Changing the private variables content

[Ryniek90]

Hi.
I'm writing some class, and decided to use inside private method and 
some private variables. While with method i haven't got any problem's 
with variables i have.
Maybe some example.
A class with private method and private variable:

"
 >>> class Secret(object):
    #
    def __init__(self):
        self._number = 1
    #
    def _secret(self):
        print self._number
    def showit(self):
        print "Secret number is:\n"
        self._secret()

 >>> sec = Secret()
 >>> sec.showit()
Secret number is:

1
 >>> sec._number
1
 >>> sec._number = 2
 >>> sec._number
2
 >>> sec._number += 3
 >>> sec._number
5
 >>>
"

As You can see, i made class with private method and private variable 
inside __init__ constructor. I've changed also the variable value, but 
outside class.
I've got problem with changing some variable value _inside__my_ class, 
which i'm writing.
I've searched over google, some tuts with topics about operations on 
private variables, but didn't found anything - only how to make them, 
but no how to assign new objects/override them with new content (and 
other advanced and helpful options).

If someone could help me, it would be great.

Thanks.