Finding a list of all double-underscore variables?

If you want to see magic names whether documented or not, go to the Lib directory and run:

egrep -oh '__[A-Za-z_][A-Za-z_0-9]*__' *.py | sort | uniq

That produces:

'__all__'
'__args__'
'__author__'
'__bases__'
'__builtin__'
'__builtins__'
'__cached__'
'__call__'
'__class__'
'__copy__'
'__credits__'
'__date__'
'__decimal_context__'
'__deepcopy__'
'__dict__'
'__doc__'
'__exception__'
'__file__'
'__flags__'
'__ge__'
'__getinitargs__'
'__getstate__'
'__gt__'
'__import__'
'__importer__'
'__init__'
'__ispkg__'
'__iter__'
'__le__'
'__len__'
'__loader__'
'__lt__'
'__main__'
'__module__'
'__mro__'
'__name__'
'__package__'
'__path__'
'__pkgdir__'
'__return__'
'__safe_for_unpickling__'
'__setstate__'
'__slots__'
'__temp__'
'__test__'
'__version__'

when i use

dir(object)

i got these:

'__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__has
h__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__'
, '__setattr__', '__sizeof__', '__str__', '__subclasshook__'

and i think they are the dunder names every object will have in python

The complete list used by Python is given in the Python Language Reference section 3, "Data model". Every other one is non-standard or used by third-party modules and is documented separately.