Python3 - NameError: name 'select_range' is not defined

I try to create a package that contains the main.py module and two folders (Menus and Calculations).

The main_3.py module contains:

# -*- coding: utf-8 -*-
# main_3.py
# Importación de módulos

import Menus.menu_seleccion_valor 
import Menus.menu_seleccion_rango
import Calculos.importar_excel_3
import Calculos.selec_rango_3
import Calculos.calculo_beneficios_3
import Calculos.analisis_3
import Calculos.graficos_3     
file1 = (seleccionar_valor())    
df1 = importar_excel(file1)
ini, fin = elegir_periodo_analisis(df1)
selec_index, ini, fin = seleccionar_rango(df1, ini, fin)

The module "selec_range_3" contains:

def seleccionar_rango(df, ini, fin):
    mask = df['Fecha'] >= "2017-09-05" 
    cols = ["Fecha", "Valor"]
    df_aux = df.loc[mask, cols]
    selec_index  = df[ini:fin]
    print  ("selec_index = ", selec_index )
    return (selec_index, ini, fin )

When I run main_3 (), I get the following error

File "E:/Python/Mi_Cartera_funciones/main_3.py", line 24, in <module>
    selec_index, ini, fin = seleccionar_rango(df1, ini, fin)

NameError: name 'seleccionar_rango' is not defined

I can not see what my mistake is.