Many objects, and what they share

What you will be able to do

After this lesson you can

• keep many objects in a list, loop over them, and sort them by an attribute,
• tell a class attribute from an object attribute,
• explain why two names can be one object, and
• avoid the shared default list.

Many objects

A class is useful because you can make many objects from it. A class of students has many students. A library has many books. You keep them in a list, and use the loops, max and sorted that you know.

Predict first. Which student has the best average? Then run it.

• The list students holds three Student objects. The loop for s in students gives one object in each round, and s.average() calls its method.
• max(students, key=by_average) finds the object whose average is the biggest. The key function takes one object and returns the thing to compare. This is the same key= that you used to sort words by length.
• sorted(students, key=by_average, reverse=True) puts the objects in order.
• The list comprehension [s.name for s in students if s.average() > 70] picks the names of the students above 70.

Objects are just values. You can keep them in lists and dictionaries, give them to functions, and return them.

Shared by all, or one for each?

So far each attribute belonged to one object, set with self.. But sometimes a value belongs to the whole class. Every dog belongs to the same species. And we may want to count how many dogs were made. For that, write the attribute in the class body, outside any method.

Two names, one object

A variable holds a reference to an object, like a label on it. You met this with lists.

Making a copy needs work. The line Box(list(a.items)) makes a new list and a new object, so c is independent. When an object holds a list, think about whether you share it or copy it.

The shared default: a famous trap

The cure is the pattern in FixedTeam: use None as the default, and make a new list inside, when the default is used:

self.members = [] if members is None else members

Now each object gets its own list. The rule: never use a list, a dictionary or a set as a default value. Use None.

The default value is worked out once, when def runs, not each time the method is called. That is why a list default is shared.

Common mistakes

• Using a list as a default. All the objects share it.
• Changing a class attribute through an object and making a new object attribute instead.
• Thinking b = a is a copy.
• Forgetting the parentheses of a method in a key function. key=Student.average is fine, key=s.average() is not.
• Putting self. before a class attribute in the class body. Names in the class body have no self.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Students and averages. You write a class and a function that finds the best student.
2. Build. Open the problem Class or object attribute? and the problem The shared default.
3. Stretch. Open the problem Sort the objects and sort a list of dogs by age, then by name.

Remember these

• Keep objects in lists or dictionaries. Use loops, max and sorted with a key function.
• A class attribute is shared. An object attribute belongs to one object.
• A write through an object makes an object attribute. It does not change the class.
• A variable is a reference. b = a gives the same object a second name.
• Never use a list as a default value. Use None.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How do you find the student with the highest average in a list of objects?

2. After rex.species = "Wolf", what is Dog.species?

3. Why is members=[] a bad default?