Why Python’s Double‑Underscore Variables Aren’t Truly Private (And What Really Happens)

In Python classes, attributes and methods are normally accessible directly through an instance.

To hide an internal attribute from external access, prefix its name with two underscores (__), which triggers name‑mangling and makes it a private variable that can only be accessed inside the class.

Although external code cannot directly access a private attribute, it can still be accessed or modified indirectly via class methods, such as getter and setter functions.

Private attributes prefixed with double underscores are not absolutely inaccessible; they can be accessed using the mangled name _ClassName__attribute.

Python also exhibits an interesting phenomenon where external variables can shadow class variables. Assigning a value to an instance attribute creates the attribute on the instance if it does not already exist, potentially masking a class attribute of the same name and causing side effects.

The following images illustrate code examples of private attributes, getter/setter methods, and attribute shadowing:

Another example demonstrates why deleting a dict attribute can raise an error: CPython is implemented in C, where strings are immutable character arrays, but dictionaries are stored in non‑contiguous memory that can be modified in place, leading to different behavior when attempting to delete attributes.