No Private Class Members in Python

1 minute read

TIL that Python does not have the ability to make class members (methods, data fields) private.

Scenario

I’ve been building a small python class that calculates the information gain for a particular set of decisions, with respect to a random variable. This was inspired by a midterm for the Georgia Tech AI course I’m currently taking.

In this python class, I only wanted to expose one method as publically accessible through the API. There is a helper method that is a class member (it relies on class data) which I wanted to make private.

It’s all in the name?

I initially found a stack overflow post that noted making a method private in python is all in the name.

By prepending two underscores to the beginning of a class method name, the method becomes inaccessible on the class instance itself.

class TestClass:
    def __init__(self, data):
        self.data = data

    def __privatemethod(self):
        print "is it private?"

    def publicmethod(self):
        print "definitely public"

tc = TestClass([1,2,3])
tc.publicmethod() # prints "definitely public"
tc.__privatemethod() # throws AttributeError: TestClass instance has no attribute '__privatemethod'

The thrown AttributeError makes it appear at first glance that the __privatemethod method is inaccessible off of the class instance.

Unfortunately, though, that’s not the case.

No Private Class Members in Python

I learned from reading the python class docs that python doesn’t actually support making class members private.

Prepending double underscore to the beginning of a class name is a convention used to indicate the method should be regarded as private, and is not meant to be used outside of the class context.

The double underscore mangles the class name by additionally prepending _ClassName to the member name. This makes it more difficult to access a “private” member of the class, but not impossible.

class TestClass:
   """
   Same as above
   ...
   """

tc = TestClass([1,2,3])
tc.publicmethod() # prints "definitely public"
tc._TestClass__privatemethod() # prints "is it private?"

Therefore, it is actually impossible to make class members private in python. The mangling convention makes it more difficult to access members that should be considered private, but they can still be accessed by persistent developers.

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