Harness Tkinter Callbacks in Loops: Capture the Widget via Lambda to Avoid cget Errors

Learn how to fix Tkinter callbacks in loops by passing the widget instance, not an index. Use lambda default arguments to access button text via cget reliably.

Passing the right object to a tkinter callback is easy to get wrong when you create widgets in a loop. The typical symptom: you expect a widget in your handler, but you actually pass an integer or nothing meaningful to work with. Below is a minimal pattern that triggers the issue and a compact fix that keeps widget access explicit and reliable.

Problem setup

You might create a single button with a direct variable reference and pass that into the callback:

btn_one = ttk.Button(app, text="Button 1", command=lambda: on_press(btn_one))

When switching to a loop, it’s tempting to pass an index instead of the widget:

tk.Button(app, text=f"Button {idx}", command=lambda x=idx: on_press(x)).pack()

But the handler expects a widget-like object and reaches for its text:

def on_press(widget):
    label_txt = widget.cget('text')
    ttk.Label(app, text=label_txt).pack()

The result is obvious: the first variant passes a widget and works, the second passes a number and breaks when calling cget.

Why this breaks

The callback signature expects something with widget semantics, specifically supporting cget('text'). In the looped version you hand over an integer, so there’s no way to access the button’s attributes. The problem isn’t that the looped button “has no name”; the issue is that the callback doesn’t receive the widget reference.

Fix: capture the widget reference in the callback

The straightforward solution is to create the button, keep its reference, and bind it to the command via a default argument in the lambda. This mirrors the first working approach, but inside the loop.

def on_press(w):
    ttk.Label(app, text=w["text"]).pack()
for n in range(1, 5):
    b = tk.Button(app, text=f"Button {n}")
    b.pack()
    b.config(command=lambda w=b: on_press(w))

This way the handler always receives the actual widget instance and can safely read w["text"] (equivalent to w.cget("text")).

Do buttons created in a loop have names?

Yes, every tkinter widget has a default internal name. For buttons in the root window, those names look like “.!button”, “.!button2”, “.!button3”, and so on.

Every tkinter widget has a default internal name. For buttons in the root window, those names are like ".!button", ".!button2", ".!button3", etc.

This is separate from keeping a Python variable pointing at the widget. The fix above doesn’t rely on internal names; it explicitly passes the widget instance to the callback.

Why this matters

Handlers that receive the right object are simpler to reason about. You avoid surprising type mismatches, keep access to widget state straightforward, and make the code consistent whether you create a single button or dozens in a loop.

Takeaways

When wiring tkinter commands in loops, bind the widget instance into the lambda so the callback gets the object it needs. Internal names exist, but the reliable everyday approach is to pass the reference directly and work with widget attributes like ["text"] or cget("text"). Keeping this discipline prevents subtle bugs and keeps your UI code predictable.