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The author describes the second line of code in Listing 7 as follows:
"The second line tells Tk to put the focus on our entry widget. That way the cursor will start in that field, so the user doesn't have to click in it before starting to type."
And finally , the author describes the third line of code in Listing 7 as follows:
"The third line tells Tk that if the user presses the Return key (Enter on Windows) anywhere within the root window, that it should call our calculate routine, the same as if the user pressed the Calculate button."
The calculate function, which is shown in Listing 8 , is executed whenever the user presses the calculate button or hits the Enter key on the keyboard as described above .
| Listing 8 . The calculate function. |
|---|
def calculate(*args):
try:value = float(feet.get())
meters.set((0.3048 * value * 10000.0 + 0.5)/10000.0)except ValueError:
pass |
Any value that is entered into the Entry widget is stored in the StringVar object referred to by feet as shown in Listing 3 and Listing 4 and as described above .
The StringVar object has a method named get that returns the string stored in the object. The code in the try block of Listing 8 calls the get method on the StringVar object and attempts to convert the string to type float .
If the string cannot be converted to a float , a ValueError is thrown and control is passed to the pass statement. This causes the function to exit without doing anything.
If the string can be converted to a float , that float value is stored in the variable named value .
A StringVar object also has a method named set that can be used to store a value in the object. The code in the try block performs the arithmetic to convert the value in feet to a value in meters and then calls the set method on meters to store that value in the StringVar object referred to by meters in Listing 3 .
Storing a value in that StringVar object causes the value to appear in the Label immediately to the left of the word meters in Figure 2 due to the textvariable argument in Listing 5 . Once again, if you connect a widget to a StringVar object using a textvariable argument, a change in the contents of either will cause a corresponding change in the other.
Finally, the last statement in the program, which is shown in Listing 9 , causes Tk to enter its event-handling loop and causes everything to run. The code in this loop will monitor for an event indicating that the user has pressed the calculate button. When that happens, the calculate function will be executed causing the conversion from feet to meters to take place and causing the resultsof the calculation to be displayed as shown in Figure 2 . This is an infinite loop that will continue to run and monitor for events untilthe program is terminated.
I encourage you to copy the code from Listing 9 . Execute the code and confirm that you get results similar to those shown in Figure 2 or Figure 4 . Experiment with the code, making changes, and observing the results of your changes. Make certain that youcan explain why your changes behave as they do.
A complete listing of the program is shown in Listing 9 .
| Listing 9 . Complete program listing. |
|---|
# Illustrates a Python GUI using tkinter.
# Published by Mark Roseman at TkDocs under a Creative Commons
# Attribution-NonCommercial-ShareAlike 2.5 Canada (CC BY-NC-SA 2.5 CA) license.#------------------------------------------------------------------------------
from tkinter import *from tkinter import ttk
def calculate(*args):try:
value = float(feet.get())meters.set((0.3048 * value * 10000.0 + 0.5)/10000.0)
except ValueError:passroot = Tk()
root.title("Feet to Meters")mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=0, row=0, sticky=(N, W, E, S))mainframe.columnconfigure(0, weight=1)
mainframe.rowconfigure(0, weight=1)feet = StringVar()
meters = StringVar()feet_entry = ttk.Entry(mainframe, width=7, textvariable=feet)
feet_entry.grid(column=2, row=1, sticky=(W, E))ttk.Label(mainframe, textvariable=meters).grid(column=2, row=2, sticky=(W, E))
ttk.Button(mainframe,text="Calculate", command=calculate).grid(column=3, row=3, sticky=W)
ttk.Label(mainframe, text="feet").grid(column=3, row=1, sticky=W)ttk.Label(mainframe, text="is equivalent to").grid(column=1, row=2, sticky=E)
ttk.Label(mainframe, text="meters").grid(column=3, row=2, sticky=W)for child in mainframe.winfo_children(): child.grid_configure(padx=5, pady=5)
feet_entry.focus()root.bind('<Return>', calculate)
root.mainloop() |
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
-end-
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