>>> class CachedPropertyTest(object):
...
... @cachedproperty('_foo_cache')
... def foo(self):
... return 23
...
>>> instance = CachedPropertyTest()
>>> instance.foo
23
>>> instance._cached_properties
['_foo_cache']
>>> clear_cachedproperties(instance)
>>> instance._cached_properties
[]
>>> hasattr(instance, '_foo_cache')
False
"""
naked_instance = removeSecurityProxy(instance)
cached_properties = getattr(naked_instance, '_cached_properties', [])
for property_name in cached_properties:
delattr(naked_instance, property_name)
naked_instance._cached_properties = []
def is_cached(instance, attrname):
"""Return True if attrname is cached on instance.
>>> class CachedPropertyTest(object):
...
... @cachedproperty('_foo_cache')
... def foo(self):
... return 23
...
>>> instance = CachedPropertyTest()
>>> instance.foo
23
>>> is_cached(instance, '_foo_cache')
True
>>> is_cached(instance, '_var_cache')
False
"""
naked_instance = removeSecurityProxy(instance)
return safe_hasattr(naked_instance, attrname)
class CachedPropertyForAttr:
"""Curry a decorator to provide arguments to the CachedProperty."""
def __init__(self, attrname):
self.attrname = attrname
def __call__(self, fn):
return CachedProperty(self.attrname, fn)
class CachedProperty:
# Used to detect not-yet-cached properties.
sentinel = object()
def __init__(self, attrname, fn):
self.fn = fn
self.attrname = attrname
def __get__(self, inst, cls=None):
if inst is None:
return self
cachedresult = getattr(inst, self.attrname, CachedProperty.sentinel)
if cachedresult is CachedProperty.sentinel:
result = self.fn(inst)
cache_property(inst, self.attrname, result)
return result
else:
return cachedresult
if __name__ == '__main__':
import doctest
doctest.testmod()
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