Python Language Classes Properties
Example
Python classes support properties, which look like regular object variables, but with the possibility of attaching custom behavior and documentation.
class MyClass(object):
def __init__(self):
self._my_string = ""
@property
def string(self):
"""A profoundly important string."""
return self._my_string
@string.setter
def string(self, new_value):
assert isinstance(new_value, str), \
"Give me a string, not a %r!" % type(new_value)
self._my_string = new_value
@string.deleter
def x(self):
self._my_string = None
The object's of class MyClass will appear to have have a property .string, however it's behavior is now tightly controlled:
mc = MyClass()
mc.string = "String!"
print(mc.string)
del mc.string
As well as the useful syntax as above, the property syntax allows for validation, or other augmentations to be added to those attributes. This could be especially useful with public APIs - where a level of help should be given to the user.
Another common use of properties is to enable the class to present 'virtual attributes' - attributes which aren't actually stored but are computed only when requested.
class Character(object):
def __init__(name, max_hp):
self._name = name
self._hp = max_hp
self._max_hp = max_hp
# Make hp read only by not providing a set method
@property
def hp(self):
return self._hp
# Make name read only by not providing a set method
@property
def name(self):
return self.name
def take_damage(self, damage):
self.hp -= damage
self.hp = 0 if self.hp <0 else self.hp
@property
def is_alive(self):
return self.hp != 0
@property
def is_wounded(self):
return self.hp < self.max_hp if self.hp > 0 else False
@property
def is_dead(self):
return not self.is_alive
bilbo = Character('Bilbo Baggins', 100)
bilbo.hp
# out : 100
bilbo.hp = 200
# out : AttributeError: can't set attribute
# hp attribute is read only.
bilbo.is_alive
# out : True
bilbo.is_wounded
# out : False
bilbo.is_dead
# out : False
bilbo.take_damage( 50 )
bilbo.hp
# out : 50
bilbo.is_alive
# out : True
bilbo.is_wounded
# out : True
bilbo.is_dead
# out : False
bilbo.take_damage( 50 )
bilbo.hp
# out : 0
bilbo.is_alive
# out : False
bilbo.is_wounded
# out : False
bilbo.is_dead
# out : True
