@property
@property
is a built-in decorator that transforms instance methods into properties.
In practice properties are used just like value attributes - they can be accessed with dotted name without parentheses.
(Script available)
class Vector2(object):
def __init__(self, x, y):
self.x = x
self.y = y
def __str__(self):
return "({}, {})".format(self.x, self.y)
@property
def array(self):
return self.x, self.y
@array.setter
def array(self, val):
self.x, self.y = val
@property
def length(self):
return math.sqrt(sum(i ** 2 for i in self.array))
@length.setter
def length(self, val):
ratio = val / self.length
self.x *= ratio
self.y *= ratio
@length.deleter
def length(self):
self.x = 0
self.y = 0
if __name__ == '__main__':
v = Vector2(1, 3)
print(v.array)
v.array = 2, 2
print(v)
print(v.length)
v.length += math.sqrt(2)
print(v)
del v.length
print(v)
For a property, the setter or deleter don’t have to be defined, so you may create read-only properties.
(Script available)
class C(object):
def __init__(self, x):
self.__private = x
@property
def x(self):
return self.__private
if __name__ == '__main__':
c = C(1)
print(c.x)
try:
print(c.__private)
except AttributeError as e:
print(repr(e))
try:
c.x = 2
except AttributeError as e:
print(repr(e))
@cached_property
@cached_property is available in both the
cached-property package
and the Django package.
Instead of being a property with setter and deleter, it is just a getter that evaluates and replaces it self with the result when it is first accessed.
It is most useful in two cases:
There is a resource-heavy calculation that should only be done on demand.
A calculated value that does not change while changes are made to the class instance.
Especially when both are true.
(Script available)
import sys
import time
from cached_property import cached_property
class C(object):
def __init__(self, x):
self.__x = x
@property
def x(self):
return self.__x
@cached_property
def squared(self):
print('Calculating...', file=sys.stderr)
time.sleep(1)
return self.x ** 2
if __name__ == '__main__':
c = C(10)
print(c.squared)
print(c.squared)