I'm learning about encapsulation and abstraction in python and i came across the property function and decorator. The common example is something like this.
class Celsius():
def __init__(self, temperature = 0):
self.set_temperature(temperature)
def to_fahrenheit(self):
return (self._temperature * 1.8) + 32
def get_temperature(self):
print("Getting value")
return self._temperature
def set_temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
print("Setting value")
self._temperature = value
temperature = property(get_temperature,set_temperature)
I dont understand why the property function is assigning the descriptor to temperature
and not self.temperature
. Isn't it suppouse to create a getter or setter functionality to a Instance, not to the class?
something like
self.temperature = property(get_temperature,set_temperature)
using
test = Celsius()
pprint(test.__dict__)
returns that the instance object just have the self._temperature
attribute (Which we are trying to make private).
using pprint(Celsius.__dict__)
returns that is actually the class that have the temperature attribute that we are accessing when using the objects, which to my understanding doesn't make sense since i am interested in creating functionality to the instance and access the instance atributes, not the class attributes.
Thanks in advance :)
There's no self outside the method. This is defining a class attribute.
@Barmar has a helpful comment above.
The commonly used class function parameters self
and cls
are not available outside of class functions. In this case, you are defining a static member of the class. It is implied by default that vars (like your temperature =
) defined this way are static members of the class.
See some useful tips on static members here: Are static class variables possible in Python?
But why is the syntax like that. Isnt should be assigning the property to a instance variable?
Note the below. The behavior of class_static_var = 4
is similar to how someone may view cls.class_static_var = 4
, however it is not similar to self.instance_var = 4
. To define a instance var, you can use the __init__
method
class DemoClass:
def __init__(self):
self.instance_var = 3
class_static_var = 4