PCPP1: GUI Programming

GUI questions target Tkinter's structure: which geometry manager takes which options, how callbacks receive arguments, and what Entry.get() actually returns. Most stems show a small window built from a few lines and ask for the output or the exception, so trace the widget tree and the event loop rather than guessing.

Structure and layout

• One tk.Tk() root per process; additional windows are tk.Toplevel. Without root.mainloop() the window never processes events.
• pack(side=..., fill=..., expand=True) stacks widgets; grid(row=, column=, sticky='nsew', padx=, pady=) places them on a grid; place uses absolute coordinates.
• Never mix pack and grid inside the same container. Grid cells only grow if you set root.columnconfigure(0, weight=1).

Widgets and variables

• Label, Button, Entry (single-line, .get() always returns str), Text (multi-line, indices such as '1.0' and 'end'), Listbox, Canvas, Frame; tkinter.ttk supplies themed variants.
• Checkbutton and Radiobutton link to IntVar/StringVar; create one with var = tk.StringVar(value='x') and read it with .get().
• command=handler receives the function object and passes no arguments; bind('<Return>', handler) passes an event exposing .widget, .x, .y, .keysym.

Worked example

def convert(*_):
    try:
        out.set(f'{float(celsius.get()) * 9 / 5 + 32:.1f}')
    except ValueError:
        out.set('?')

celsius = tk.StringVar()
out = tk.StringVar()
tk.Entry(root, textvariable=celsius).grid(row=0, column=0)
tk.Button(root, text='Convert', command=convert).grid(row=0, column=1)

Reasoning: command=convert hands over the callable, whereas command=convert() would run it once at build time and pass None back. The *_ parameter lets the same function serve a <Return> binding, which supplies an event object. State lives in the variables, so the callback needs no widget reference.

Traps

• An uncaught ValueError from float('') is printed to stderr while the GUI stays unchanged, so guard every conversion.
• Creating a second Tk() inside a callback spins up a second interpreter and behaves unpredictably; use Toplevel.