Python Property Objects

Python Property Objects are used to perform different tasks in python programming by the learners. Find out more about this feature here.

Python Property Objects: Using the @property decorator for read-write properties

If you want to use @property to implement custom behavior for setting and getting, use this pattern:

class Cash(object):
def init(self, value):
self.value = value
@property
def formatted(self):
return '${:.2f}'.format(self.value)
@formatted.setter
def formatted(self, new):
self.value = float(new[1:])

To use this:

wallet = Cash(2.50)
print(wallet.formatted) $2.50
print(wallet.value)
2.5
wallet.formatted = '$123.45'
print(wallet.formatted)
$123.45
print(wallet.value)
123.45

Using the @property decorator

The @property decorator can be used to define methods in a class which act like attributes. One example where this can be useful is when exposing information which may require an initial (expensive) lookup and simple retrieval thereafter.

Given some module foobar.py:

class Foo(object):
def init(self):
self.__bar = None
@property
def bar(self):
if self.__bar is None:
self.__bar = some_expensive_lookup_operation()
return self.__bar
Then

from foobar import Foo
foo = Foo()
print(foo.bar) # This will take some time since bar is None after initialization
42
print(foo.bar) # This is much faster since bar has a value now
42

Python Property Objects: Overriding just a getter, setter or a deleter of a property object

When you inherit from a class with a property, you can provide a new implementation for one or more of the property getter, setter or deleter functions, by referencing the property object on the parent class:

class BaseClass(object):
@property
def foo(self):
return some_calculated_value()
@foo.setter
def foo(self, value):
do_something_with_value(value)
class DerivedClass(BaseClass):
@BaseClass.foo.setter
def foo(self, value):
do_something_different_with_value(value)

You can also add a setter or deleter where there was not one on the base class before.

Using properties without decorators

While using decorator syntax (with the @) is convenient, it also a bit concealing. You can use properties directly, without decorators. The following Python 3.x example shows this:

class A:
p = 1234
def getX (self):
return self._x
def setX (self, value):
self._x = value
def getY (self):
return self._y
def setY (self, value):
self._y = 1000 + value # Weird but possible
def getY2 (self):
return self._y
def setY2 (self, value):
self._y = value
def getT (self):
return self._t
def setT (self, value):
self._t = value
def getU (self):
return self._u + 10000
def setU (self, value):
self._u = value - 5000

x, y, y2 = property (getX, setX), property (getY, setY), property (getY2, setY2)
t = property (getT, setT)
u = property (getU, setU)
A.q = 5678
class B:
def getZ (self):
return self.z_
def setZ (self, value):
self.z_ = value
z = property (getZ, setZ)
class C:
def init (self):
self.offset = 1234
def getW (self):
return self.w_ + self.offset
def setW (self, value):
self.w_ = value - self.offset
w = property (getW, setW)
a1 = A ()
a2 = A ()
a1.y2 = 1000
a2.y2 = 2000
a1.x = 5
a1.y = 6
a2.x = 7
a2.y = 8
a1.t = 77
a1.u = 88
print (a1.x, a1.y, a1.y2)
print (a2.x, a2.y, a2.y2)
print (a1.p, a2.p, a1.q, a2.q)
print (a1.t, a1.u)
b = B ()
c = C ()
b.z = 100100
c.z = 200200
c.w = 300300
print (a1.x, b.z, c.z, c.w)
c.w = 400400
c.z = 500500
b.z = 600600

print (a1.x, b.z, c.z, c.w)

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