Does underscore has any special built-in meaningin Python ?

[Terry Reedy]

Benjamin Kaplan wrote:
> On Wed, Jul 29, 2009 at 1:59 PM, dandi kain <dandi.kain at gmail.com> wrote:
>> Hello everybody,
>> I have just started learning Python.I heard its simple so I pick a
>> presentation [1] and tried to work on it.But when it comes to
>> underscores leading and trailing an object I dont understand any.I
>> look through the python manual also but that was not helping .I
>> searched some forums and I still dont have a clear picture.
>>
>> What is the functionality of __ or _ , leading or trailing an object ,
>> class ot function ? Is it just a naming convention to note special
>> functions and objects , or it really mean someting to Python ?
> 
> It's just a convention for the most part. A single leading underscore
> is used for "private" attributes. Two leading underscores will affect
> the code- it mangles the variable name so that you don't have to worry
> about the value being overwritten by a subclass. For instance
> """
> class Foo(object) :
>    def __init__(self) :
>        self.__bar = ''
> 
> foo = Foo()
> ""
> will store the attribute as foo._Foo__bar.
> 
> Also, the "magic methods"- the ones that are used for operations and
> built-in stuff, all have two leading and two trailing underscores.
> These are things like __add__ (+), __eq__ (=), __cmp__ (old way for
> comparisons), __len__ (len), __str__ (str), and so on.

For this last, see
http://docs.python.org/dev/py3k/reference/datamodel.html#special-method-names