Python @property decorator

class Celsius:
    def __init__(self, temperature = 0):
        self.temperature = temperature

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32

# Basic method of setting and getting attributes in Python
class Celsius:
    def __init__(self, temperature=0):
        self.temperature = temperature

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32


# Create a new object
human = Celsius()

# Set the temperature
human.temperature = 37

# Get the temperature attribute
print(human.temperature)

# Get the to_fahrenheit method
print(human.to_fahrenheit())

37
98.60000000000001

The extra decimal places when converting into Fahrenheit is due to the floating-point arithmetic error. To learn more, visit Python Floating-Point Arithmetic Error.

Whenever we assign or retrieve any object attribute like temperature, as shown above, Python searches it in the object’s built-in __dict__ dictionary attribute.

>>> human.__dict__
{'temperature': 37}

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Using Getters and Setters

Suppose we want to expand the ease of use of the Celsius class defined above. We know that the temperature of any object can’t reach beneath – 273.15 degrees Celsius (Absolute Zero in Thermodynamics)

How about we update our code to actualize this worth imperative.

A conspicuous answer for the above limitation will be to conceal the quality temperature (make it private) and characterize new getter and setter techniques to control it. This can be done as follows:

# Making Getters and Setter methods
class Celsius:
    def __init__(self, temperature=0):
        self.set_temperature(temperature)

    def to_fahrenheit(self):
        return (self.get_temperature() * 1.8) + 32

    # getter method
    def get_temperature(self):
        return self._temperature

    # setter method
    def set_temperature(self, value):
        if value < -273.15:
            raise ValueError("Temperature below -273.15 is not possible.")
        self._temperature = value

As we can see, the above method introduces two new get_temperature() and set_temperature() methods.

Furthermore, the temperature was replaced with _temperature. An underscore _ at the beginning is used to denote private variables in Python.

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Now, let’s use this implementation:

# Making Getters and Setter methods
class Celsius:
    def __init__(self, temperature=0):
        self.set_temperature(temperature)

    def to_fahrenheit(self):
        return (self.get_temperature() * 1.8) + 32

    # getter method
    def get_temperature(self):
        return self._temperature

    # setter method
    def set_temperature(self, value):
        if value < -273.15:
            raise ValueError("Temperature below -273.15 is not possible.")
        self._temperature = value


# Create a new object, set_temperature() internally called by __init__
human = Celsius(37)

# Get the temperature attribute via a getter
print(human.get_temperature())

# Get the to_fahrenheit method, get_temperature() called by the method itself
print(human.to_fahrenheit())

# new constraint implementation
human.set_temperature(-300)

# Get the to_fahreheit method
print(human.to_fahrenheit())

Output

37
98.60000000000001
Traceback (most recent call last):
  File "<string>", line 30, in <module>
  File "<string>", line 16, in set_temperature
ValueError: Temperature below -273.15 is not possible.

This update successfully implemented the new limitation. We are no longer permitted to set the temperature underneath – 273.15 degrees Celsius.

Note: The private factors don’t really exist in Python. There are just standards to be followed. The language itself doesn’t apply any restrictions.

>>> human._temperature = -300
>>> human.get_temperature()
-300

However, the bigger problem with the above update is that all the programs that implemented our previous class have to modify their code from obj.temperature to obj.get_temperature() and all expressions like obj.temperature = val to obj.set_temperature(val).

This refactoring can cause problems while dealing with hundreds of thousands of lines of codes.

All in all, our new update was not backward compatible. This is where @property comes to rescue.

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The property Class

A pythonic way to deal with the above problem is to use the property class. Here is how we can update our code:

# using property class
class Celsius:
    def __init__(self, temperature=0):
        self.temperature = temperature

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32

    # getter
    def get_temperature(self):
        print("Getting value...")
        return self._temperature

    # setter
    def set_temperature(self, value):
        print("Setting value...")
        if value < -273.15:
            raise ValueError("Temperature below -273.15 is not possible")
        self._temperature = value

    # creating a property object
    temperature = property(get_temperature, set_temperature)

We added a print() function inside get_temperature() and set_temperature() to clearly observe that they are being executed.

The last line of the code makes a property object temperature. Simply put, property attaches some code (get_temperature and set_temperature) to the member attribute accesses (temperature).

Let’s use this updated code:

# using property class
class Celsius:
    def __init__(self, temperature=0):
        self.temperature = temperature

    def to_fahrenheit(self):
        return (self.temperature * 1.8) + 32

    # getter
    def get_temperature(self):
        print("Getting value...")
        return self._temperature

    # setter
    def set_temperature(self, value):
        print("Setting value...")
        if value < -273.15:
            raise ValueError("Temperature below -273.15 is not possible")
        self._temperature = value

    # creating a property object
    temperature = property(get_temperature, set_temperature)


human = Celsius(37)

print(human.temperature)

print(human.to_fahrenheit())

human.temperature = -300

Output

Setting value...
Getting value...
37
Getting value...
98.60000000000001
Setting value...
Traceback (most recent call last):
  File "<string>", line 31, in <module>
  File "<string>", line 18, in set_temperature
ValueError: Temperature below -273 is not possible

As we can see, any code that retrieves the value of temperature will automatically call get_temperature() instead of a dictionary (__dict__) look-up. Similarly, any code that assigns a value to temperature will automatically call set_temperature().

We can even see above that set_temperature() was called even when we created an object.

>>> human = Celsius(37)
Setting value...

Can you guess why?

The reason is that when an item is made, the __init__() strategy gets called. This strategy has the line self.temperature = temperature. This articulation consequently calls set_temperature().

Correspondingly, any entrance like c.temperature automatically calls get_temperature(). This is the thing that the property does. Here are a few more examples.

>>> human.temperature
Getting value
37
>>> human.temperature = 37
Setting value

>>> c.to_fahrenheit()
Getting value
98.60000000000001

By using property, we can see that no change is required in the execution of the worth imperative. Along these lines, our usage is in reverse compatible.

Note: The actual temperature value is put away in the private _temperature variable. A temperature characteristic is a property object which gives an interface to this private variable

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The @property Decorator

In Python, property() is a built-in function that creates and returns a property object. The syntax of this function is:

property(fget=None, fset=None, fdel=None, doc=None)

where,

fget is function to get value of the attribute
fset is function to set value of the attribute
fdel is function to delete the attribute
doc is a string (like a comment)
As seen from the implementation, these function arguments are optional. So, a property object can simply be created as follows.

>>> property()
<property object at 0x0000000003239B38>

A property object has three methods, getter(), setter(), and deleter() to specify fget, fset and fdel at a later point. This means, the line:

# make empty property
temperature = property()
# assign fget
temperature = temperature.getter(get_temperature)
# assign fset
temperature = temperature.setter(set_temperature)

# Using @property decorator
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):
        print("Setting value...")
        if value < -273.15:
            raise ValueError("Temperature below -273 is not possible")
        self._temperature = value


# create an object
human = Celsius(37)

print(human.temperature)

print(human.to_fahrenheit())

coldest_thing = Celsius(-300)

Setting value...
Getting value...
37
Getting value...
98.60000000000001
Setting value...
Traceback (most recent call last):
  File "<string>", line 29, in <module>
  File "<string>", line 4, in __init__
  File "<string>", line 18, in temperature
ValueError: Temperature below -273 is not possible

The above implementation is simple and efficient. It is the recommended way to use the property.

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Please feel free to give your comment if you face any difficulty here.

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