On Mon, Jul 02, 2018 at 11:54:08AM +1000, Chris Roy-Smith wrote: > Hi, > > I'm trying to understand working with objects. > > If I have grasped things correctly a widget is an object.
In Python, all values are objects. (That's not the case in all languages.) > So why can I > assign the widget, or use it stand alone? See sample code below That depends on the object, but in general, you can use any object you like even if it doesn't survive the experience. Python knows enough to only access objects which are still alive. That's *incredibly common* for small, lightweight values like floats and strings. We might say: print(math.sin(1.3*math.pi) + 1) which creates (and then destroys) the following transient objects: * the float 1.3 * then the float 4.084070449666731 (multiplying the above by pi) * then the float -0.8090169943749473 (calling sin on the above) * then 0.19098300562505266 (adding 1) none of which survive more than a few microseconds. By the time the final value is printed, all of those transient objects will have been reclaimed for recycling by the garbage collector, their memory ready for re-use. The same applies for big, complex objects. But under normal circumstances, the bigger and more complex, the more unlikely you are to treat it as a transient, disposable object. (Creating lightweight objects is cheap, but the bigger the object, the less cheap it is.) In the case of tkinter, things are even more murky. Like many (all?) GUI frameworks, tkinter does a lot of behind the scenes clever stuff that makes it easier to use at the cost of being less comprehensible. And so: > # as I understand it this will create an instance of the button widget > called b1 > b1=Button(main, text='instantce', command= lambda b='goodbye' : > print(b)).grid(row=1, column=0) Actually, no, if you print the value of b1 -- or print its repr, print(repr(b1)) -- you might be in for a surprise. Your b1 is not actually the button, but the None object. But don't worry, the button object itself is safely attached to the TK main window, as if by magic. (That's tkinter's magic: the first argument to the Button constructor is the window to attach it to.) > # but here I haven't made an instance, but all seems well > Button(main, text='test1', command=lambda a='hello' > :print(a)).grid(row=0, column=0) Indeed. You don't have to hold onto the button, because the main window does it for you. -- Steve _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
