Desktop Applications 🖥️

Event-driven programming

A command-line program runs top to bottom and stops. A GUI program sets up a window, registers what should happen when things are clicked, and then hands control to a loop that waits for the user. Your code becomes a set of responses rather than a sequence.

import tkinter as tk

root = tk.Tk()
root.title("Ahoy")

count = 0


def on_click():
    """This runs when the button is pressed. Not before."""
    global count
    count += 1
    label.config(text=f"Insults learned: {count}")


label = tk.Label(root, text="Insults learned: 0", font=("Helvetica", 16))
label.pack(padx=40, pady=20)

tk.Button(root, text="Learn an insult", command=on_click).pack(pady=(0, 20))

root.mainloop()          # hands control to tkinter. Nothing after this runs until the window closes

mainloop() is the equivalent of the game loop from Lesson 48: it waits for events and dispatches them to your functions. Everything you write is a callback.

The widgets you will actually use

import tkinter as tk
from tkinter import ttk          # themed widgets: they look native

root = tk.Tk()
root.title("Crew manager")
root.geometry("420x360")

name = tk.StringVar(value="Guybrush")
role = tk.StringVar(value="captain")
active = tk.BooleanVar(value=True)

frame = ttk.Frame(root, padding=16)
frame.pack(fill="both", expand=True)

ttk.Label(frame, text="Name").grid(row=0, column=0, sticky="w", pady=4)
ttk.Entry(frame, textvariable=name, width=24).grid(row=0, column=1, pady=4)

ttk.Label(frame, text="Role").grid(row=1, column=0, sticky="w", pady=4)
ttk.Combobox(frame, textvariable=role, values=["captain", "lookout", "cook"],
             state="readonly").grid(row=1, column=1, pady=4)

ttk.Checkbutton(frame, text="Currently aboard", variable=active).grid(
    row=2, column=0, columnspan=2, sticky="w", pady=8)

listbox = tk.Listbox(frame, height=6)
listbox.grid(row=3, column=0, columnspan=2, sticky="nsew", pady=8)
frame.rowconfigure(3, weight=1)
frame.columnconfigure(1, weight=1)


def add_member():
    status = "aboard" if active.get() else "ashore"
    listbox.insert("end", f"{name.get()} ({role.get()}, {status})")
    name.set("")


ttk.Button(frame, text="Add to crew", command=add_member).grid(
    row=4, column=0, columnspan=2, pady=4)

root.mainloop()

Layout: pick one and stick to it

Dialogs and files

import tkinter as tk
from tkinter import filedialog, messagebox
from pathlib import Path

root = tk.Tk()
root.withdraw()          # hide the main window; we only want dialogs


def open_and_count():
    path = filedialog.askopenfilename(
        title="Choose a text file",
        filetypes=[("Text files", "*.txt"), ("All files", "*.*")],
    )
    if not path:
        return          # the user cancelled, which is not an error

    try:
        words = len(Path(path).read_text(encoding="utf-8").split())
    except UnicodeDecodeError:
        messagebox.showerror("Cannot read", "That does not look like a text file.")
        return

    if messagebox.askyesno("Result", f"{words} words. Save a report?"):
        target = filedialog.asksaveasfilename(defaultextension=".txt")
        if target:
            Path(target).write_text(f"{words} words\n", encoding="utf-8")

Note that cancelling returns an empty string, not an exception. Handling the cancel case is the most commonly forgotten branch in GUI code, and it produces the classic "the app crashed when I pressed cancel".

The rule that keeps a GUI usable

import threading
import tkinter as tk
from tkinter import ttk

root = tk.Tk()
status = ttk.Label(root, text="Ready")
status.pack(padx=20, pady=20)


def slow_work():
    """Runs on a background thread, so the window keeps repainting."""
    import time
    time.sleep(3)
    # Never touch widgets from another thread. Schedule it on the main one:
    root.after(0, lambda: status.config(text="Done"))


def start():
    status.config(text="Working...")
    threading.Thread(target=slow_work, daemon=True).start()


ttk.Button(root, text="Start", command=start).pack(pady=(0, 20))
root.mainloop()

Other options

import tkinter as tk
from tkinter import ttk

CONVERSIONS = {
    "Celsius to Fahrenheit": lambda c: c * 9 / 5 + 32,
    "Fahrenheit to Celsius": lambda f: (f - 32) * 5 / 9,
    "Kilometres to Miles": lambda km: km * 0.621371,
    "Miles to Kilometres": lambda mi: mi / 0.621371,
}

root = tk.Tk()
root.title("Converter")

value = tk.StringVar()
choice = tk.StringVar(value=list(CONVERSIONS)[0])
result = tk.StringVar(value="-")

frame = ttk.Frame(root, padding=16)
frame.grid(sticky="nsew")

ttk.Entry(frame, textvariable=value, width=16).grid(row=0, column=0, padx=4)
ttk.Combobox(frame, textvariable=choice, values=list(CONVERSIONS),
             state="readonly", width=24).grid(row=0, column=1, padx=4)
ttk.Label(frame, textvariable=result, font=("Helvetica", 16)).grid(
    row=1, column=0, columnspan=2, pady=12)


def convert(*_):
    try:
        number = float(value.get())
    except ValueError:
        result.set("Enter a number" if value.get() else "-")
        return
    result.set(f"{CONVERSIONS[choice.get()](number):.2f}")


value.trace_add("write", convert)      # convert as they type
choice.trace_add("write", convert)

root.mainloop()