Why Python’s Double Underscore Doesn’t Guarantee True Privacy

In Python classes, attributes and methods can be accessed and modified directly through instances.

To hide internal attributes from external access, prefix the variable name with two underscores (__).

When an instance variable starts with __, Python treats it as a private variable that is only accessible inside the class; external code cannot directly access it via the class or instance.

Code example:

Although external code cannot directly access a private attribute, it can be accessed and modified indirectly through class methods.

Class internal methods getname and setname:

Is a double‑underscore private attribute always inaccessible from outside? Actually, it is not:

Python exhibits two interesting phenomena: external variables can shadow class variables, so accessing class attributes via an instance requires caution.

Like regular Python variables, assigning to an instance attribute creates the attribute on the instance if it does not already exist.

However, if a class attribute with the same name exists, this creates a side effect where the instance attribute hides the class attribute.

This behavior persists in Python 3.x.

Example code:

Why does deleting a dict attribute sometimes raise an error?

The reason lies in CPython’s implementation: Python is written in C.

In C there is no native string type; strings are represented as character arrays stored in contiguous memory and are immutable. Any modification creates a new array without altering the original.

In contrast, a dict is stored as a collection of non‑contiguous memory blocks that can be modified in place, so attempting to delete an attribute like f.x in the second example triggers an error.