Week 2 - You Try It, Part 2 [Optional]
Python Object to model a Scratch Sprite

Code Activity

Earlier I showed you working code for two classes:

• CreditCard.py  (and it you like, here is the Wallet code to go with it.)
• Fraction.py

While we will not make a habit of having you write classes in this course - mostly just using them - I think it is worth TRYING to write a class from scratch.  In fact, let's have you write a Python version of the Scratch Sprite that we designed together last week.

Using the CreditCard and Fraction examples from above, create a Python Class called Sprite which you save in a file named Sprite.py.  This class should provide all of the data and functionality that was outlined in the class diagram shown below:

Most of these are fairly straight forward because they are simple operations.  But a few of them have some error checking and one even has (heaven forbid) trigonometry. And non-standard trigonometry at that.  Since my MAIN learning outcome with this is really just to get you to explore with the structure of Python class code, I will give you the option to create one of two versions.

A class called SimpleSprite in a file called SimpleSprite.py

• This version does not do any error checking about the validity of parameters. 
  • If the user says  mySprite.setX(5000) then it is allowed. 
  • If the current direction is 90 and the user says mySprite.turnClockwise(100) then it goes to 190 rather than -170 (which is what Scratch would do).
• This version only allows move to happen if the current direction value is one of the four cardinal directions - 0, 90, 180, and -90 for the direction.  In other words, a move() will change x or y but not both.

Once you write your Sprite you can download this tester program and run it to see if your outcomes agree with what I was expecting.

If you REALLY want to test yourself:

A class called Sprite in a file called Sprite.py

• This version does everything that the SimpleSprite did but adds some additional functionality that makes it a little closer to the true Scratch Sprite:
  • If a user moves or sets an X/Y value that put it beyond the border of the stage (-240 to +240 for x and -180 to +180 for y).  It just sets the appropriate value to that limit.
    • mySprite.setX(5000) would end up with x=240
  • If a change to the angle [either through pointInDirection(), turnClockwise() or turnCounterClockwise() ] were to cause an angle that is not between -180 to +180 inclusively than the value should "wrap" appropriately.
    • mySprite.pointInDirection(200) should end up with direction=-160
    • mySprite.pointInDirection(160) with mySprite.turnClockwise(400) should end up with direction=-160
  • If a sprite is pointing in any valid direction (-180 to +180) and a move() is executed it should calculate the accurate trigonometry of the move.
    • mysprite.pointInDirection(30) followed by mySprite.move(100) should create x=50 and y=86.6.
    • Note,  python has math.sin() and math.cos() in the math libarary. HOWEVER, they expect radians. Fortunately, there is a math.radians() which converts degrees to radians
      • import math
      • rad = math.radians(45)
      • print(math.cos(rad))
      • print(math.sin(rad))
      •  
      • produces:
      • 0.7071... as expected

Once you write your Sprite you can download this tester program and run it to see if your outcomes agree with what I was expecting.