Objects you already use

What you will be able to do

After this lesson you can

• say what an object, a class, an attribute and a method are,
• read the dot in word.upper() and in z.real,
• tell a method call from an attribute, and
• list what an object can do with dir.

You already use objects

You have written word.upper() and numbers.sort() many times. The dot says: "the action upper of the thing word". That thing, word, is an object. An object keeps some data, and knows some actions that work on that data. The data are called attributes. The actions are called methods.

Predict first, then run it.

• type("python") is <class 'str'>. The word class is the new idea. A class is a kind of thing: all strings are of the class str, all lists of the class list. An object is one example of a kind. "python" is one object of the class str.
• word.upper() is a method: an action that a string knows how to do. It is called with brackets.
• numbers.sort() is a method of a list. It changes the list, and gives back None.
• (3 + 4j).real is an attribute: a piece of data that belongs to the object. A complex number keeps a real part and an imaginary part. We read it without brackets.
• dir(word) lists the names that a string knows. A string has about 45 public methods, and you have met only a few.
• word.upper == "PYTHON" is False, because word.upper without brackets is the method itself, not its answer. The brackets call it.

The vocabulary

An object is a package of data and actions. A class is the description of the package: what data it has, and what it can do. You write the description once, and you can make as many objects from it as you like.

Attributes are read without brackets. A method is called with them. The object carries both, so z knows its own parts.

Why not just use functions?

You can do a lot with functions and dictionaries, and you have. So why classes?

Think about a student with a name and some marks. With a dictionary, you write {"name": "Asha", "marks": [72, 85]}, and a function average(student). The data is in one place and the function is in another. Nothing stops someone from putting a text where the marks should be, or from calling average with a dictionary that has no marks.

With a class, the data and the actions live together. A Student object knows its own marks, and student.average() works on those marks. The class can also protect its data, with rules: "a mark must be from 0 to 100". As programs grow, this keeps things tidy. Large programs are built from objects that each look after themselves.

Common mistakes

• Forgetting the brackets of a method. word.upper is the method, and word.upper() is its answer.
• Using brackets for an attribute. z.real() is a TypeError, because a number is not something to call.
• Confusing the class and the object. str is a class. "python" is an object of it.
• Thinking that only classes you write are classes. Every type that you know is a class.

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 problem Which is which? and say whether each name is a class, an object, an attribute or a method.
2. Build. Open the lab Look inside an object. You use type and dir and the checker confirms the answers.
3. Stretch. Open the problem Why not just functions? and write two sentences in your own words.

Remember these

• Every value is an object. It has attributes (data) and methods (actions), reached with a dot.
• A class is a kind of thing. An object is one example of it.
• A method is called with brackets. An attribute is read without.
• type(x) gives the class of x, and dir(x) lists what it can do.

Check yourself

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

1. What is the class of "python"?

2. What is the difference between word.upper and word.upper()?

3. Which of these is an attribute: z.real or numbers.sort()?