Underscore vs Double underscore with variables and methods [duplicate]

From PEP 8:

• _single_leading_underscore: weak “internal use” indicator. E.g.

  from M import *

  does not import objects whose name starts with an underscore.

• single_trailing_underscore_: used by convention to avoid conflicts with Python keyword, e.g.

  Tkinter.Toplevel(master, class_='ClassName')

• __double_leading_underscore: when naming a class attribute, invokes name
  mangling (inside class FooBar, __boo becomes _FooBar__boo; see below).

• __double_leading_and_trailing_underscore__: “magic” objects or
  attributes that live in user-controlled namespaces. E.g. __init__,
  __import__ or __file__. Never invent such names; only use them
  as documented.

Also, from David Goodger’s Code Like a Pythonista:

Attributes: interface, _internal, __private

But try to avoid the __private form. I never use it. Trust me. If you
use it, you WILL regret it later.

Explanation:

People coming from a C++/Java background are especially prone to
overusing/misusing this “feature”. But __private names don’t work the
same way as in Java or C++. They just trigger a name mangling whose
purpose is to prevent accidental namespace collisions in subclasses:
MyClass.__private just becomes MyClass._MyClass__private. (Note that
even this breaks down for subclasses with the same name as the
superclass, e.g. subclasses in different modules.) It is possible to
access __private names from outside their class, just inconvenient and
fragile (it adds a dependency on the exact name of the superclass).

The problem is that the author of a class may legitimately think “this
attribute/method name should be private, only accessible from within
this class definition” and use the __private convention. But later on,
a user of that class may make a subclass that legitimately needs
access to that name. So either the superclass has to be modified
(which may be difficult or impossible), or the subclass code has to
use manually mangled names (which is ugly and fragile at best).

There’s a concept in Python: “we’re all consenting adults here”. If
you use the __private form, who are you protecting the attribute from?
It’s the responsibility of subclasses to use attributes from
superclasses properly, and it’s the responsibility of superclasses to
document their attributes properly.

It’s better to use the single-leading-underscore convention,
_internal. “This isn’t name mangled at all; it just indicates to
others to “be careful with this, it’s an internal implementation
detail; don’t touch it if you don’t fully understand it”. It’s only a
convention though.