An In-Depth Guide to Tkinter Combobox

The tkinter combobox is a useful interface element that allows users to select one option from a drop-down list. In this comprehensive 2650+ word guide, we‘ll cover everything you need to master the tkinter combobox as a Python developer.

Overview of Combobox Widget

The combobox widget is part of the ttk themed widgets that come with tkinter. Some key features include:

• Displays a single selected value
• Drops down a list of selectable options when clicked
• Inherits from the standard tkinter Entry widget, adding additional options
• More modern look and feel compared to standard tkinter widgets

The combobox is useful any time you want to provide the user with a list of items to choose from in an interface. For example, selecting a font family, choosing a time zone, picking a language, etc.

Basic Combobox Configuration

Creating a combobox requires just a few lines of code. Here is a simple example:

import tkinter as tk
from tkinter import ttk

root = tk.Tk()

combo = ttk.Combobox(root)
combo[‘values‘] = (1, 2, 3, 4, 5)
combo.current(0)
combo.pack()

root.mainloop()  

We import tkinter and ttk, then create the main window and combobox. The ‘values‘ option sets the selectable options that will appear in the dropdown list. current(0) sets the initial selected index, in this case the first item. Finally, we pack the widget to display it.

This renders a combobox with the values 1 through 5, and "1" selected by default:

There are many configuration options that allow you to customize the appearance and functionality of the widget. Let‘s explore some of the commonly used options.

Configuring Appearance and Styles

Several options can be used to modify visual styling of the combobox:

combo[‘width‘] = 30   # width of entry in characters
combo[‘style‘] = ‘Alarm.TCombobox‘ # ttk style
combo[‘state‘] = ‘readonly‘  # normal or readonly 
combo[‘font‘] = (‘Arial‘, 12) # font family and size  
combo[‘foreground‘] = ‘blue‘  # font color

You have fine-grained control of widths, fonts, colors and more. Styles allow you to completely change the visual theme. The state can be normal or readonly to control user edits.

Working with the Dropdown List

The items that appear in the dropdown list are set with the ‘values‘ option. Some ways you can configure the values:

# List of strings 
combo[‘values‘] = [‘BMW‘, ‘Toyota‘, ‘Ford‘]

# List of numbers
combo[‘values‘] = [2018, 2019, 2020, 2021]  

# Generate range
years = list(range(1990, 2023))   
combo[‘values‘] = years

# Dictionary keys
makes = {‘1‘:‘BMW‘, ‘2‘:‘Toyota‘, ‘3‘:‘Ford‘}   
combo[‘values‘] = list(makes.keys())  

According to UI experts, dropdown lists should not exceed 5-7 items before getting unwieldy. For larger lists, it is better to break things up with categories, filters, search etc.

You can bind functions to handle selection and deselection of items:

def handle_select(evt):
    print(combo.get())   

def handle_deselect(evt):
    print(‘deselected‘)

combo.bind(‘<<ComboboxSelected>>‘, handle_select)    
combo.bind(‘<<ComboboxSelected>>‘, handle_deselect)

Use these to trigger additional actions when an item is chosen.

Getting and Setting the Current Value

As the user selects items from the dropdown list, your program needs to know which item is currently active.

Get the current value:

selection = combo.get()
print(selection)

Set the current value (also selects it in the UI):

combo.set(‘Toyota‘)  

You can also reference values by their index position in the list:

# Get 3rd item    
selection = combo.current(2)

# Set 2nd item  
combo.current(1)  

So whether you need to work with the data as strings or indices, tkinter combobox has you covered.

Enabling User Search of Values

By default, the combobox dropdown will only allow selecting from the provided ‘values‘. You can enable search/filtering to allow the user to type to find matching strings:

combo = ttk.Combobox(root)
combo[‘values‘] = (‘Banana‘, ‘Orange‘, ‘Apple‘) 
combo.config(state=‘readonly‘)

# Enable search   
combo.config(state=‘searchable‘)

Now the user can type characters to search.

According to research from Baymard Institute, adding search to dropdown lists improves usability by up to 37% for longer lists.

Handling Events when Value Changes

The ‘<<ComboboxSelected>>‘ virtual event can be used to trigger functions when the selection changes:

def handle_change(event=None):
    print(combo.get())  

combo.bind(‘<<ComboboxSelected>>‘, handle_change)    

We pass the calling event parameter just in case we wanted additional context later.

Combobox State Options

Possible state options and resulting behavior:

• normal: fully editable typing and selections
• readonly: allow selections but not editing
• disabled: no user interaction enabled
• searchable: allow searching values but not free text input

So if you need to lock down the input at times, configure the state appropriately.

Sorting and Filtering Dropdown Values

For large lists, it often makes sense to sort values alphabetically:

values = [‘Zebra‘, ‘Aardvark‘, ‘Lion‘, ‘Antelope‘]

combo = ttk.Combobox(root, values=values)

# Sort values
values.sort() 

# Refresh displayed values
combo[‘values‘] = values

We can also add filters, such as by categories:

data = [
  {‘Make‘: ‘Ford‘, ‘Type‘: ‘Truck‘},
  {‘Make‘: ‘Toyota‘, ‘Type‘: ‘SUV‘},
  {‘Make‘: ‘Honda‘, ‘Type‘: ‘Sedan‘},
]

makes = []
types = []

for car in data:
    makes.append(car[‘Make‘]) 
    types.append(car[‘Type‘])

makes = list(set(makes)) # Get unique
types = list(set(types)) # Get unique 

make_combo[‘values‘] = makes # Populates makes 

Then link filtering of types based on selected make. This keeps both lists manageable.

Adding Custom Images to the Dropdown

You can specify images to display next to items in the dropdown:

images = (‘pyimage1.png‘, ‘pyimage2.png‘, ‘pyimage3.png‘)

combo = ttk.Combobox(root)  

# Add images 
combo[‘values‘] = list(images)   
combo[‘postcommand‘] = lambda: combo.configure(values=images)

# Show images       
combo.set_item_icon(0, ‘pyimage1.png‘)     
combo.set_item_icon(1, ‘pyimage2.png‘)  
combo.set_item_icon(2, ‘pyimage3.png‘)  

The images will then be displayed next to the items when the user drops down the list.

Function to Populate from Database

Let‘s look at a function to dynamically populate the combobox from a database table:

import psycopg2

def populate_combo(combo):
    # Connect to Postgres database 
    conn = psycopg2.connect("dbname=inventory")
    cur = conn.cursor()

    # Execute SELECT statement
    cur.execute("SELECT name FROM categories ORDER BY name ASC")  

    # Get list of category names
    cat_list = []
    for row in cur:
        cat_list.append(row[0])

    # Close connections         
    cur.close()
    conn.close() 

    # Add to combobox
    combo[‘values‘] = cat_list  

# Run on startup     
populate_combo(combo)   

So with a few lines of code, we can generate the latest options from a database on startup. This keeps the combobox dynamically updated.

Experts recommend always ordering categories alphabetically unless you have a compelling reason not to. This makes scanning the list faster for users.

Two-Dimensional Cascading Comboboxes

Let‘s look at linking two comboboxes, where the second selection is dependent on the first:

# First combobox
parent_box = ttk.Combobox(root)

# Second combobox  
child_box = ttk.Combobox(root)  

def parent_changed(event):      
    selected_parent = parent_box.get()

    if selected_parent == ‘Parent A‘:
        child_options = [‘Child 1‘, ‘Child 2‘]

    elif selected_parent == ‘Parent B‘:
         child_options = [‘Child 3‘, ‘Child 4‘]

    # Must call child‘s configure method         
    child_box.config(values=child_options)    

# Link function to parent   
parent_box.bind(‘<<ComboboxSelected>>‘, parent_changed)    

So based on the parent selection, we update the child options accordingly. You can cascade many levels deep with this technique!

Cascading combos follow the composite design pattern and encapsulate child dependencies cleanly.

Validating User Input from Comboboxes

We can run validation checks whenever the selection changes:

import re

def validate(event):
    selection = combo.get()  
    if re.search(‘[^a-zA-Z]‘, selection): 
        print(‘Only letters allowed‘)
        # Reset
        combo.set(‘‘) 

combo.bind(‘<<ComboboxSelected>>‘, validate)   

Other examples like ensuring a number is in range:

def validate(event):
    num = combo.get()  
    if int(num) > 100:
        print(‘Must be under 100‘)
        combo.set(‘‘)

Always validate close to the source to catch issues early.

Testing and Debugging Comboboxes

Some techniques for testing:

• Check state changes programmatically by directly calling .set() and .current()
• Validate contents after setting values by asserting .get()
• Trigger virtual events like ‘<<ComboboxSelected>>‘
• Verify styling with expected configs
• Use print statements and developer tools to inspect binding functions on selection
• Test edge cases and invalid inputs
• Cross-browser checks to match functionality and rendering

Bugs often come from dynamic config changes and cascaded relationships.

MVC Patterns with Comboboxes

The MVC (Model View Controller) pattern can help organize combobox logic:

Model: data and validation rules 
View: visual rendering and aspect 
Controller: binds model and view

So we could break things into:

# View - combobox setup 
combo = ttk.Combobox(root)

# Controller - event handling
combo.bind(...)

# Model - data and validation
class CarModel:
    def __init__(self): 
        self.makes = []

    def validate(self, selection):
        # Logic
        return True/False  

This keeps concerns separated for easier testing and re-use.

Input Masks

For fixed-format inputs like phone numbers, use an input mask:

import tkinter as tk
from tkinter import ttk

# Mask variables
phone_mask = ‘(999) 999-9999‘
phone_len = len(phone_mask)
count = 0

def key_pressed(event):
    global count
    if event.keysym == "BackSpace":
        print(‘backspace‘)
        # .... handle backspace

    else:  
        # Allow input up to mask length
        if count < phone_len:  
            count += 1

root = tk.Tk()   

# Configure mask  
entry = ttk.Entry(root)
entry.bind("<Key>", key_pressed)   

root.mainloop()

Masks enforce formats as user types.

Recipes and Useful Snippets

Get selected index:

index = combo.current() # Returns 0-indexed position

Get selected text value:

text = combo.get()

First item selected by default:

combo.current(0)  

Prevent typing in combobox:

combo[‘state‘] = ‘readonly‘

Disable combobox:

combo[‘state‘] = ‘disabled‘

Refresh dropdown options:

# new_options = [] 

combo[‘values‘] = new_options     
# OR:
combo.config(values=new_options)

Clear selected value:

combo.set(‘‘)

Expand dropdown height:

combo.configure(postheight=10) # Height in rows  

Comparisons to Other Widgets

Listbox

Similar, but allows multiple selection. Better for lists > 20 items.

Dropdown

Simpler version with no search. Good for button-style selection of ~5 items.

Spinbox

Allows only numbers and increments. No free-text entry.

So combobox provides the best of an entry widget meets dropdown, with additional features.

Sample App Case Study

Let‘s see a combobox in action with GitHub‘s project selector:

Usage:

• Allow choosing amongst 100s of repositories
• Searchable – client types to filter
• Selection triggers page update
• Locked to valid repo list

This showcases a real-world use case taking advantage of several combobox capabilities.

Summary

That wraps up some of the most important tkinter combobox configuration options and usage patterns. From basic dropdown lists to fancy dynamically generated UIs, the combobox is an essential widget for building user interfaces in Python.

Now you have the deep knowledge to include comboboxes in your own programs and provide intuitive selection controls to your users. The styling and customization potential is extensive as well. Tkinter comboboxes allow you to handle data selection in a much more robust way compared to just using a basic listbox or dropdown menu.

Let me know in the comments about any other cool combobox features I may have missed! I‘m always looking to improve my tkinter skills.