[Tutor] command in menu and button

alan.gauld@freenet.co.uk alan.gauld at freenet.co.uk
Tue Nov 22 23:39:35 CET 2005

>>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():

And make the menu/button reference callSystem:

>>menu.add_command(label="Open Viewer", 

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.

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