On 27/07/16 04:44, Jim Byrnes wrote:
> OOP has always driven me crazy. I read the material and follow the
> examples until I feel I understand them, but when I try to implement it
> I end up with an error filled mess.
That suggests that its not the OOP concept thats confusing
you but the language syntax. How to turn the concept into code?
> So I decided to give it another try. When I got to the chapter on
> tkinter I decided to solve all the exercises using OOP even though the
> book solutions did not use OOP. The first one went fine:
Actually not as fine as you thought. In effect you got lucky by
making a mistake that still resulted in your code doing
approximately what you expected. But it didn't really do
what you thought it did.
> import tkinter
>
> class Goodbye:
> def __init__(self):
>
> self.frame = tkinter.Frame(window)
> self.frame.pack()
You are using a global variable as your parent here. It would be
better to pass that in as an argument. Or better still to make
the call to Tk() inside the __init__ method. That's not really
an OOP thing though just a general good practice issue.
It's best to avoid relying on global variables in your
functions.
> self.goodbye_button = tkinter.Button(self.frame, text='Goodbye',
> #command=quit)
> command=lambda: quit() )
> self.goodbye_button.pack()
Here you assign quit to the button's command. That's OK because
there is a top level built-in function called quit which exits
the interpreter. It's a bit of a brutal way to exit your GUI
but it works.
But I guess you really wanted to call your quit method. Remember
to access anything in your class you have to use the self
prefix, so you should have said:
command=self.quit
or
command=lambda: self.quit()
Lambda doesn't really help in this case but it doesn't do
any harm either.
> def quit():
> self.window.destroy()
When you define a method inside a class you need to
explicitly include the self parameter. So this should be:
def quit(self):
self.window.destroy()
But there's a snag, you don't store the window inside the
class. So self.window will cause an error. You either need
a line like
self.window = window
in your__init__ method
or use the global window variable like
def quit():
window.destroy()
My preference would be to create a self.window instance variable,
inside init()then access the self.window in quit(). You would also
call mainloop() using self.window in your init()
> if __name__=='__main__':
> window = tkinter.Tk()
> myapp = Goodbye()
> window.mainloop()
So if you took my advice this section of code would look like:
if __name__=='__main__':
Goodbye()
and init() would look like:
def __init__(self):
self.window = tkinter.Tk()
self.frame = tkinter.Frame(self.window)
self.frame.pack()
self.goodbye_button = tkinter.Button(self.frame, text='Goodbye',
command=self.quit)
self.goodbye_button.pack()
self.window.mainloop()
If you read through that and understand it, it should give
you the clues as to why the second one behaved as it did.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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