@property decorator

Purpose

property objects provides an interface for getter and setter functions. The decorator creates a property object interface for the inner function.

Defintion

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

• fget : function to get value of the attribute
• fset: function to set value of the attribute
• fdel: function to delete the attribute
• doc: string - like a comment

Example without decorator

Example of property object (without decorator), temperature, running when instantiated

class Celsius:
	def __init__(self, temperature=0):
		self.temperature = temperature
	# Note '_' dentoes private variable
	def get_temperature(self):
		return self._temperature
	def set_temperature(self, value):
		print("SETTING VAL")
		if value > 200:
			raise ValueError("Temperature too high")
		self._temperature = value
	# class variable
	temperature = property(get_temperature, set_temperature)
 
test = Celsius(50)
print(test.temperature)
test.temperature = 500
# Output: 
# SETTING VAL
# 50
# Temperature too high

Note: we can now disregard using test.set_temperature(100) but instead use test.temperature = 100 to set values. Also we can compute some logic to check if the new value meets constraints (temperature being too high). This keeps all the code inside of the class function.

Example with decorator

class Celsius:
	def __init__(self, temperature=0):
		self.temperature = temperature
	# Note '_' dentoes private variable
	@property
	def temperature(self):
		return self._temperature
	@temperature.setter
	def temperature(self, value):
		print("SETTING VAL")
		if value > 200:
			raise ValueError("Temperature too high")
		self._temperature = value
 
test = Celsius(50)
print(test.temperature)
test.temperature = 500
# Output: 
# SETTING VAL
# 50
# Temperature too high

Prerequisite knowledge

• Note in python when you call class.field

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

human = Celsius()
human.temperature = 37
print(human.__dict__)
# Output: 'temperature': 37