CODEWITHSUNDEEP
pythonpython-3.xinheritancewxpython
Matthias
Matthias

Reputation: 10409

Initializing an attribute in a child class that is used in the parent class

I am using a 3rd party Python library (wxPython), which has a buggy class in one of its modules.
The problematic code section looks like this:

def OnText(self, event):
    value = self.GetValue()
    if value != self.__oldvalue:
        pass  # Here some more code follows ...
    self.__oldvalue = value

The problem is the if statement, because at the first call to this method self.__oldvalue has not been initialized yet. So for a workaround until this bug has been fixed by the library devs I thought I could fix this with a little workaround. I simply wanted to derive a child class from that faulty class and initialize self.__oldvalue in this constructor:

class MyIntCtrl(wx.lib.intctrl.IntCtrl):
    def __init__(self, *args, **kw):
        self.__oldvalue = None
        super().__init__(*args, **kw)

However, now when I use this new class MyIntCtrl instead of the original IntCtrl class, I do get exactly the same error as before:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/wx/lib/intctrl.py", line 509, in OnText
    if value != self.__oldvalue:
AttributeError: 'MyIntCtrl' object has no attribute '_IntCtrl__oldvalue'

Now I am wondering: What am I doing wrong, how else can I fix this issue in a child class?

Upvotes: 1

Views: 498

Answers (1)

Raymond Reddington
Raymond Reddington

Reputation: 1837

Any member of class which starts with __ (double underscore) is private, you can use single underscore _ or not use underscores in naming for access them in derived classes.

class Parent:
    def __init__(self):
        self.__private_field = "private field"
        self._protected_field = "protected field"
        self.public_field = "public field"

class Child(Parent):
    def __init__(self):
        pass

    def do(self):
        print(self.__private_field) # It will throw exception
        print(self._protected_field) # It will not throw exception
        print(self.public_field) # It will not throw exception

Or you can bypass private/protected members by calling them like:

print(_Parent__private_field)

Upvotes: 1

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