Why does my Python turtle shape size decrease when pressing 'shift'

Question

I am trying to create a turtle in Python so I could increase/ decrease it's size by pressing +/- on keyboard

import turtle

turtle.setup(700,500)
wn = turtle.Screen()
testing_turtle = turtle.Turtle()


size = 1
def dropsize():
    global size
    if size>1:
        size-=1
        print(size)    # To show the value of size
        testing_turtle.shapesize(size)

def addsize():
    global size
    if size<20:    # To ensure the size of turtle is between 1 to 20
        size+=1
        print(size)
        testing_turtle.shapesize(size)


wn.onkey(addsize,'+')
wn.onkey(dropsize,'-')


wn.listen()
wn.mainloop()

To press the '+' key, I will have to hold 'shift' & press '=' at the same time. The problem is when I release the shift key ( or just press the shift key), it decreases the size value by 1. Why?

Also if I put all these code into a main function:

def main():
    import turtle
    ...
    ...
    ...
    wn.onkey(addsize,'+')
    ...
    ...

    wn.mainloop()
main()

A error message show up:

NameError: name 'size' is not defined

I had called 'size' a global variable but why it is not defined now?

Answer 1

You need to use 'plus' and 'minus' to bind the functions to the key-release event of the + and - key:

wn.onkey(addsize,'plus')
wn.onkey(dropsize,'minus')

To solve the NameError exception, either place your size variable otside your main loop or use the nonlocal statement instead of global:

def addsize():
    nonlocal size
    if size<20:    # To ensure the size of turtle is between 1 to 20
        size+=1
        print(size)
        testing_turtle.shapesize(size)