python - tkinter button in class can't call function

Question

Total noob, seriously and angrily struggling with Python...

What I'm trying to do SHOULD be simple:

1. Make a button.
2. Connect that button go a function.
3. Click button --> run function.

The problem comes when we have to use CLASS (which, no matter how much I read, study - or even pay to take classes continues to make zero sense to me)...

I've tried every concieveable combination of putting this little convert() function IN the class, of adding self.convert or root.convert - and NONE of it works. And, I am clueless why - or what to try next.

Here's the code:

from tkinter import *
from tkinter.ttk import Frame, Button, Style


def convert():
    print("clicked")
    kg = entry_kg.get()
    print(kg)



class Example(Frame):

    def __init__(self):
        super().__init__()

    self.initUI()    # initiate the GUI
    # -------------------------------


    def initUI(self):

        self.master.title("Weight Converter")
        self.pack(fill=BOTH, expand=True)
        # -------------------------------------

        frame_kg = Frame(self)   # frame for Kilograms
        frame_kg.pack(fill=X)

        lbl_kg = Label(frame_kg, text="Kilograms", width=16)
        lbl_kg.pack(side=LEFT, padx=5, pady=5)

        entry_kg = Entry(frame_kg)
        entry_kg.pack(fill=X, padx=(5, 30), expand=True)
        # ------------------------------------------------

        frame_btn = Frame(self)    # frame for buttons
        frame_btn.pack(fill=BOTH, expand=True, padx=20, pady=5)

        btn_convert=Button(frame_btn, text="Convert", command=convert)
        btn_convert.pack(side=LEFT, padx=5, pady=5)

        # -------------------------------------------

def main():

    root = Tk()
    root.geometry("300x200+300+200")
    app = Example()
    root.mainloop()


if __name__ == '__main__':
    main()

What am I doing wrong?

How to do it right?

The seemingly arbitrary and useless over-complication of a simple task is seriously maddening...

Answer 1

If you want your functions to be designed outside of the class, but want to call it from a button defined in the class, the solution is to create a method in your class which takes the input values and then passes them to the external function.

This is called decoupling. Your function is decoupled from the implementation of the UI, and it means that you are free to completely change the implementation of the UI without changing the function, and vice versa. It also means that you can reuse the same function in many different programs without modification.

The overall structure of your code should look something like this:

# this is the external function, which could be in the same
# file or imported from some other module
def convert(kg):
    pounds = kg * 2.2046
    return pounds

class Example(...):
    def initUI(self):
        ...
        self.entry_kg = Entry(...)
        btn_convert=Button(..., command=self.do_convert)
        ...

    def do_convert(self):
        kg = float(self.entry_kg.get())
        result = convert(kg)
        print("%d kg = %d lb" % (kg, result))