“public” or “private” attribute in Python ? What is the best way?

Typically, Python code strives to adhere to the Uniform Access Principle. Specifically, the accepted approach is:

• Expose your instance variables directly, allowing, for instance, foo.x = 0, not foo.set_x(0)
• If you need to wrap the accesses inside methods, for whatever reason, use @property, which preserves the access semantics. That is, foo.x = 0 now invokes foo.set_x(0).

The main advantage to this approach is that the caller gets to do this:

foo.x += 1

even though the code might really be doing:

foo.set_x(foo.get_x() + 1)

The first statement is infinitely more readable. Yet, with properties, you can add (at the beginning, or later on) the access control you get with the second approach.

Note, too, that instance variables starting with a single underscore are conventionally private. That is, the underscore signals to other developers that you consider the value to be private, and they shouldn’t mess with it directly; however, nothing in the language prevents them from messing with it directly.

If you use a double leading underscore (e.g., __x), Python does a little obfuscation of the name. The variable is still accessible from outside the class, via its obfuscated name, however. It’s not truly private. It’s just kind of … more opaque. And there are valid arguments against using the double underscore; for one thing, it can make debugging more difficult.