A parameter called evt

26 Jan, 2016

Dudley B writes:

I'm a Business Analyst trying to learn how to code. Luckily there's Python, a suitable language for absolute beginners like me. Thank God also for your book, Python for Kids. It's a great book which explains Python fundamentals in a fun and easy way. In Chapter 14 of the book, page 208, there is a parameter called "evt":

def turn_left(self, evt):
    self.x = -2
def turn_right(self, evt):
    self.x = 2

I can't figure out what this parameter is for, and why it's there since there is no value passed on to it. Please help me understand why "evt" is needed for those functions.

The evt parameter isn't brilliantly explained in that chapter, now that I look back at it. If you go back to the section titled "Making an object React to something" in Chapter 12, there's a description there of event bindings. The function described takes a parameter called event. After you've "told" tkinter about the function (that's the binding part), when an event occurs (such as the mouse being moved, or keyboard being pressed) it calls that function with an object containing more information (such as what sort of event it was).

The parameter evt on page 208, is simply a shortened name for the parameter event mentioned in the earlier chapter. In fact, the name isn't important at all -- you could even call it bob, or aardvark, or anything else you like.

By the way, another way of thinking about the tkinter event binding is that it's like a contract between two people. Let's say between a surfer, and an old man living on a cliff above the sea. The surfer says to the old man, "send me a message and let me know how high the surf is, whenever it changes". The function is that contract, and the message about the surf is the parameter value.

Hopefully that helps.