classes in Python for a beginner

Question

Could you please help me to find the problem? Python code that works:

class ParamWindow:
    def __init__(self, b):
        self.a = b
        print self.a

params = ParamWindow(8)
print params.a

this prints 8 and 8. Ok. Then I do:

class ParamWindow:
    def __init__(self, parent, b):
        self = wx.Frame(parent = parent, id=-1, title="Parameters")
        self.a = b
        print self.a

params = ParamWindow(None, 8)
print params.a

and it says "ParamWindow instance has no attribute 'a'". Why has not it? I told him that self is Frame and then added a field "a" (no error at this point) but when i ask to print it (error at print line), it forgets that "a" exists... Where am I wrong? Thanks.

Ferdinand Beyer · Accepted Answer · 2011-11-03 11:50:24Z

6

def __init__(self, parent, b):
    self = wx.Frame(parent = parent, id=-1, title="Parameters")

Here you reassign self, so you end up having no reference to the ParamWindow instance any more! You should never do that! What are you trying to achieve?

    self.a = b

Here, you assign a to self, which is now the Frame, not the ParamWindow. ParamWindow.a never gets defined and you get the error later on.

Maybe you want to inherit from Frame? If so, your code should look like this:

class ParamWindow(wx.Frame):
    def __init__(self, parent, b):
        # Initialize the superclass (wx.Frame).
        super(ParamWindow, self).__init__(parent=parent, id=-1, title="Parameters")
        self.a = b
        print self.a

edited Nov 3, 2011 at 11:50

answered Nov 3, 2011 at 10:10

Ferdinand Beyer

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7 Comments

Bite code Over a year ago

+1. Never assign something to self unless you really know what you are doing.

Andrew Lazarev Over a year ago

yeah, but the error is returned at the moment of addressing to "a" but not at the assignment. Is it normal? Why does not it say smth bad on the assignment line? And in total: how should I write to obtain a Frame with an additional field "a" ? Thank you.

Daniel Roseman Over a year ago

@AndrewLazarev because self now has nothing to do with the original instance. You can assign anything you like to it, but it won't affect the instance.

Andrew Lazarev Over a year ago

@Ferdinand Beyer, oook, I C! Thanks a lot. Could you please just tell me: if "super" in a key word, why does not my eclipse highlight it? :)

Katriel Over a year ago

It's not a keyword. It's just a built in function.

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