T-Th 9:05
T-Th 11:15
in Olin 155

CS 1110: Introduction to Computing Using Python

Fall 2012

Encapsulation and Properties

There is a PDF version of these instructions, if you would prefer to have that instead.

This lab gives you some experience with encapsulation and abstraction. It also contains a recursion problem, to give you some more practice for the upcoming exam. The concepts of this lab were, believe it or not, part of assignments A1 and A3 in previous semesters of this course.

The lab this week is a little longer than the last one. But you should definitely try to finish it before next week because it provides some good practice for the exam (particularly the recursion question).

Requirements For This Lab

There is just one file necessary for this lab: lab9.py. You should download this file and put it in a new directory. You will note that this file contains a class Person as a well as a function test. The test function is essentially a unit test for this lab. In many cases, it is not necessary to have a separate file for a unit test; you can just put the test in the same file as everything else. To run the unit test on this class, simply type

  python lab9.py

to run the module as an application. If you run it right now, you will see that it fails right away, because Person is incomplete.

In addition, we are using simple assert statements for our unit test, not assert_equals from cunittest. This is also okay; it cuts down on the number of files we give you. Outside of this class, in the real world, this would be an acceptable way to construct a unit test.

In this lab, you will modify the class Person in lab9.py, You do not need to modify the unit tests, just the class. When you are done, simply show these two files your lab instructor. As always, you should try to finish the lab during your section. However, if you do not finish during section, you have until the beginning of lab next week to finish it. You should always do your best to finish during lab hours; remember that labs are graded on effort, not correctness.

Lab Instructions

The class Person is meant to represent an entry into a geneological database. The object stores the name of the person, as well the mother and father, and the children for that person. The mother and father are additional Person objects stored in fields (much like the Worker object could store another Worker object in its boss fields). The children attribute is a list of Person objects. When you hook Person objects together, you will get a family tree much like the picture shown below.

If you look at the class Person, you will see hidden fields for all of these attributes, as well as properties that provide getters (and sometimes setters) for each of them. Most of the properties are immutable, except for mother and father. And the setters for those are not yet implemented.

Task 1: Complete the setters for mother and father

Note that while mother and father are mutable, children is not. That is because we need these values to be consistent with one another. If object q is in the mother field of object p, then object p should be in the list of children of object q.

This is very similar to the problem with farenheit and celsius from the Temperature class shown in class. You are used to invariants being limited to the type or range of a single attribute. But invariants can also specify the relationship between two attributes. In that case, the setter for each attribute should ensure that the relationship between the two attributes remains preserved.

In this example, when we change either the mother or the father attribute, we need to change the children in the mother or father as well. We have spelled out how to do this in the comments of these setters. The final code should five lines long; each comment corresponds to a line of code. The code for each setter will be identical except for self._mother being replaced by self._father and vice versa.

Even though the comments spell out how to do this, here are some more hints to make this go quickly.

Use list methods to modify the children attribute

You want to use the following to methods to add or remove children:

Method Description
children.append(p) Adds p to this list of children.
children.remove(p) Removes the first occurence of p from this list.

In the case of remove, each child should appear at most once in the list of children. Therefore, a single remove will remove all occurrences.

Do not use children; use _children

You will notice that every time you call the getter for children it makes a copy of the list. So if you try to use


it will add to the copy and not the original list of children. If you want to add to the actual list of children, use the field _children instead.

Why do we do this? Because we want children to be immutable. Even though there is no setter for children, you could still modify the contents of children since a list is itself mutable. This negates the whole purpose of making children immutable. So the getter gives a copy of _children, which can be modified without affecting this object.

Remember to modify the children of the parents

You might be tempted to write the following:


If you do this, you are claiming that the current object (self) has itself as its own child. While this is the subject of many interesting science fiction novels, it is not what we want here. Modify the _children fields in the parents.

The setter has to handle None

None is a valid value for a mother or father. None means that the appropriate parent is unknown. Even if we had an actual value for that parent at one time, we may reassign it to None later (perhaps on the revelation that this person was adopted). You have to be prepared for this, as you cannot access the _children field in None

Testing it Out

To test out your code, simply run lab9.py as an application. If you have done it correctly, you should see the following output:

Family Tree
Smith,John [0 kids]
   Evans,Ellen [1 kid]
      O'Reilly,Ellen [1 kid]
         Chase,Heather [1 kid]
         O'Reilly,Dan [1 kid]
      Evans,John [1 kid]
   Smith,John [1 kid]
      Dare,Jane [1 kid]
         Brown,Eva [1 kid]
      Smith,John [1 kid]
         Grey,Pamela [1 kid]
         Smith,John [1 kid]

Properties mother and father working correctly

The program should then fail immediately afterwards. That is okay; that error message is about the test for the next task.

Task 2: Complete the method familyNamed

Recall the famiy tree created when we link objects together.

Up until now, our use of recursion has been limited to sequences. But we can use recursion on this family tree as well. Note that "ancestor" has a recursion definition.

  • The mother or father of a Person an ancestor
  • The mother or father of an ancestor is an ancestor

Given this recursive definition, we can use recursion to define the following function.

    def familyNamed(self,name):
        """Returns: number of family with name as first name.
        The number of family members includes this person 
        (e.g. self) as well as all family members. You should 
        implement this method recursively.
        Precondition: name is a string."""

In the picture above, p.familyNamed('John') is 5 (remember to count p as well), while p.familyNamed('Ellen') is 2.

Write this recursive function. In order to figure out how to do this, you might find the method fullstr above it to be quite useful. This is a recurisve procedure that creates a string representing the entire family tree, indenting four spaces at each generation. In fact, you should recognize it, as it is what produced the output in the previous task.

When you are finished, run lab9.py again as an application. If you did it correctly, the output should end with the message

  Method familyNamed working correctly

At this point show your lab9.py to your instructor.