Hey there! Let's tackle your two Tkinter questions step by step.

How to use winfo_width() #

winfo_width() is a method available on all Tkinter widgets. It returns the actual current displayed width of the widget in pixels. You just call it directly on the widget instance, like this:

import tkinter
root = tkinter.Tk()
label = tkinter.Label(root, text="Hello World")
label.pack()
# Wait for the widget to render first, then get width
root.update_idletasks()
print(label.winfo_width())  # Prints the actual pixel width of the label
root.mainloop()

Note: You need to make sure the widget has been rendered (or at least its size calculated) before calling this method—otherwise you'll get an incorrect initial value (usually 0 or a tiny default number).

Difference between width and reqwidth #

Let's break down these two related concepts:

• width: This is a configuration option you set explicitly when creating a widget (for example, Label(root, text="Hi", width=15)). For text-based widgets like Label or Button, this value is measured in character units (based on the widget's font). For container widgets like Frame, it's measured in pixels. It's your manual override for the widget's width—Tkinter will try to use this width unless content forces it to expand.
• reqwidth (requested width): This is the minimum width the widget needs to display its content properly, calculated automatically by the widget itself based on its content (text, images, child widgets, etc.). You can get this value with winfo_reqwidth(). Unlike winfo_width(), this value is available as soon as the widget is created (even before it's rendered on screen), since it's based on internal content calculations rather than actual display size.

The root cause of the issue #

Your original code calls Win.winfo_width() and Welcome.winfo_width() before the window and widget have finished calculating their actual sizes:

import tkinter
Win=tkinter.Tk()
Welcome=tkinter.Label(Win,text="Welcome.")
# At this point, the window hasn't been rendered—Win.winfo_width() returns a tiny default value (often 1)
# Welcome also hasn't been rendered, so Welcome.winfo_width() returns 0 or a minimal default
Welcome.place(x=Win.winfo_width()//2-Welcome.winfo_width()//2,y=16)
Win.update()  # This refreshes display, but the position was already set incorrectly

When you create a Tkinter window and widget, they don't immediately calculate their final sizes. Those calculations are part of "idle tasks" that Tkinter queues up to run later (before the main loop starts or when the UI is idle). So when you call winfo_width() early, you're getting uninitialized values, which leads to the label being placed at x=0 (or near it) instead of the center.

Why reqwidth and update_idletasks() fix it #

• Using winfo_reqwidth(): As mentioned earlier, reqwidth is the widget's internally calculated minimum width for its content. This value is available right after the widget is created, no need to wait for rendering. So Welcome.winfo_reqwidth() gives you the correct width needed to display "Welcome." without relying on the widget being drawn.
• Calling update_idletasks(): This method forces Tkinter to process all pending idle tasks immediately—including calculating the window's actual size based on its content and system defaults. If you call Win.update_idletasks() before Win.winfo_width(), you'll get the real current width of the window, making your centering calculation accurate.

Here's a corrected version of your code that combines both fixes:

import tkinter
Win=tkinter.Tk()
Welcome=tkinter.Label(Win,text="Welcome.")

# Process idle tasks to calculate window size
Win.update_idletasks()

# Calculate center position using real window width and widget's requested width
Welcome.place(x=Win.winfo_width()//2 - Welcome.winfo_reqwidth()//2, y=16)

Win.mainloop()

内容的提问来源于stack exchange，提问作者Programmer S