tkinter: GUI programs in Nil Python

A Nil Python program gets a desktop window with import tkinter, as in CPython. PXX's tkinter module is lib/pcl/tkinter.pas. It binds the system's Tcl/Tk 8.6 directly: each widget call becomes one Tcl command, which is also how CPython's tkinter works underneath. Widgets are objects that take keyword options, command= and bind take Python functions, and the widget variables (StringVar, BooleanVar) work as in CPython.

It is a subset of tkinter, not all of it. Everything below was measured on Linux x86-64. A name the module does not have is a compile error that names it, never a call that silently does nothing (see Limits).

For a Pascal program, use the PCL instead.

What you need

• At run time, the Tk 8.6 library: sudo apt install libtk8.6. No development headers are needed.
• To run a GUI program without a desktop (on a server, or in a test), Xvfb: sudo apt install xvfb, then start the program with xvfb-run -a.

An example

A window with an entry, a check box, a button and a label. The button's command is a Python function. Instead of waiting for a person, after() types a name, presses the button, ticks the box, prints what the widgets now hold, and closes the window, so the program also runs headless.

import tkinter as tk

root = tk.Tk()
root.title("tkinter hello")
name = tk.StringVar(value="")
remember = tk.BooleanVar(value=False)

entry = tk.Entry(root, textvariable=name, width=20)
entry.pack(padx=8, pady=4)
check = tk.Checkbutton(root, text="Remember me", variable=remember)
check.pack()
greeting = tk.Label(root, text="(nobody yet)")
greeting.pack(pady=4)


def greet():
    greeting.configure(text="Hello, " + name.get())


button = tk.Button(root, text="Greet", command=greet)
button.pack(pady=4)


def play():
    name.set("PXX")
    button.invoke()
    remember.set(True)
    print("entry:", entry.get())
    print("label:", greeting.cget("text"))
    print("checked:", remember.get())
    print("children:", len(root.winfo_children()))
    root.destroy()


root.after(300, play)
root.mainloop()
print("closed")

./pxx tk_hello.npy tk_hello
xvfb-run -a ./tk_hello

entry: PXX
label: Hello, PXX
checked: True
children: 4
closed

Built with pin v451 (compiler sha256 d9b7226769cc) through ./pxx and run under xvfb-run on 2026-09-29. CPython 3.14.4 prints the same five lines for the same file. Drop the after() line to keep the window open.

More programs are in examples/tk/, listed on the Examples page.

What works

The module has these names. Each widget takes the options listed in lib/pcl/tkinter.pas as keyword arguments, in any subset.

tkinter.font.Font(family=, size=, weight=, slant=) or Font(font=spec) has measure, metrics and actual, and a Font can be passed as any widget's font=. It is a shim, and from tkhtmlview import HTMLLabel, HTMLScrolledText renders HTML into a Tk text widget (see Nil Python: Shims).

A command or binding can be a plain function, a bound method or a lambda. A bound function receives an event object with tkinter's field names, such as event.width for <Configure> and event.widget, the widget the binding was made on (None for bind_all), and a yscrollcommand function receives Tk's two fractions as strings, as in CPython. examples/tk/callbacks.npy shows each of these, and it printed its .expected output with v451.

A class can extend a widget and pass options on with super().__init__(parent, height=h, **kwargs). When Tk cannot start, for example with no display, tk.Tk() raises tk.TclError with Tk's reason, so the usual except tk.TclError: fallback to a text interface works. examples/tk/style_and_short_options.npy and examples/tk/no_display_raises_tclerror.npy match CPython's output; both were checked on 2026-10-20 with the compiler built from master (not yet in a pin). So was examples/tk/canvas_items_dialog_and_events.npy: a live graph moved with coords(), a scrolling side bar of clickable tiles, Style.map, font measuring and a modal dialog closed through wait_window. And so was examples/tk/sidebar_cards_and_hover_rows.npy: card frames with a highlight border, wrapped labels, uniform grid columns, rows taken out with pack_forget, a window icon, and a group of widgets bound with lambdas in a loop to act as one hover button.

coords(item, x0, y0, ...) takes at most four numbers. For a longer polyline pass the list, coords(item, pts), which CPython takes too.

Limits

Measured with pin v451 on 2026-09-29, each with a one-line program:

• Not in the module: Listbox, Radiobutton, Scale, Spinbox, IntVar and DoubleVar, among others. tk.Listbox(root) stops the build with "no member Listbox came of the qualifier tk".
• from tkinter import * does not compile ("expected expression"). This is true of every module in Nil Python, not only tkinter. Import the names you use, or use import tkinter as tk.
• tk.X and tk.Y are not there ("no member X came of the qualifier tk"). Write the option as a string: pack(fill="x").
• Write from tkinter import messagebox, not import tkinter.messagebox. The dotted form is refused ("no unit named tkinter_messagebox").
• Only Linux x86-64 has been run. Tk is a system library, so a program needs libtk8.6 wherever it runs. The binary names libtcl8.6 and libtk8.6 as required libraries, so without them it does not start at all: an except ImportError around import tkinter cannot catch their absence. aarch64 cross-compiles the same way but has not been run.
• Memory: with v451, every event delivered to a bound function leaked its event object, and each winfo_children() call leaked each child it returned. A program that handled 300 <Configure> events (the census build of examples/tk/event_and_children_are_released.npy) ended with 2,431 allocations still live on v451, against 25 after the fix. Fixed after v451 (6658668a8b, in no pin yet).