Class notes for Introduction Python_week 3 and 4

1. in Canvas, the origin is always on upper-left corner.
2. On a 300 by 200 pixel canvas, the pixel coordinates range from 0, 0 in the top left to 299, 199 in the bottom right.

3.draw handler needs to be registered.
4. use Brackets [], to define the position of the texts.  The position is the lower-left corner of the string.

5. print s1[0]  是从0开始数一个string， 第一个字母是第0个。
6. print s1[-1] 是print 最后一个字母。
7. print s1[0:7] 是print 前7个字母，不包括7所代表的的那个字母，是开区间。
8. s2[:13] 前13位。

9. str（123）把 数字变成string；  int()就是string转成整数

10. position = [50, 60]      position[0] = 50     position[1] = 60

11. decomposition the program is important. draw is draw, timer is timer, message is message. One has nothing to do with another.

12. don't forget start the frame.   frame.start() . same for timer.   timer1.start()  timer2.start()

WEEK 4

1. list   [] empty list;  [1,2,3] or ['hello', 'goodbye'];  position = [4, 9]; even other list inside list.
 [[1,3], ['a', 'b'], []]

2. len() return the length of the list.

3.  access the list print l[0]    index start from 0   .   l2[1:3] 是l2里的第二项和第三项，不包裹第四项。

4. l2[0] = 'cheese'

5. chr() turns the key codes number into caracter.

6. in Python power use ** 2  it is square

7. In python, for update vector, we need update x and y separately.

8. a is a list, and b = a,  if change a element in a,       b is changed too.

9. a is a list,   b = list(a) , b is the same list as a , but when updating a ,   b unchanged.

10. In a function , we do not need "global" if using mutation. We only need it when doing assignment.

11. type([1,2,3]) is list,     type((1,2,3)) is tuple.  tuple is not mutable.  If you want to protect your data, you'd better save it as a tuple.