variable naming query

[loial]

I'ma a newbie python user and would like clarification on variable
naming conventions.

What is the difference between

self.myvariable
self._myvariable
self.__myvariable

and when should I use each of them?

 

[Ben Finney]

What is the difference between

self.myvariable

Indicates to the reader that the attribute 'myvariable' is available
for use as part of the interface of the object.

Prefer this style unless you have good reason in a particular case to
do otherwise.

self._myvariable

Indicates to the reader that the attribute '_myvariable' should not be
used as part of the interface to the object.

self.__myvariable

Indicates to the reader that the attribute '__myvariable' is not
available by that name outside the object, and name mangling is
automatically done to discourage its use from outside the object.

and when should I use each of them?

Use each of them to indicate the above conditions where appropriate.

Note that none of them will change the nature of the attribute, and
Python will allow use of any of them by the correct name. There is no
such thing as "limited-access" attributes in Python; we rely on the
maxim that "we're all consenting adults here". If an attribute exists
in the current scope, it is available for any use regardless of what
name you give it.

 

[Neil Cerutti]

Indicates to the reader that the attribute '__myvariable' is
not available by that name outside the object, and name
mangling is automatically done to discourage its use from
outside the object.

From _Python Reference Manual (2.3.2) Reserved Classes of
Identifiers:

__*

Class-private names. Names in this category, when used
within the context of a class definition, are re-written to
use a mangled form to help avoid name clashes between
``private'' attributes of base and derived classes.

Further, from the _Python Tutorial (9.6) Private Variables_:

(Buglet: derivation of a class with the same name as the base
class makes use of private variables of the base class
possible.)

In other words, it's a misfeature that's best avoided.