How to Wire Dependent Tkinter ComboBoxes: Map Machine Choices to Tasks and Update Values Correctly

Learn to build dependent ComboBoxes in Tkinter: use a clean mapping, bind events, and update ttk.Combobox values with StringVar for dynamic, reliable menus.

Building a small Tkinter tool to record machine maintenance is straightforward until you need dependent inputs: one ComboBox that drives the options of another. A common pitfall is updating only a Python variable and assuming the UI will reflect it. It won’t. Below is a focused walkthrough of the issue and a clean, reliable way to wire dependent ComboBox widgets in Tkinter.

Problem

The goal is to show a task list in a second ComboBox based on the chosen machine in the first ComboBox. The callback figures out which list to use, but the UI doesn’t refresh and the second dropdown keeps showing the initial placeholder.

import tkinter as tk
from tkinter import ttk
tasks_pool = ['Maybe it will work this time?']
def machine_task_select(event):
    """generate task combobox based on machine selection"""
    global tasks_pool
    jf_tasks = ['Coolant', 'Waylube', 'Spindle Oil', 'Laser Reflector', 'Air Filter', 'Check Unlcamp/Clamp', 'Check ATC']
    dia_tasks = ['Coolant', 'Spindle Cone', 'Clean Blume Laser', 'Check Spindle Oil', 'Check Grease', 'Check Fluid Press', 'Check Chiller', 'Clean Chip Basket Recovery Tray', 'Electrical Cabinet AC Air Filter', 'Column Air Filter']
    tak_tasks = ['Coolant', 'Spindle Oil', 'Waylube', 'Air Filter']
    act3_tasks = ['Coolant', 'Spindle Cone', 'Clean Blume Laser', 'Spindle Oil','Waylube', 'Coupling Lube', 'Air Filter']
    hart_tasks = ['Coolant', 'Oil', 'Air Filter']
    herm_tasks = ['Coolant', 'Grease', 'Air Filter']
    choice = str(box_machine.get())
    if choice == machines[0] or choice == machines[1] or choice == machines[2] or choice == machines[3]:
        tasks_pool = jf_tasks
    elif choice == machines[4] or str(box_machine.get()) == machines[5]:
        tasks_pool = dia_tasks
    elif choice == machines[6]:
        tasks_pool = tak_tasks
    elif choice == machines[7]:
        tasks_pool = act3_tasks
    elif choice == machines[8]:
        tasks_pool = hart_tasks
    elif choice == machines[9]:
        tasks_pool = herm_tasks
    else:
        tasks_pool = ['Am I Insane?']
    return tasks_pool
app = tk.Tk()
app.title("Maintenance Recorder")
app.resizable(width=False, height=False)
panel = tk.Frame(master=app)
panel.pack()
machines = ['JF1', 'JF2', 'JF1500', 'JF1600', 'Diamond 1', 'Diamond 2', 'Takumi B8', 'Active 3000', 'Hartford', 'Hermle C400U']
box_machine = ttk.Combobox(master=panel, width=50, values=machines, state='readonly')
box_machine.bind('<>', machine_task_select)
lbl_machine = tk.Label(master=panel, text='Machine:')
box_task = ttk.Combobox(master=panel, width=50, values=tasks_pool, state='readonly')
lbl_task = tk.Label(master=panel, text='Task Completed:')
lbl_machine.grid(row=0, column=0, sticky='e')
box_machine.grid(row=0, column=1, sticky='w')
lbl_task.grid(row=4, column=0, sticky='e')
box_task.grid(row=4, column=1, sticky='w')
app.mainloop()

What’s really going on

The callback computes which list of tasks should be shown, but the ComboBox never gets told to use that new list. Updating a Python variable doesn’t mutate widget state. Tkinter widgets are not bound to arbitrary globals; to change what a ttk.Combobox displays, you need to explicitly update its values via widget configuration and, if you use a textvariable, set it as well.

There’s also a maintainability issue: the long if/elif chain can be replaced by a direct mapping from machine to task list. That makes the UI update trivial and less error-prone.

Solution

Use a dictionary to map the first ComboBox’s selection to the second ComboBox’s options. Bind a callback to the first ComboBox that updates the second ComboBox’s values and selects a default item. This keeps behavior predictable and the code compact.

import tkinter as tk
from tkinter import ttk
def set_secondary_options(_evt) -> None:
    """Update the values in secondary_box when primary_box changes"""
    picked = sel_primary.get()
    options = mapping[picked]
    secondary_box.config(values=options)
    sel_secondary.set(options[0])
ui = tk.Tk()
# the keys populate the first combobox; each key's list populates the second
mapping = {
    'foo': ['fee', 'fi', 'fo', 'fum'],
    'bar': ['fizz', 'buzz'],
    'baz': ['zip', 'zap', 'zoop'],
}
first_keys = list(mapping.keys())
second_defaults = mapping[first_keys[0]]
# variables that hold current selections
sel_primary = tk.StringVar(ui, value=first_keys[0])
sel_secondary = tk.StringVar(ui, value=second_defaults[0])
# first combobox with a callback to update the second
primary_box = ttk.Combobox(ui, values=first_keys, textvariable=sel_primary)
primary_box.bind('<<ComboboxSelected>>', set_secondary_options)
# second combobox
secondary_box = ttk.Combobox(ui, values=second_defaults, textvariable=sel_secondary)
primary_box.pack(side='left', padx=5, pady=5)
secondary_box.pack(side='left', padx=5, pady=5)
ui.mainloop()

Why this matters

Dependent inputs are everywhere in internal tools and data-entry utilities. If the UI doesn’t update deterministically, users face stale choices and inconsistent records. Explicitly reconfiguring widget state when an event fires is the difference between a field that looks right and a field that is right.

Closing thoughts

When wiring dependent ComboBox widgets in Tkinter, rely on a mapping structure for clarity, bind to the '<<ComboboxSelected>>' event of the driving control, and update the dependent widget with .config(values=...). If you maintain a default selection, set a StringVar to a valid option immediately after updating values. For readability, follow PEP 8: prefer lower_case_names for functions and variables and reserve CamelCaseNames for classes like Frame and Label. With these small practices, dynamic dropdowns remain simple, predictable, and easy to extend.