problem with inheritance in python

Question

in school we got this class file:

class Konto:
    def __init__(self, nummer):
        self.__nr = nummer
        self.__stand = 0
        self.__minimum = -1000.0

    def getStand(self):
        return self.__stand

    def getNr(self):
        return self.__nr

    def einzahlen(self, betrag):
        self.__stand = self.__stand + betrag

    def auszahlen(self, betrag):
        if self.__stand - betrag >= self.__minimum:
            self.__stand = self.__stand - betrag
        else:
            print("Auszahlung nicht möglich!")

class Sparkonto(Konto):
    def __init__(self, nummer):
        Konto.__init__(self, nummer)
        self.__zinssatz = None
        self.__minimum = 0
        self.__maxAuszahlung = 2000.0

    def setZinssatz(self, zinssatz):
        self.__zinssatz = zinssatz

    def getZinssatz(self):
        return self.__zinssatz

    def auszahlen(self, betrag):
        if betrag <= self.__maxAuszahlung:
            Konto.auszahlen(self, betrag)
        else:
            print("Auszahlung nicht möglich!")

    def zinsenGutschreiben(self):
        zinsen = self.__stand * (self.__zinssatz / 100)
        self.einzahlen(zinsen)

When I run this test programm:

#Test
from sparkonto import *
s = Sparkonto(1)
s.einzahlen(1000)
print(s.getStand())
s.setZinssatz(4)
print(s.getZinssatz())
s.zinsenGutschreiben()
print(s.getStand())
s.auszahlen(2500)
print(s.getStand())

I get this error

1000
4
Traceback (most recent call last):
  File "/home/malte/home/py3/sparkonto/test.py", line 8, in <module>
    s.zinsenGutschreiben()
  File "/home/malte/home/py3/sparkonto/sparkonto.py", line 44, in zinsenGutschreiben
AttributeError: 'Sparkonto' object has no attribute '_Sparkonto__einzahlen'
>>>

We do not know what we are doing wrong. Any guess?

I think you should try as much as possible to write your code using english — Daniel Băluţă
– Daniel Băluţă, Commented Jun 22, 2010 at 6:23
@daniel: we got this from our teacher and I think she does this because most of my class is to dumb — Malte Schledjewski
– Malte Schledjewski, Commented Jun 22, 2010 at 6:39
ok I see, but for your future development I strongly recommend you to use english :) — Daniel Băluţă
– Daniel Băluţă, Commented Jun 22, 2010 at 6:49

otherchirps · Accepted Answer · 2010-06-22 06:41:01Z

2

Daniel was halfway there, you do need to change self.__einzahlen -> self.einzaheln, as he said.

Also, self.__stand belongs to the parent class. With the double underscore in the name, it gets mangled used anywhere else. But you don't need to use self.__stand directly. Konto gives you getStand().

Try something like this:

def zinsenGutschreiben(self):
    zinsen = self.getStand() * (self.__zinssatz / 100)
    self.einzahlen(zinsen)

answered Jun 22, 2010 at 6:41

otherchirps

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3 Comments

Daniel Băluţă Over a year ago

just to be sure that I've understood. If you prefix your variable names with '__' than your variable cannot be accessed from derived classes? It's like private in C++?

John La Rooy Over a year ago

@Daniel, not quite, the name of the attribute is mangled to eg. _Konto__stand and is perfectly accessable under that name. It does ensure that a subclass doesn't accidently overwrite that name.

otherchirps Over a year ago

It can be accessed, but with a double underscore, the name is mangled when used from the derived classes. It's just a convention, giving you a hint that the author wants to treat it as "private". In this case, you could have also used "self._Konto__stand", but why bother when there's an "official" getStand() function?

Daniel Băluţă · Accepted Answer · 2010-06-22 06:18:37Z

1

self.__einzahlen(zinsen) -> self.einzahlen(zinsen)

answered Jun 22, 2010 at 6:18

Daniel Băluţă

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4 Comments

Daniel Băluţă Over a year ago

why not? __einzahlen function is not defined in base class.

Ignacio Vazquez-Abrams Over a year ago

@Malte: There is no such thing as "private" in Python.

Malte Schledjewski Over a year ago

we learned that leading double underscore means private but i often don't think that my teacher knows how python works

Ignacio Vazquez-Abrams Over a year ago

Leading double underscores does not mean private. There is no such thing as "private" in Python.

Ignacio Vazquez-Abrams · Accepted Answer · 2010-06-22 06:23:03Z

1

Double leading underscores invoke name mangling, using the current class's name. Use a single leading underscore instead.

answered Jun 22, 2010 at 6:23

Ignacio Vazquez-Abrams

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