question about using setter, getter and _

[akbarza]

hi
in below code:

# from: https://virgool.io/@GreatBahram/once-for-all-property-sovwcuakw7aq
# code about getter, setter
class Person:
    def __init__(self,username):
        self.username=username
    #getter
    @property
    def username(self):
        return self._username

    #setter
    @username.setter
    def username(self,name):
        if not isinstance(name,str):
            raise TypeError('name must be a string')
        self._username= name.lower()

        
p=Person('Ali')
p.username
# output will be : 'ali'
p.username='mohammad'
p.username
#output will be: 'mohammad'

what is differnce between username and _username or between self.username and slef._username?
plz, explain.
thanks