Question

What is the purpose of python variables __something__
e.g. __name__, __file__

I often see them in real python programs.
Is there a list of these variables?

thank you

Answer 1

PEP8 covers this:

In addition, the following special forms using leading or trailing
underscores are recognized (these can generally be combined with any
case convention):

• _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.

The final bullet point is pertinent.

Is there a list of these variables?

I doubt that there is an official list of all such double leading and trailing underscore names, but I could not say so for sure.