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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()

Miscellaneous

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
Disclaimers:

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-

Questions & Answers

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
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Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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