Let's start with Python decorators.
A Python decorator is a function that helps to add some additional functionalities to an already defined function.
In Python, everything is an object. Functions in Python are first-class objects which means that they can be referenced by a variable, added in the lists, passed as arguments to another function, etc.
Consider the following code snippet.
def decorator_func(fun):
def wrapper_func():
print("Wrapper function started")
fun()
print("Given function decorated")
# Wrapper function add something to the passed function and decorator
# returns the wrapper function
return wrapper_func
def say_bye():
print("bye!!")
say_bye = decorator_func(say_bye)
say_bye()
# Output:
# Wrapper function started
# bye!!
# Given function decorated
Here, we can say that the decorator function modified our say_bye function and added some extra lines of code to it.
Python syntax for decorator
def decorator_func(fun):
def wrapper_func():
print("Wrapper function started")
fun()
print("Given function decorated")
# Wrapper function add something to the passed function and decorator
# returns the wrapper function
return wrapper_func
@decorator_func
def say_bye():
print("bye!!")
say_bye()
Let's go through everything with a case scenario. But before that, let's talk about some OOP principles.
Getters and setters are used in many object-oriented programming languages to ensure the principle of data encapsulation(which is seen as the bundling of data with the methods that operate on these data.)
These methods are, of course, the getter for retrieving the data and the setter for changing the data.
According to this principle, the attributes of a class are made private to hide and protect them from other code.
Yup, @property is basically a pythonic way to use getters and setters.
Python has a great concept called property which makes the life of an object-oriented programmer much simpler.
Let us assume that you decide to make a class that could store the temperature in degrees Celsius.
class Celsius:
def __init__(self, temperature = 0):
self.set_temperature(temperature)
def to_fahrenheit(self):
return (self.get_temperature() * 1.8) + 32
def get_temperature(self):
return self._temperature
def set_temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
self._temperature = value
Refactored Code, Here is how we could have achieved it with 'property.'
In Python, property() is a built-in function that creates and returns a property object.
A property object has three methods, getter(), setter(), and delete().
class Celsius:
def __init__(self, temperature = 0):
self.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)
Here,
temperature = property(get_temperature,set_temperature)
could have been broken down as,
# make empty property
temperature = property()
# assign fget
temperature = temperature.getter(get_temperature)
# assign fset
temperature = temperature.setter(set_temperature)
Point To Note:
- get_temperature remains a property instead of a method.
Now you can access the value of temperature by writing.
C = Celsius()
C.temperature
# instead of writing C.get_temperature()
We can go on further and not define names get_temperature and set_temperature as they are unnecessary and pollute the class namespace.
The pythonic way to deal with the above problem is to use @property.
class Celsius:
def __init__(self, temperature = 0):
self.temperature = temperature
def to_fahrenheit(self):
return (self.temperature * 1.8) + 32
@property
def temperature(self):
print("Getting value")
return self.temperature
@temperature.setter
def temperature(self, value):
if value < -273:
raise ValueError("Temperature below -273 is not possible")
print("Setting value")
self.temperature = value
Points to Note -
- A method that is used for getting a value is decorated with "@property".
- The method which has to function as the setter is decorated with "@temperature.setter", If the function had been called "x", we would have to decorate it with "@x.setter".
- We wrote "two" methods with the same name and a different number of parameters, "def temperature(self)" and "def temperature(self,x)".
As you can see, the code is definitely less elegant.
Now, let's talk about one real-life practical scenario.
Let's say you have designed a class as follows:
class OurClass:
def __init__(self, a):
self.x = a
y = OurClass(10)
print(y.x)
Now, let's further assume that our class got popular among clients and they started using it in their programs, They did all kinds of assignments to the object.
And one fateful day, a trusted client came to us and suggested that "x" has to be a value between 0 and 1000; this is really a horrible scenario!
Due to properties, it's easy: We create a property version of "x".
class OurClass:
def __init__(self,x):
self.x = x
@property
def x(self):
return self.__x
@x.setter
def x(self, x):
if x < 0:
self.__x = 0
elif x > 1000:
self.__x = 1000
else:
self.__x = x
This is great, isn't it: You can start with the simplest implementation imaginable, and you are free to later migrate to a property version without having to change the interface! So properties are not just a replacement for getters and setters!
You can check this Implementation here
propertyis actually a class (not a function), although it does probably does call the__init__()method when you make an object, of course. Usinghelp(property)from the terminal is insightful.helpis also a class for some reason. – Beneficiary@propertyworks, but fail to address why you would use it. The question "why use@propertyin Python" is the equivalent of "why use accessors (getter/setters/deleters) in Java", as@propertyis the pythonic equivalent to the same concept. These complementary threads have some helpful discussions stackoverflow.com/questions/1568091/… stackoverflow.com/questions/6618002/… – Jerriself._x, which is protected. But using the property defined above, one can set and read the attribute just by typingc = C(), c.x = 99so letting every programmers to use that attribute as if it were NOT protected. I understood how @property works. But I still don't understand which is the benefit of using it the example above – Marauding