[Tutor] command in menu and button

[Double Six]

Hi John and Alan,

I got it! Thank you both for explaining this situation. 

Thanks,
Joe



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---- On Tue, 22 Nov 2005, alan.gauld at freenet.co.uk
(alan.gauld at freenet.co.uk) wrote:

> >>I'm puzzled by the 'command' option in menu and Button of
> >>Tkinter. With the following lines,
> 
> The command value needs to be a *reference* to a function.
> 
> That is not the function call itself but a reference to the
function
> that will be \called.
> 
> Let me illustrate the difference:
> 
> def f(): print 'Its me!'
> 
> f()   # prints the message
> 
> g = f   # this assigns a reference to f
> 
> g()  # this now calls that reference, 
> so calling g() is the same as calling f()
> 
> 
> >>menu.add_command(label="Open Viewer", 
>                    command=os.system("Open my viewer &"))
> 
> Here you assign the result of the os.system() 
> call to command, in fact you want to assign 
> a reference to a call of os.system which will 
> be executed when the menu/button is activated.
> 
> The more straightforward way to do that is to 
> define a short function that calls os.system:
> 
> def callSystem():
>    os.system(Mycommand)
> 
> And make the menu/button reference callSystem:
> 
> >>menu.add_command(label="Open Viewer", 
>                    command=callSystem)
> 
> Notice no parens, just the name of the function.
> 
> Because we can wind up with loads of these little 
> wrapper functions there is a shortcut called lambda.
> With lambda we can avoid defining a new mini function:
> 
> >>menu.add_command(label="Open Viewer", 
>                    command=lambda : os.system("Open my viewer
&"))
> 
> the thing that follows the lambda is what gets 
> executed when the widget activates.
> 
> Does that help?
> 
> Alan G.
> http://www.freenetpages.co.uk/hp/alan.gauld