Why can I not get the value of tkinter variables? - python-3.x

Question

I am trying to add a configuration window in my coupled oscillator simulation. But when I want to get the value of tkinter variables to set up the system, I only get ''.
I tried to change the type from tk.DoubleVar() to tk.StringVar() but nothing seems to works...
Here the code for the configuration frame for one object. Then I create two of them and add them to the configuration frame.
import tkinter as tk
class MassConfig(tk.Frame):
def __init__(self, root, **kwargs):
tk.Frame.__init__(self, root, kwargs)
self.grid()
# Variables :
self.weight = tk.StringVar()
self.vitesse = tk.StringVar()
self.position = tk.StringVar()
self.lbl_weight = tk.Label(self, text='Masse :')
self.lbl_weight.grid(row=0, column=0)
self.entry_weight = tk.Entry(self, textvariable=self.weight, width=5)
self.entry_weight.grid(row=0, column=1)
self.lbl_vit = tk.Label(self, text='Vitesse :')
self.lbl_vit.grid(row=1, column=0, pady=5)
self.entry_vit = tk.Entry(self, textvariable=self.vitesse, width=5)
self.entry_vit.grid(row=1, column=1)
self.lbl_pos = tk.Label(self, text='Position :')
self.lbl_pos.grid(row=2, column=0)
self.entry_pos = tk.Entry(self, textvariable=self.position, width=5)
self.entry_pos.grid(row=2, column=1)
Here the code of the configuration frame, which has two 'MassConfig' frame inside.
import tkinter as tk
from gui.masse_config import MassConfig
class Configuration(tk.Frame):
def __init__(self, root, **kwargs):
tk.Frame.__init__(self, root, **kwargs)
self.root = root
self.grid()
self.update()
self.is_alive = True
self.data = {}
self.m1_frame = tk.LabelFrame(self, text='Masse 1', width=200, height=200)
self.m1_frame.grid(column=0, row=0, padx=(10, 5), pady=(10, 10))
self.m2_frame = tk.LabelFrame(self, text='Masse 2', width=200, height=200)
self.m2_frame.grid(column=1, row=0, padx=(5, 10), pady=(10, 10))
self.config_1 = MassConfig(self.m1_frame)
self.config_2 = MassConfig(self.m2_frame)
self.config_1.grid(padx=10, pady=10)
self.config_2.grid(padx=10, pady=10)
self.btn_validation = tk.Button(self, text='Valider', command=self.validation)
self.btn_validation.grid(row=3, column=1, padx=10, pady=(0, 10), sticky=tk.E)
self.update()
def validation(self):
print(":", self.config_1.position.get())
self.data['Mass 1'] = {}
self.data['Mass 1']['Position'] = self.config_1.position.get()
self.data['Mass 1']['Vitesse'] = self.config_1.vitesse.get()
self.data['Mass 1']['Masse'] = self.config_1.weight.get()
self.data['Mass 2'] = {}
self.data['Mass 2']['Position'] = self.config_2.position.get()
self.data['Mass 2']['Vitesse'] = self.config_2.vitesse.get()
self.data['Mass 2']['Masse'] = self.config_2.weight.get()
self.is_alive = False
self.root.destroy()
I want to have the value of the tkinter variables self.weight, self.vitesse and self.position from the MassConfig class in a dictionary (self.data from Configuration class). But nothing come out, except ''.