There's no trick -- the widget is centered in the area allocated to it by default. Simply place a label in a cell without any sticky attributes and it will be centered.

Now, the other question is, how to get the area it is allocated to be centered. That depends on many other factors, such as what other widgets are there, how they are arranged, etc.

Here's a simple example showing a single centered label. It does this by making sure the row and column it is in takes up all extra space. Notice that the label stays centered no matter how big you make the window.

import Tkinter as tk

class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        label = tk.Label(self, text="This should be centered")
        label.grid(row=1, column=1)
        self.grid_rowconfigure(1, weight=1)
        self.grid_columnconfigure(1, weight=1)

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).grid(sticky="nsew")
    root.grid_rowconfigure(0, weight=1)
    root.grid_columnconfigure(0, weight=1)
    root.mainloop()

You can get a similar effect by giving a weight to all rows and columns except the one with the label.

import Tkinter as tk

class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        label = tk.Label(self, text="This should be centered")
        label.grid(row=1, column=1)

        self.grid_rowconfigure(0, weight=1)
        self.grid_rowconfigure(2, weight=1)
        self.grid_columnconfigure(0, weight=1)
        self.grid_columnconfigure(2, weight=1)

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).grid(sticky="nsew")
    root.grid_rowconfigure(0, weight=1)
    root.grid_columnconfigure(0, weight=1)

    root.mainloop()

grid by default will center a widget in the space allocated to it. The problem is that you've not configured the column to grow to fill the window. Also by default, a row or column will be just large enough to fit the widgets in it.

To do that, you need to give column 0 a non-zero weight:

mw.grid_columnconfigure(0, weight=1)

You can use .place() for your label since your frame and your label have different parents. In place() you can use anchor="center" specify the startingpoint of your "anchor" with: x and y. Here is a working example:

app = Tk()
f = Frame(app,bg="yellow",width=50,height=50)
f.grid(row=0,column=0,sticky="NW")
f.grid_propagate(0)
f.update()
l = Label(f,text="123",bg="yellow")
l.place(x=25, y=25, anchor="center")
app.mainloop()

You put the label in column 1 and told it to span two columns. Therefore it is in column 1 and column 2.

I assume you want it to span all three columns, so the solution is to move the label to column zero and have it span three columns:

b.grid(row=0, column=0, columnspan=3)

The problem is that you are reading the documentation for the tkinter label but you are using a ttk label. This is why you should not use wildcard imports -- when two modules export objects with the same name (eg: tkinter.Label and ttk.Label) it becomes difficult to know which one is being used in your code. The default for a ttk label is to be aligned left, but the tkinter label is aligned center, and the order of your imports means that you're using a ttk Label.

The quick fix for your example is to explicitly set the anchor option for the ttk label (eg: label.configure(anchor="center")).

You should also fix your imports so that this problem doesn't happen to you again. Instead of doing a wildcard import (eg: from tkinter import *) you should explicitly import the module as a unit, optionally with a shorter name. Once you do that, you need to prefix your widgets with the name of the module.

For example, given these import statements:

import tkinter as tk
from tkinter import ttk

... you would then create a ttk label with ttk.Label(...), and a tkinter label with tk.Label(...) which makes your code much easier to understand, and it removes all ambiguity.

I found the solution using this method

grid_columnconfigure(index, weight)

Setting the weight to "1", the column will use the whole available width.

import tkinter as tk

class GUI(tk.Tk):
   
    def __init__(self):
        
        tk.Tk.__init__(self)
        
        self.frame1 = tk.LabelFrame(self, text="frame #1")
        self.frame1.grid(row=0, column=0)
        
        self.l1f1 = tk.Label(self.frame1, text="vvvvvvvvvveeeeeeeerrrryyyyyy "
                             "llllllllooooonnnngggg tttttteeeeeexxxxxxtttttt")
        self.l1f1.grid(row=0, column=0)
        
        self.frame2 = tk.LabelFrame(self, text="frame #2")
        self.frame2.grid(row=1, column=0, sticky="ew")
        
        self.frame2.grid_columnconfigure(0, weight=1)
        self.frame2.grid_columnconfigure(1, weight=1)
        
        self.l1f2 = tk.Label(self.frame2, text="  row 0 col 0  ")
        self.l1f2.grid(row=0, column=0)
        
        self.l2f2 = tk.Label(self.frame2, text="  row 0 col 1  ")
        self.l2f2.grid(row=0, column=1)

        self.update()        
        
mygui = GUI()
mygui.mainloop()