[Tutor] passing values to a callback

[Dave Angel]

On 07/13/2013 01:48 PM, Alan Gauld wrote:
> On 13/07/13 16:08, Brett Wunderlich wrote:
>
>> commanded by a Button. Once the button is pressed the values in the
>> fields should result in a calculation - the values should all be numeric
>
> You'll need to convert between strings and numbers, the GUI displays
> strings (with one exception, see below)
>
>> class App:
>>
>>      def __init__( self, master ):
>>          frame = Frame( master )
>>          frame.grid()
>>          entry1 = Entry( frame, bg = "white", width = 3 ).grid(row = 0,
>> column = 0 )
>
> You want to store a reference to the entrey in your app so this should
> read:
>
>             self.entry1 = Entry( frame, ....)

Whoops.  Brett had chained the reference with his call to grid(), so 
this would store a None.  The line needs to be split up:

         self.entry1 = Entry( frame, bg = "white", width = 3 )
         self.entry1.grid(row = 0, column = 0 )

Otherwise, Alan's answer is right-on.


If you want to give an initial string value, then add:
         self.entry1.insert(0, "42")

>
>>      def calculate( self ):
>>          print 1
>
> Now you can access then entry objects using self
>
>             print self.entry1.get()
>
> Or convert to an int (or float) as required.
>
> Finally you can write back to the entry widget the result string
>            self.entry.insert(0,resultString)
>
> This is a common requirement so there is a bit of magic that Tk
> allows namely you can specify a StringVar or IntVar variable in
> your GUI definition and link it to an entry. The Tk core will
> then keep the content ogf the variable and widget in synch
> (and do the int conversion if appropriate).
>
> In your init method you can set up a StringVar using
>
>        self.Entry1Var = StringVar()
>        self.entry1['textvariable'] = self.Entry1Var
>
> or an int var like:
>
>        self.Entry2Var = IntVar()
>        self.entry1['textvariable'] = self.Entry2Var
>
>
> And you can read/change the value by get() or set()
> actions on the variables.
>
> There are also DoubleVar and BooleanVar available too.
> You can use them on some other types of widgets, but not all.
>
> Personally I don't find them that big a help and often just access the
> widget directly.
>
> HTH


-- 
DaveA