How to pass an instance attribute to a button command in tkinter

Question

I'm making a simple gui that has three buttons:

select a file path to file
select a file path to a separate file
run a function, passing the file paths selected from buttons 1 and 2.

I have this so far:

import tkinter as tk
from tkinter.filedialog import askopenfilename
import pathlib

root = tk.Tk()

class App:

    def __init__(self, main):
        myFrame = tk.Frame(main)
        myFrame.pack()
        self.btn_1= tk.Button(main, text="Choose file", command=self.choose_data).pack()
        self.btn_2= tk.Button(main, text="Choose file", command=self.choose_data).pack()
        self.run_button = tk.Button(main, text="Run", command=lambda: self.run_analysis(self.btn_1, self.btn_2)).pack()

    def choose_data(self):
        global data_file
        data_file = pathlib.Path(askopenfilename(filetypes=[("Excel files", ".xls .xlsx")]))
        return data_file

    def run_analysis(self,a,b):
        do_something(a,b)

a = App(root)

root.mainloop()

The first buttons work as expected. And I can see the filepath is stored in a variable that I can print from the choose_data function

When I try to use these attributes from the run function I get this error

ValueError: Invalid file path or buffer object type: <class 'NoneType'>

When I print the attributes from a function within the init method, they are None. So from that and the error, I know thats where I'm going wrong.

I think its because the init method is beng called before the attributes are defined? And so when I call the run_analysis method it does not work?

I tried making data_file a global variable so that it would allow me to use it anywhere, but thats not working for me.

Any help would be appreciated

Thanks

Thanks for response. I accidentally omitted the return statement in the choose data function. The same error persists. I have updated the question. — Aless
– Aless, Commented Dec 2, 2020 at 21:17
You haven't shown how you're actually trying to access that variable, or even the full error message. Without a minimal reproducible example it can be difficult to figure out what you're doing and what the solution would be. But it's safe to say that adding the return keyword isn't a magic fix; you need to actually use functions correctly. Also, your self.btn_1 and self.btn_2 are saved as just None as well. — TigerhawkT3
– TigerhawkT3, Commented Dec 2, 2020 at 21:28

Flavio Moraes · Accepted Answer · 2020-12-02 21:28:35Z

Ok, the reason the attributes of your function are printing None is because they are None. When you define self.btn_1= tk.Button(main, text="Choose file", command=self.choose_data).pack() the variable self.btn_1 receive the return of the method pack. Since pack does not return anything, it is None. Then you pass self.btn_1 and self.btn_2 (both are None) to the method run_analysis.

First of all, if you want btn1 to be the button instead of Noneyou have to do it with two commands:

self.btn_1 = tk.Button(main, text="Choose file", command=self.choose_data)
self.btn_1.pack()

but be aware: it is the button and not the the result of the callback function it calls.

Second: if btn1 and btn2 are attributes of your object (they are saved in self) and the run_analysis is a non-static method (it has access to self) there are no necessity of passing the buttons as attributes of your method.

Third, I believe what you want to do is to read two data files and pass the data files to the do_something. In this case, the best thing to do is telling the choose_data which button is being pressed and saving the files in different variables according to it. You can do it like this:

class App:

    def __init__(self, main):
        myFrame = tk.Frame(main)
        myFrame.pack()
        self data_files = [None,None]
        self.btn_1= tk.Button(main, text="Choose file", command=lambda: self.choose_data(0)).pack()
        self.btn_2= tk.Button(main, text="Choose file", command=lambda: self.choose_data(1)).pack()
        self.run_button = tk.Button(main, text="Run", command=self.run_analysis).pack()

    def choose_data(self, index):
        self.data_files[index] = pathlib.Path(askopenfilename(filetypes=[("Excel files", ".xls .xlsx")]))

    def run_analysis(self):
        do_something(*self.data_files)

Thank you very much, you have solved my problem! I will be researching into what you've written in detail. Thanks for sharing

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