Tkinter: puzzlement with creating a canvas widget

[meijeru]

I have successfully created a widget of type Canvas, but when I want to use its attributes I get a NoneType error

root = tk.Tk()
frame = tk.Frame(root)
frame.grid()
panel = tk.Canvas(frame, width=panel_width, height=panel_height, bg="silver").grid(row=0, column=0)
btn = tk.Button(frame, text="Quit", command=root.destroy).grid(row=1, column=0)

The canvas is created, but the name is not recognized in the next statement:

panel.create_line(0, 20, panel_width, 20, fill="red", width=2)

I get an error:
AttributeError: 'NoneType' object has no attribute 'create_line'

I must be doing something wrong... Thanks in advance for any help

[deanhystad]

.grid() returns None. When you call x = Widget(params).grid(row=0, column=0), x == None.

Python is not designed to daisy chain method calls. The accepted pattern is:

x = Widget(params)
x.grid()

Like you did with frame.

You should be debugging this kind of problem yourself. Better for learning and a lot faster.

The error says panelis None, so lets check the value of panel.

root = tk.Tk()
frame = tk.Frame(root)
frame.grid()
panel = tk.Canvas(frame, width=panel_width, height=panel_height, bg="silver").grid(row=0, column=0)
print(panel)
btn = tk.Button(frame, text="Quit", command=root.destroy).grid(row=1, column=0)

Output:
None

Not much code between creating panel and printing panel, so break up the code so it runs a line at a time.

root = tk.Tk()
frame = tk.Frame(root)
frame.grid()
panel = tk.Canvas(frame, width=panel_width, height=panel_height, bg="silver")
print(panel)
panel.grid(row=0, column=0)
print(panel)
btn = tk.Button(frame, text="Quit", command=root.destroy).grid(row=1, column=0)

Output:
.!frame.!canvas
.!frame.!canvas

Now the code runs.

In this code btn is also None:

btn = tk.Button(frame, text="Quit", command=root.destroy).grid(row=1, column=0)

It's likely you don't need a handle to btn, so you can write like this.

tk.Button(frame, text="Quit", command=root.destroy).grid(row=1, column=0)

[meijeru]

Thanks.