Python @property

Fluent Python Verse 9.6 speaks of hashable Class,

In order for the Vector2d class to be hashed, the following conditions are available:

(1) Implement __hash__ method

(2) Implement __eq__ method

(3) Make vector2d vector immutable

How do I make a vector of vector2d class instances read-only? You can use the property as follows:

classvector2d:def __init__(self, x, y): self._x=x self.__y=y @property #The @property decorator marks the getter method of a property.    defx (self):returnself._x @property #The @property decorator marks the getter method of a property.    defy (self):returnSelf.__y    def __hash__(self):returnHash (self._x) ^ hash (self.__y)    def __eq__(self, Other):returnhash (self) = =Hash (Other)def __iter__(self):return(I forIinch(Self._x, self.)__y))

We tried to modify X or Y in the console:

>>>Importexample9_7>>> V1 = example9_7.vector2d (3, 4)>>>v1.x3>>> v1.x = 4Traceback (most recent): File"<stdin>", Line 1,inch<module>Attributeerror:can'T set attribute>>> V1.Y = 5Traceback (most recent): File"<stdin>", Line 1,inch<module>Attributeerror:can'T set attribute

This is what we want to do, but why did you add the @properly adorner and become read-only? We need to have a deeper understanding of the property.

An Example to Begin with:

Before that, let's take a look at the property's application scenario:

Let's say we've written a class about temperature:

class Celsius:     def __init__ (Self, temperature=0):         = temperature    def  Get_fahrenheit (self):        return self.temperature * 1.8 + 32

And this class is becoming very popular, is called by many users, one day, a user ran to suggest that the temperature should not be lower than the absolute temperature-273 degrees Celsius, he asked us to achieve this limit.

In order to achieve the user's requirements, we update to v1.1:

classCelsius:def __init__(Self, temperature=0): Self.__temperature=TemperaturedefGet_fahrenheit (self):returnSelf.__temperature* 1.8 + 32defget_temperature (self):returnSelf.__temperature    defset_temperature (self, value):ifValue <-273:            RaiseValueError ("temperature below-273 is not possible.") self.__temperature= value

The user's requirements are realized can have a problem here, the user's code is still the way to get the temperature:

c = Celsius (PNs)
= C.temperature

And there are hundreds of lines of code in the code that have to be changed to:

C.set_temperature () c.get_temperature ()

This is a headache for the user, because our changes are not backward compatible.

The Power of @property:

For this problem, the more pythonic solution is as follows:

1 classCelsius:2     def __init__(Self, temperature=0):3Self.__temperature=Temperature4 5     defGet_fahrenheit (self):6         returnSelf.__temperature* 1.8 + 327 8     defget_temperature (self):9         returnSelf.__temperatureTen  One     defset_temperature (self, value): A         ifValue <-273: -             RaiseValueError ("temperature below-273 is not possible.") -Self.__temperature=value the  -     Temprature = Property (Get_temperature, Set_temperature)

This way, users still have access to temprature as before:

>>> C1 = Property_demo. Celsius (Ten)>>> c1.temprature10>>> c1.temprature = 20>>>  C1.temprature20

As a result, we have achieved both the limitations of termperature and the Guaranteed backwards compatibility

Digging deeper into property:

Python @property