Exercise 10.1

Each of the following expressions raises an exception. Copy each expression into your file (one at a time), run the file to see the error that is raised, and then put the expression in a try/except block that deals with the specific form of exception raised. For example, if the expression was

import happiness

this would generate an ImportError (unless you somehow defined a happiness module), so the resulting code would be:

try:
    import happiness
except ImportError as ie:
    print('dealt with ImportError:', ie)

The expressions:

1. 'hi' + 4
2. 10 / 0
3. 'person'.find_first('e')
4. [0,1,2]['summer']
5. ['hi','there','student'][5]
6. print(name)
7. 9 <= (3, 4)
8. f = open('missingFile.txt')

Exercise 10.2

Add code to the Planet class to maintain the invariant that the radius, mass and distance of a planet from the sun must be positive numbers. If the value provided by the calling program does not meet the invariant, raise an appropriate exception (e.g., ValueError('Planet numeric properties must be positive') )

Now, add testing code at the bottom of the file (that will not be run if the file is imported) to verify that you can correctly construct valid planets, and that an exception is raised if you attempt to construct an invalid planet.

Note: You do not need to add tests for the other methods in the class, but you should create at least two valid planets without raising an exception, and you should check that creating 3 planets that are invalid for different reasons each raise an exception.

Exercise 10.3

Repeat exercise 10.2, but now for the Sun class. Again, the radius and mass should be positive numbers, but now the temperature should also be greater than absolute zero (-273.15°C).

Also add appropriate tests in this file that will not be run when the module is imported.

Exercise 10.4

To check that an object is actually the type we are looking for, we can use the function isinstance(). This function has two arguments: the object and the class. Replace the body of the add_sun method with the following:

if isinstance(a_sun, Sun):
    self._sun = a_sun
else:
    raise TypeError('add_sun(): cannot add ' + str(type(a_sun)) + ' to solar system')

Similarly, add code to the add_planet() method to verify that the given parameters are an appropriate type, raising a TypeError with a helpful message if they are not. As in previous exercises, add some testing code to your file.

Exercise 10.5

Create a new file in your lab10 package called driver.py. This will start as an empty file, and will become the place that we bring together the graphical components of our task.

The job of the driver is to set up our output window and get everything set up in our solar system. Copy and paste the following code into driver.py :

import turtle
from solar_system import *

window = turtle.Screen()
window.setworldcoordinates(-1, -1, 1, 1)

ss = Solar_System()
ss.add_sun(Sun("SUN", 8.5, 1000, 5800))
ss.add_planet(Planet("EARTH", .475, 5000, 0.6, 'blue'))

# Keep the window open until it is clicked
window.exitonclick()
			

Take a minute to understand/guess what this code is doing (i.e., add some comments once you figure out a piece to remind yourself what is happening).

As written, this code will not run because it assumes updates to our classes to support graphics that we have not yet made. Add the following changes to get things running appropriately:

• planet.py
  1. Import the turtle graphics package
  2. Add a parameter and associated instance variable for the color of the planet
  3. Add instance variables for the x and y coordinates of the planet (but do not allow the calling program to change them)
     • Initialize the x coordinate to be the distance from the sun
     • Initialize the y coordinate to zero
  4. Add a turtle as an instance variable (each planet will have its own turtle that is responsible for drawing its position... it should be constructed here) and make the following adjustments:
     1. Pick the turtle's pen up
     2. Change the color of the turtle to self._color
     3. Change the shape of the turtle to 'circle' (i.e. self._turtle.shape('circle'))
     4. Change the size of the turtle using self._turtle.shapesize(self._radius, self._radius)
     5. Change the location of the turtle using goto to self._x, self._y
  5. In the main code section of your code:
     1. Create a turtle window
     2. Update your tests of the Planet class to account for the additional color parameter.
     3. Make it so the window doesn't close until you click it
• sun.py
  1. Import the turtle graphics package
  2. Add instance variables for the x and y coordinates of the sun (but do not allow the calling program to change them):
     • Initialize the x coordinate to zero
     • Initialize the y coordinate to zero
  3. Add a turtle as an instance variable and make the same adjustments as you made for the planet class.
• Save all of your classes!

With each of these changes made, you should be able to run your driver program and see a nice visualization of your solar system. Note: don't get too excited. Planets don't actually move... :-(

Exercise 10.6

Add code that prompts the user for each of the following:

• Name of the planet
• Radius of planet
• Mass of the planet
• Distance of planet from the sun
• Desired color of the planet

Using the entered values, create a new planet and add it to the solar system. Your code should never crash (that is, be ready for any possible exception). Further, verify that the new planet appears in the visualization appropriately.

For example, entering Mars, distance: 0.51; mass: 3000; radius: 0.2; color: red, produces a reasonable approximation of the planet mars.