using private method in your constructor python

Question

I am trying to write a class that allows users to register on a platform with a username, but checks that this username does not exist in the database.

I have this code:

class Freelancer:
    """Leaving this blank for now while I explore the functionality """
    number_of_sales = 0
    available_niches = ["Graphic design", "Art", "Data Analysis", "Music", "Business", "Writing and Translations"]
    usernames = []

    def __init__(self, username):
        self.username = _check_username(username)
    
    def _check_username(self, username):
        if self.username in Freelancer.usernames:
            print("This username already exist on our database, consider choosing another one")
        else:
            self.username = username
            Freelancer.usernames.append(self.username)
            print("You have successfully setup a username on our platform")

which I tested like so:

David = Freelancer("dave23")

But I got an exception: NameError: name '_check_username' is not defined. What is wrong with the code? How can I call a private method from __init__?

Need: `self.username = self.__check_username(username)` to i.e. needs 'self.' [call methods within class](https://stackoverflow.com/questions/5615648/python-call-function-within-class). Need `__` (i.e. double dash) to make method private [Private Methods](https://www.geeksforgeeks.org/private-methods-in-python/) — DarrylG, Apr 11 '20 at 12:04

FAHAD SIDDIQUI · Answer 1 · 2020-04-11T12:17:27.860

3

You missed self before calling private method.

def __init__(self, username):
    self.username = self._check_username(username)

If it is still gives error like: 'Freelancer' object has no attribute 'username' define username variable

edited Apr 11 '20 at 12:17

answered Apr 11 '20 at 12:09

FAHAD SIDDIQUI

631
4
22

1

Need double underscores to make method private, otherwise it's still accessible by public i.e. `bill = Freelancer('bill'); bill._check_username('bill')` works [see](https://www.geeksforgeeks.org/private-methods-in-python/) – DarrylG Apr 11 '20 at 12:21
1

@DarrylG the name mangling caused by a leading double underscore is not proper "privacy". Using a single leading underscore is the standard convention, as Python lacks real functionality for `private` attributes. – Karl Knechtel Oct 18 '22 at 18:27

score -1 · Answer 2 · answered Oct 15 '22 at 11:51

First of all, __init__ is not the constructor. Note that __init__(self) already has the object (self), so it was already constructed before calling __init__. (The real constructor in Python is __new__). Python does not have private methods, no matter how many underscores you add.

This should work:

def __init__(self, username):
    self.username = self.check_username(username) 

def check_username(self, username):
   ...

You could add the leading underscore to indicate that the method is for internal use only.

There is however another mistake in the code. It should be:

def _check_username(self, username):
    if self.username in self.usernames:  # <-----
        print("This username already exist on our database, consider choosing another one")

    else:            
        self.usernames.append(self.username)  # <----- 
        return username

You are manipulating the class. That means that if the class is instantiated again usernames is not empty.

"You are manipulating the class." This is **intentional**. The goal of OP's code is to remember the usernames of all instances that are created. — Karl Knechtel, Oct 18 '22 at 18:28
@KarlKnechtel right, it could be. But the code is a bit weird anyway. if `self.usernames()` was a call to a database it would make more sense. — Alex, Oct 19 '22 at 09:24

using private method in your constructor python

2 Answers2