Python's descriptor protocol can seem a bit esoteric at first; however, it can be invaluable in handling everyday idioms and patterns - something that the Django framework authors have taken advantage of in numerous occasions (e.g.: auth middleware).
One such idiom I see and use often and would like to generalize is the attribute-existence check-or-set routine illustrated here:
def get_foo(self):
if not hasattr(self, '_foo'):
self._foo = init_foo()
return self._foo
Rather than coding this up multiple times (either for a given class or across
many unrelated classes), I would prefer to delegate this repetitive work to a
descriptor and remain DRY. The means to this end is implemented as a
variation on the Python property construct, and is intentionally over simplistic
(I leave the details of the heavy lifting up to the reader).
The basic premise shown in source here is simply straight-forward Python, a quick and dirty example of how it could be utilized within a Django context is shown here:
from django.db import models
from cacheprop import CacheProperty2
ARTIFACT_TYPES = (
('F', _('File')),
('D', _('Directory')),
('A', _('Alias')),
)
class Artifact(models.Model):
# model fields
name = models.CharField(maxlength=64)
type = models.CharField(maxlength=1, choices=ARTIFACT_TYPES)
file_metadata = CacheProperty2(
lambda self: self.filemetadata_set.get(artifact__id=self.id)
)
class FileMetadata(models.Model):
byte_size = models.IntegerField()
artifact = models.ForeignKey(Artifact, unique=True)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 | #!/usr/local/bin/python
'''
Sample test code:
>>> f = Foo()
>>> print f.x
1
>>> print f.y
2
>>> print f.z
2007-08-05 00:00:00
>>> print vars(f)
{'_z': datetime.datetime(2007, 8, 5, 0, 0), '_y': 2, '_x': 1}
>>>
>>> b = Bar()
>>> print b.x
1
>>> print b.y
2
>>> print b.z
2007-08-05 00:00:00
>>> print vars(b)
{'_z': datetime.datetime(2007, 8, 5, 0, 0), '_y': 2, '_x': 1}
>>>
>>> z = Baz()
>>> print z.x
1
>>> print z.y
2
>>> print z.z
2007-08-05 00:00:00
>>> print vars(z)
{}
'''
from datetime import datetime
#------------------------------------------------------------------
def simple_curry(f, *args, **kws):
'''Simple helper function for pre-bundling a function with some
known arguments.'''
def inner_curry(self):
return f(self, *args, **kws)
return inner_curry
#==================================================================
class CacheProperty(object):
'''Caching mechanism that uses the supplied instance's
namespace for storage.'''
#--------------------------------------------------------------
def __init__(self, cache_var_name, initializer):
self.initializer = initializer
self.var = cache_var_name
#--------------------------------------------------------------
def __set__(self, *args):
raise AttributeError, u'Read-only attribute'
#--------------------------------------------------------------
def __get__(self, instance, cls):
if instance is None:
raise AttributeError(
u'%s must be accessed via instance' % (self.var,)
)
if not hasattr(instance, self.var):
# print '... No Cache'
if callable(self.initializer):
setattr(instance, self.var, self.initializer(instance))
else:
setattr(instance, self.var, self.initializer)
return getattr(instance, self.var)
#==================================================================
class CacheProperty2(object):
'''Caching descriptor that stores the cached data itself.'''
__NIL = object()
#--------------------------------------------------------------
def __init__(self, initializer):
self.initializer = initializer
self.cache = self.__NIL
#--------------------------------------------------------------
def __set__(self, *args):
raise AttributeError, u'Read-only attribute'
#--------------------------------------------------------------
def __get__(self, instance, cls):
if instance is None:
raise AttributeError, u'Instance level access only'
if self.cache == self.__NIL:
# print '... No Cache'
if callable(self.initializer):
self.cache = self.initializer(instance)
else:
self.cache = self.initializer
return self.cache
#==================================================================
class Foo(object):
'''A typical representation of the attribute caching mechanism
in verbose fashion.'''
#--------------------------------------------------------------
def expensive_function(self, arg):
# do something expensive here
return arg
#--------------------------------------------------------------
def _get_x(self):
if not hasattr(self, '_x'):
self._x = self.expensive_function(1)
return self._x
#--------------------------------------------------------------
def _get_y(self):
if not hasattr(self, '_y'):
self._y = self.expensive_function(2)
return self._y
#--------------------------------------------------------------
def _get_z(self):
if not hasattr(self, '_z'):
self._z = datetime(2007,8,5)
return self._z
#--------------------------------------------------------------
x = property(_get_x)
y = property(_get_y)
z = property(_get_z)
#==================================================================
class Bar(object):
'''Exhibit "A" for caching attributes, in this case the data is
stored directly within the instance.'''
#--------------------------------------------------------------
def expensive_function(self, arg):
# do something expensive here
return arg
#--------------------------------------------------------------
x = CacheProperty('_x', simple_curry(expensive_function, 1))
y = CacheProperty('_y', simple_curry(expensive_function, 2))
z = CacheProperty('_z', lambda self: datetime(2007,8,5))
#==================================================================
class Baz(object):
'''Exhibit "B" for caching attributes, in this case the data is
contained within the descriptor.'''
#--------------------------------------------------------------
def expensive_function(self, arg):
# do something expensive here
return arg
#--------------------------------------------------------------
x = CacheProperty2(simple_curry(expensive_function, 1))
y = CacheProperty2(simple_curry(expensive_function, 2))
z = CacheProperty2(lambda self: datetime(2007,8,5))
#------------------------------------------------------------------
def test():
# doctest does not seem to work in 2.3
import doctest
doctest.testmod()
###################################################################
if __name__ == '__main__':
test()
|
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