question about using setter, getter and _

[Gribouillis]

what is differnce between username and _username or between self.username and slef._username?

_username is just another attribute name. It could be replaced by any other name such as hidden_data

class Person:
    def __init__(self,username):
        self.username=username
    #getter
    @property
    def username(self):
        return self.hidden_data

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


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

In this code, the name is stored in the Person object as the member self.hidden_data. When the code using this class calls p.username, that value is returned by the property getter function. When the client code writes p.username = 'Mohammad' the value is converted to lower case and then stored in the object by the property setter function.

Client code is not supposed to access directly p.hidden_data, but the Python language does not forbid that. Therefore it is a convention among Python programmers that a variable which name starts with an underscore is not meant to be accessed directly by client code. It is a "private" variable known from the Person class but not from client code. That's why a name such as _hidden_data or _username is preferred.

Note that the statement self.username = username in the __init__() function invokes the property setter to check that the argument is a string and to store the lowercase value of this argument in the variable self.hidden_data.