python protected and private vars

Topics

• oops
• python internals

In Python, we are “mature consenting adults”. There are no actual “private” variables and by doing obj.__dict__, one can see all the attributes of an obj. Nevertheless, conceptually, there are standards to follow:

• If a var is like: self._value = value, then it is protected. External callers and subclasses are expected (but not enforced) not to use this variable
• If a var is named as: self.__value = value, then it is private. In Python, internally they are name-mangled and stored as _ClassName__value, so caller can’t directly do obj.__value to access it. This is used majorly inside libraries to avoid var name clashes, since users of that library can have similar var names (especially when word is very common such as value, model, total etc)

Following code illustrates this pretty clearly:

@dataclass
class BaseClass:
    __value: int
 
 
class SubClass(BaseClass):
    def get_value_v1(self):
        return self.__value
 
    def get_value_v2(self):
        return self._BaseClass__value
 
subcls = SubClass(3)
print(subcls.get_value_v2()) # Success: Prints 3
print(subcls.get_value_v1()) # Error: Throws `AttributeError: ...`

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