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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."

Now back to the calculate function

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

The input value in feet

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 .

Either save the float or pass

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 .

Calculate and display the result in meters

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.

Make everything run

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.

Run the program

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.

Complete program listing

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.

Housekeeping material
  • Module name: Itse1359-1610-GUI Programming
  • File: Itse1359-1610.htm
  • Published: 10/28/14
  • Revised: 01/31/16

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.


Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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