parameter error when inheriting from tkinter object

Question

I have a class, call it Test, which has a variable var. It uses this variable in its __init__ mathod, and maybe looks something like this:

class Test(object):
    var = 0
    def __init__(self):
        print(self.var)

In order to change this variable before I initialise the class, this seemed logical

test = Test
test.var = 42
test.__init__(test)

...and behaves like I expected it to. (printing 42)
However, when the code looks like this:

class Test(Canvas):
    var = 0
    def __init__(self, parent):
        Canvas.__init__(self, parent)
        self.pack()
        print(self.var)

test = Test
test.var = 42
test.__init__(test, frame) #tkinter frame I had made elsewhere

... it throws an error at the canvas.__init__ line, complaining that

TypeError: _options() missing 1 required positional argument: 'cnf'

I doubt this is a problem with the cnf parameters, as the class works fine when called 'normally', as in test = Test(). I have a feeling the parameters are being passed differently. Can someone shed some light?
Thanks in advance

What do you think test = Test does? It's not clear what you are trying to accomplish. Why are you calling __int__? That is highly unusual. — Bryan Oakley
– Bryan Oakley, Commented Apr 18, 2017 at 17:20
I wanted to change the the class' variable before I initialised the class, like in the first example. Is this bad practise? In which case, what is a better way of doing it? — EriktheRed
– EriktheRed, Commented Apr 18, 2017 at 17:24
The first variable ("self") must be an instance of a class, not the class itself. ie the result of Test() or test(). And when you create the instance, __init__() is called automatically, so you don't need that anyway. — Novel
– Novel, Commented Apr 18, 2017 at 17:24
@Jonathan " when you create the instance" do you mean the test = Test? Because nothing is printed there (else it would have printed 0, not 42), so I assumed __init__ wasn't being called... — EriktheRed
– EriktheRed, Commented Apr 18, 2017 at 17:33
Hmm, so do you think this is the wrong approach, or is there some way of getting around it? @Jonathan — EriktheRed
– EriktheRed, Commented Apr 18, 2017 at 17:45

Bryan Oakley · Accepted Answer · 2017-04-18 18:41:20Z

1

You seem to have a misunderstanding of how python classes work.

In your code you're doing test = Test, which does nothing but make test point to the class Test. It does not create an instance of Test.

If you want to change the value of a class value, there's no need to do an assignment to a temporary variable first. For example. to create a class that has var set to zero, and you later want to change it to 42, you can do it this way:

class Test(object):
    var = 0
...
Test.var = 42

Also, you should never call __init__ directly, except perhaps when calling the function of the superclass with the __init__ of a subclass. Instead, you create an instance of the class which will automatically call the __init__ for you:

test = Test()

This all works the same whether you inherit from a tkinter class or any other class.

When you run the following example, it will print out 42:

import tkinter as tk

class Test(tk.Canvas):
    var = 0
    def __init__(self, parent):
        tk.Canvas.__init__(self, parent)
        self.pack()
        print(self.var)

Test.var = 42

root = tk.Tk()
test = Test(root)

Note that because this is a class variable, changing it once will change it for all instances.

answered Apr 18, 2017 at 18:41

Bryan Oakley

389k53 gold badges582 silver badges739 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

EriktheRed Over a year ago

Thank you Bryan! Yes, this was what was confusing me; I did not realise that I could change the variable before I had created the instance. Helpful answer!

Bryan Oakley Over a year ago

@EriktheRed: the code in your question changes the variable before creating an instance. Your code does not create an instance anywhere.

Collectives™ on Stack Overflow

parameter error when inheriting from tkinter object

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related