here is my code

    from Tkinter import *

Main window

    App = Tk()
    App.title("Converter")
    App.geometry('360x180')

Menu

    kind = ['Temperature', 'Length', 'Weight']
    all = StringVar()
    all_menu = OptionMenu(App, all, *kind)
    all_menu.grid(row=0, column=1, pady=5)

Get index

    index = all.get()
    if index == 'Temperature' :
        scales = ['Fahrenheit','Celsius']
    elif index == 'Length' :
        scales = ['Mile','Kilometer']
    else: 
        scales = ['Pound','Kilogram']

The scale of the length to be used for conversion

    _from = StringVar()
    from_menu = OptionMenu(App, _from, *scales)
    from_menu.grid(row=1, column=1, pady=5)

In between label

    lbl = Label(App, text=' convert to ')
    lbl.grid(row=1, column=2, pady=5)

The scale of the length to convert the value to

    to_ = StringVar()
    to_menu = OptionMenu(App, to_, *scales)
    to_menu.grid(row=1, column=3, pady=5)

Entry pre-label

    numL = Label(App, text='Enter: ')
    numL.grid(row=2, column=0, columnspan=1, pady=5)

Entry field

    numE = Entry(App)
    numE.grid(row=2, column=1, columnspan=1, pady=5)

In between Entry field and Converter function

    equal = Label(App, text=' = ')
    equal.grid(row=2, column=2, pady=5)

Converter temperature function

    def Convert_Temperature():
        froM = _from.get()
        tO = to_.get()
        num = int(numE.get())
    
        if froM == 'Fahrenheit' and tO == 'Celsius':
            converted_num = (num - 32) * 5 / 9
        elif froM == 'Celsius' and tO == 'Fahrenheit':
            converted_num = (num * 9 / 5)   32 
        else:
            converted_num = num
    
        conv_numL = Label(App, text=round(converted_num, 4), width=10)
        conv_numL.grid(row=2, column=3, pady=5)

Converter length function

    def Convert_Length():
        froM = _from.get()
        tO = to_.get()
        num = int(numE.get())
    
        if froM == 'Mile' and tO == 'Kilometer':
            converted_num = num * 1.609
        elif froM == 'Kilometer' and tO == 'Mile':
            converted_num = num / 1.609  
        else:
            converted_num = num
    
        conv_numL = Label(App, text=round(converted_num, 4), width=10)
        conv_numL.grid(row=2, column=3, pady=5)

Converter weight function

    def Convert_Weight():
        froM = _from.get()
        tO = to_.get()
        num = int(numE.get())
    
        if froM == 'Pound' and tO == 'Kilogram':
            converted_num = num / 2.205
        elif froM == 'Kilogram' and tO == 'Pound':
            converted_num = num * 2.205  
        else:
            converted_num = num
    
        conv_numL = Label(App, text=round(converted_num, 4), width=10)
        conv_numL.grid(row=2, column=3, pady=5)

Convert button

    if index == 'Temperature':
        conB = Button(App, text='Convert', command=Convert_Temperature)
    elif index == 'Length':
        conB = Button(App, text='Convert', command=Convert_Length)
    else:
        conB = Button(App, text='Convert', command=Convert_Weight)
    conB.grid(row=3, column=1, pady=5)

Clear text function

    def clear_text():
        numE.delete(0, END)
        _from.set("")
        to_.set("")
        conv_numL = Label(App, text="", width=10)
        conv_numL.grid(row=2, column=3, pady=5)

Clear text button

    btn = Button(App, text="Restart", command=clear_text)
    btn.grid(row=3, column=3, pady=5)

App.mainloop()

full code

from tkinter import *
# Main window
App = Tk()
App.title("Converter")
App.geometry('360x180')

# Menu
kind = ['Temperature', 'Length', 'Weight']
all = StringVar()
all_menu = OptionMenu(App, all, *kind)
all_menu.grid(row=0, column=1, pady=5)

# Get index
index = all.get()
if index == 'Temperature' :
    scalesa = ['Fahrenheit','Celsius']
elif index == 'weight' :
    scalesa = ['Pound','Kilogram']
else: 
    scalesa = ['Mile','Kilometer']
    

# The scale of the length to be used for conversion
_from = StringVar()
from_menu = OptionMenu(App, _from, *scalesa)
from_menu.grid(row=1, column=1, pady=5)

# In between label
lbl = Label(App, text=' convert to ')
lbl.grid(row=1, column=2, pady=5)

# The scale of the length to convert the value to
to_ = StringVar()
to_menu = OptionMenu(App, to_, *scalesa)
to_menu.grid(row=1, column=3, pady=5)

# Entry pre-label
numL = Label(App, text='Enter: ')
numL.grid(row=2, column=0, columnspan=1, pady=5)

# Entry field 
numE = Entry(App)
numE.grid(row=2, column=1, columnspan=1, pady=5)

# In between Entry field and Converter function
equal = Label(App, text=' = ')
equal.grid(row=2, column=2, pady=5)

# Converter temperature function
def Convert_Temperature():
    froM = _from.get()
    tO = to_.get()
    num = int(numE.get())

    if froM == 'Fahrenheit' and tO == 'Celsius':
        converted_num = (num - 32) * 5 / 9
    elif froM == 'Celsius' and tO == 'Fahrenheit':
        converted_num = (num * 9 / 5)   32 
    else:
        converted_num = num

    conv_numL = Label(App, text=round(converted_num, 4), width=10)
    conv_numL.grid(row=2, column=3, pady=5)

# Converter length function
def Convert_Length():
    froM = _from.get()
    tO = to_.get()
    num = int(numE.get())

    if froM == 'Mile' and tO == 'Kilometer':
        converted_num = num * 1.609
    elif froM == 'Kilometer' and tO == 'Mile':
        converted_num = num / 1.609  
    else:
        converted_num = num

    conv_numL = Label(App, text=round(converted_num, 4), width=10)
    conv_numL.grid(row=2, column=3, pady=5)

# Converter weight function
def Convert_Weight():
    froM = _from.get()
    tO = to_.get()
    num = int(numE.get())

    if froM == 'Pound' and tO == 'Kilogram':
        converted_num = num / 2.205
    elif froM == 'Kilogram' and tO == 'Pound':
        converted_num = num * 2.205  
    else:
        converted_num = num

    conv_numL = Label(App, text=round(converted_num, 4), width=10)
    conv_numL.grid(row=2, column=3, pady=5)

# Convert button
if index == 'Temperature':
    conB = Button(App, text='Convert', command=Convert_Temperature)
elif index == 'Length':
    conB = Button(App, text='Convert', command=Convert_Length)
else:
    conB = Button(App, text='Convert', command=Convert_Weight)
conB.grid(row=3, column=1, pady=5)

# Clear text function
def clear_text():
    numE.delete(0, END)
    _from.set("")
    to_.set("")
    conv_numL = Label(App, text="", width=10)
    conv_numL.grid(row=2, column=3, pady=5)

# Clear text button
btn = Button(App, text="Restart", command=clear_text)
btn.grid(row=3, column=3, pady=5)

App.mainloop()

# Label -> print text
# Grid -> Locate
# Entry -> table

CodePudding user response：

You need to learn about event-driven programming which tkinter is based on.

You need to update the options of from_menu and to_menu inside a function which is triggered whenever selection of all_menu is changed. It can be done by using the command option of OptionMenu.

Also you need to call the corresponding conversion based on the selection of all_menu, from_menu and to_menu inside a function which is triggered by clicking the Convert button.

Below is modified code:

from tkinter import *

# Main window
App = Tk()
App.title("Converter")
App.geometry('450x180')

def update_menu(optmenu, optvar, options):
    menu = optmenu['menu']
    # clear current options
    menu.delete(0, 'end')
    # populate new options
    for item in options:
        menu.add_command(label=item, command=lambda value=item: optvar.set(value))
    # reset selection
    optvar.set('')

def update_conversion_menus(index):
    if index == 'Temperature' :
        scalesa = ['Fahrenheit','Celsius']
    elif index == 'Weight' :
        scalesa = ['Pound','Kilogram']
    else:
        scalesa = ['Mile','Kilometer']
    update_menu(from_menu, from_var, scalesa)
    update_menu(to_menu, to_var, scalesa)

# Menu
kind = ['Temperature', 'Length', 'Weight']
kind_var = StringVar()
all_menu = OptionMenu(App, kind_var, *kind, command=update_conversion_menus) # used command option
all_menu.grid(row=0, column=1, pady=5)

# The scale of the length to be used for conversion
from_var = StringVar()
from_menu = OptionMenu(App, from_var, None)
from_menu.grid(row=1, column=1, pady=5)

# In between label
lbl = Label(App, text=' convert to ')
lbl.grid(row=1, column=2, pady=5)

# The scale of the length to convert the value to
to_var = StringVar()
to_menu = OptionMenu(App, to_var, None)
to_menu.grid(row=1, column=3, pady=5)

# Entry pre-label
numL = Label(App, text='Enter: ')
numL.grid(row=2, column=0, columnspan=1, pady=5)

# Entry field
numE = Entry(App)
numE.grid(row=2, column=1, columnspan=1, pady=5)

# In between Entry field and Converter function
equal = Label(App, text=' = ')
equal.grid(row=2, column=2, pady=5)

# result of conversion
conv_numL = Label(App, width=10)
conv_numL.grid(row=2, column=3, pady=5)

# Converter temperature function
def Convert_Temperature(froM, tO, num):
    if froM == 'Fahrenheit' and tO == 'Celsius':
        num = (num - 32) * 5 / 9
    elif froM == 'Celsius' and tO == 'Fahrenheit':
        num = (num * 9 / 5)   32
    return num

# Converter length function
def Convert_Length(froM, tO, num):
    if froM == 'Mile' and tO == 'Kilometer':
        num *= 1.609
    elif froM == 'Kilometer' and tO == 'Mile':
        num /= 1.609
    return num

# Converter weight function
def Convert_Weight(froM, tO, num):
    if froM == 'Pound' and tO == 'Kilogram':
        num /= 2.205
    elif froM == 'Kilogram' and tO == 'Pound':
        num *= 2.205
    return num

def do_conversion():
    try:
        froM = from_var.get()
        tO = to_var.get()
        num = float(numE.get().strip()) # exception may be raised on invalid input

        # do corresponding conversion based on selections
        index = kind_var.get()
        if index == 'Temperature':
            converted_num = Convert_Temperature(froM, tO, num)
        elif index == 'Length':
            converted_num = Convert_Length(froM, tO, num)
        else:
            converted_num = Convert_Weight(froM, tO, num)

        # show the conversion result
        conv_numL.config(text=round(converted_num, 4))
    except Exception as e:
        print(e)

Button(App, text='Convert', command=do_conversion).grid(row=3, column=1, pady=5)

# Clear text function
def clear_text():
    numE.delete(0, END)
    from_var.set("")
    to_var.set("")
    conv_numL.config(text="")

# Clear text button
btn = Button(App, text="Restart", command=clear_text)
btn.grid(row=3, column=3, pady=5)

App.mainloop()

Note that I have changed some variable names as _from or to_ are not good variable names and all is a built-in function.