Adding variables to a class

Question

Given a class Mitarbeiter with the constructor __init__ which is creating 3 variables. Can additional variables be added by another method in python? Example below:

class Mitarbeiter:
    species = "Homo sapiens"
    
    #Klassenmethoden ändern nur Klassenattribute
    #statische Methoden verwenden werder Instanz- noch Klassenattribute
    #konstruktor

    
    def __init__ (self, vorname, name, personalnummer):
        self.__vorname = vorname
        self.__name = name
        self.__personalnummer = personalnummer

    def _def_abteilung(self, abteilung):
        self.__abteilung = abteilung
    
    def _set_vorgesetzter (self, vorgesetzter):
        self.__vorgesetzer = vorgesetzter

    @property
    def get_abteilung(self,):
        return self.__abteilung

when I create an Object of Mitarbeiter with the variables vorname, name and personalnummer I can call later the method _def_abteilung to set some more information to the Object Mitarbeiter.

ma1 = Mitarbeiter("Franz", "Bauer", 736) #--> Works
ma1._def_abteilung("Test") #--> Works
ma1.abteilung #--> does not work, any Ideas?

By Python convention, methods and variables beginning with an underscore aren't meant to be accessed from outside the class (i.e. using an instance). Also you never defined a variable or method named `abteilung` (without any underscores). — iBug, Dec 12 '22 at 18:00
The "`_name` is private" convention is a _convention_, but "`__name`" mangles the name so `ma1.__abteilung` literally doesn't exist. https://medium.com/analytics-vidhya/python-name-mangling-and-how-to-use-underscores-e67b529f744f — Pranav Hosangadi, Dec 12 '22 at 18:03
don't ever write getters and setters like that, use @property. and fix the naming of your methods, no need to add `_def_`, `_set_` (or `_get` for properties) — Alex, Dec 31 '22 at 13:07

Pranav Hosangadi · Accepted Answer · 2022-12-12T18:27:07.133

The "_name is private" convention is a convention (privacy isn't enforced in any way),
"__name" mangles the name so ma1.__abteilung literally doesn't exist outside the class.
See What is the meaning of single and double underscore before an object name? and Python Name Mangling and How to Use Underscores

You can set attributes for an object anywhere -- if you changed self.__abteilung to self.abteilung, that name wouldn't be mangled by the interpreter and you would see the abteilung attribute defined on all Mitarbeiter objects.

Consider the following TestClass example:

class TestClass:
    def __init__(self, a):
        self.a = a

    def set_b(self, b):
        self.b = b

    def set_mangled(self, m):
        self.__m = m
        print("Mangled", self.__m)

    def get_mangled(self):
        return self.__m

The a attribute is set in the __init__, so t.a always exists

t = TestClass(0)
print(t.a) # 0
print(t.b) # AttributeError

The b attribute is only set in the set_b method, so it only exists on an object after that method is called. The method can set an attribute of the object using the reference self. Trying to print t.b before setting it throws an AttributeError

t.set_b(10)
print(t.a) # 0
print(t.b) # 10
print(t.c) # AttributeError

The c attribute is set only once you run the line t.c = 100. As you can see, it is possible to set the attribute from outside the class. Trying to access t.c before setting it throws an AttributeError.

t.c = 100
print(t.a) # 0
print(t.b) # 10
print(t.c) # 100

The __m attribute is defined as a mangled name. Inside the class, it is accessed using self.__m. Outside the class, the name is mangled to _TestClass__m, so it is not accessible as t.__m or t.m.

t.set_mangled(1000) # Mangled: 1000
print(t.get_mangled()) # 1000
print(t._TestClass__m) # 1000
print(t.__m) # AttributeError: 'TestClass' object has no attribute '__m'
print(t.m) # AttributeError: 'TestClass' object has no attribute 'm'

Adding variables to a class

1 Answers1