Encapsulation in Python
#pythonIn OOP, setter and getter (mutator and accessor) methods is a common way to implement encapsulation which protects private variables. In Python, as we know, object properties are public, when we need to achieve property protection, the pythonic way is the property decorator.
General Class¶
A general class looks like
class Person:
def __init__(self, name: str) -> None:
self.name = name
p = Person("John")
p.name
'John'
Clearly, we can have some unexpected names...
print(Person(123).name)
print(Person([]).name)
123 []
Class with Property Protection¶
Depends on the purpose, if certain level of encapsulation is need, we can leverage the property decorator on getter and overloading setter method.
class StrictPerson:
def __init__(self, name: str) -> None:
self._name = None # where real value is
self.name = name # interface for getter and setter
@property
def name(self) -> str:
return self._name
@name.setter
def name(self, name: str) -> None:
if isinstance(name, str):
self._name = name
else:
raise ValueError("Name should be a string.")
sp = StrictPerson("Doe")
print(sp.name)
sp.name = "John Wick"
print(sp.name)
sp.name = 123
Doe John Wick
--------------------------------------------------------------------------- ValueError Traceback (most recent call last) <ipython-input-24-c4f5d3c50dca> in <module> 3 sp.name = "John Wick" 4 print(sp.name) ----> 5 sp.name = 123 <ipython-input-23-1dc003482362> in name(self, name) 13 self._name = name 14 else: ---> 15 raise ValueError("Name should be a string.") ValueError: Name should be a string.
Since it's an assignment operation with one argument in it, the overloading name setter method will be used.
Let's try to instantiate an object with unexpected name...
StrictPerson(123)
--------------------------------------------------------------------------- ValueError Traceback (most recent call last) <ipython-input-19-a65321deca47> in <module> ----> 1 StrictPerson(123) <ipython-input-17-1dc003482362> in __init__(self, name) 2 def __init__(self, name: str) -> None: 3 self._name = None # where real value is ----> 4 self.name = name # interface for getter and setter 5 6 @property <ipython-input-17-1dc003482362> in name(self, name) 13 self._name = name 14 else: ---> 15 raise ValueError("Name should be a string.") ValueError: Name should be a string.
From the traceback, we can see the self.name = name in the 4th line is infact the initial setter and the real value stores in the self._name property.