def onChange(self, test):
    print('Content is: ', test.widget.get())
    #this also works, but only when having "test" or any other other second parameter name in the def line:
    print('Content is: ', self.var.get())

def onChange(self):
    print('Content is: ', self.var.get())

import tkinter as tk
from tkinter import ttk

class GUI:
    def __init__(self):
        self.window = tk.Tk()
        self.create_widgets()

#   The following function works with "test" or any other second parameter name in the def line:
def onChange(self, test):
    print('Content is: ', test.widget.get())
    #this also works, but only when having "test" or any other other second parameter name in the def line:
    print('Content is: ', self.var.get())

##  The following function does not works with "test" or any other second parameter name in the def line:
#    def onChange(self):
#         print('Content is: ', self.var.get())


def create_widgets(self):
    # Makes an Entry widget:
    self.string_entry = ttk.Entry(self.window,width=30)
    self.string_entry.grid(row=0,column=0)

    self.var = tk.StringVar()
    self.var.set('Change me and press enter!')

    self.string_entry["textvariable"]=self.var
    self.string_entry.bind('<Key-Return>', self.onChange)

program = GUI()
program.window.mainloop()