Tagged: protected

Private, protected and public in Python

A lot of folks learn object-oriented programming with languages like C++ and Java. And while they learn, they’re told repeatedly (over and over again), that encapsulation is one of the key principles of object-oriented paradigm and that they should take advantage of it. Sure, until you get down with Python :-).

In C++ and Java, things are pretty straight-forward. There are 3 magical and easy to remember access modifiers, that will do the job (public, protected and private). But there’s no such a thing in Python. That might be a little confusing at first, but it’s possible too. We’ll have look at how do it right in Python.

Python doesn’t have any mechanisms, that would effectively restrict you from accessing a variable or calling a member method. All of this is a matter of culture and convention.

Public

All member variables and methods are public by default in Python. So when you want to make your member public, you just do nothing. See the example bellow:

class Cup:
    color    = None
    content  = None

    def fill(self, beverage):
        content = beverage

    def empty(self):
        content = None

We have a class Cup. And here’s what we can do with it:

redCup = Cup()
redCup.color = "red"
redCup.content = "tea"
redCup.empty()
redCup.fill("coffee")

All of this is good and acceptable, because all the attributes and methods are public.

Protected

Protected member is (in C++ and Java) accessible only from within the class and it’s subclasses. How to accomplish this in Python? The answer is — by convention. By prefixing the name of your member with a single underscore, you’re telling others “don’t touch this, unless you’re a subclass“. See the example bellow:

class Cup:
    color     = None
    _content  = None  # protected variable

    def fill(self, beverage):
        _content = beverage

    def empty(self):
        _content = None

Same example as before, but the content of the cup is now protected. This changes virtually nothing, you’ll still be able to access the variable from outside the class, only if you see something like this:

cup = Cup()
cup._content = "tea"

you explain politely to the person responsible for this, that the variable is protected and he should not access it or even worse, change it from outside the class.

Private

By declaring your data member private you mean, that nobody should be able to access it from outside the class, i.e. strong you can’t touch this policy. Python supports a technique called name mangling. This feature turns every member name prefixed with at least two underscores and suffixed with at most one underscore into _<className><memberName>. So how to make your member private? Let’s have a look at the example bellow:

class Cup:
    _color     = None  # protected variable
    __content  = None  # private variable

    def __init__(self, color):
        _color</span> = color

    def fill(self, beverage):
        __content = beverage

    def empty(self):
        __content = None

Our cup now can be only filled and poured out by using fill() and empty() methods. Note, that if you try accessing __content from outside, you’ll get an error. But you can still stumble upon something like this:

redCup = Cup("red")
redCup._Cup__content = "tea"

This will work, except, in case you see this, you kick your colleague, who’s responsible for it hard in the nuts.

Sources