Core Python Programming (2nd Edition)

If we try it out, we see that it saves the first value we give it but does not allow us to set it again:
>>> inst = ProtectAndHideX('foo')
Traceback (most recent call last):
File "", line 1, in ?
File "prop.py", line 5, in __init__
assert isinstance(x, int), \
AssertionError: "x" must be an integer!
>>> inst = ProtectAndHideX(10)
>>> print 'inst.x =', inst.x
inst.x = 10
>>> inst.x = 20
Traceback (most recent call last):
File "", line 1, in ?
AttributeError: can't set attribute
Here is another example, but with a setter:
class HideX(object):
def __init__(self, x):
self.x = x
def get_x(self):
return ~self.__x
def set_x(self, x):
assert isinstance(x, int), \
'"x" must be an integer!'
self.__x = ~x
x = property(get_x, set_x)
Here is the output of this example:
>>> inst = HideX(20)
>>> print inst.x
20
>>> inst.x = 30
>>> print inst.x
30
This property works because by the time the constructor is called to set the initial value of x, the getter
already saves it as ~x to self.__x.
You can even stick in a documentation string for your attribute, as shown here in this next example:
from math import pi
def get_pi(dummy):
return pi