State that changes: methods that act

What you will be able to do

After this lesson you can

• explain the state of an object,
• write methods that change the state, and methods that only answer,
• keep the rules of a thing inside its class, and
• test a class by following its state.

State

The values of an object's attributes, at a moment, are its state. A dog that is 3 years old is in a different state from the same dog at 4. A method often changes the state: birthday() adds one to the age. Other methods only look at the state and answer a question.

A bank account

A bank account is the classic example. It has an owner and a balance. It can take a deposit and give a withdrawal. And it has rules: you cannot withdraw more than the balance, and a deposit must be a positive number.

Predict first. The account starts with 500, receives 250 and gives 100. What does show() say? Then which of the three actions in the loop are refused? Then run it.

The balance after the first three actions is 650. Look at the design.

• __init__(self, owner, balance=0) has a default value: an account that is opened with no balance starts at 0. Defaults work in methods as they do in functions.
• deposit checks the rule first, and raises a ValueError if it is broken. Only then does it change the balance.
• withdraw refuses to give more than the balance. If it did not, the balance could become negative, and the account would be in a state that should be impossible.
• show only looks. It returns a text, and does not change anything.
• The loop uses getattr(account, action) to call a method whose name is in a text. It is a neat trick for a table of tests. getattr(account, "deposit") is the same as account.deposit.

The main program has no if for the rules. The class keeps its own rules. This is a great gain. Whoever uses the account, in any part of a big program, cannot break it.

Methods that answer, and methods that change

A good method does one of two jobs.

• A command changes the state, and returns nothing: deposit, withdraw, birthday.
• A question returns an answer, and changes nothing: show, area, is_available.

Mixing the two is a trap. A method called area that also changes the width would surprise everybody.

A test of a class follows the state: write the balance you expect after each call, then compare.

Test a class by following the state

To test a class, make an object, perform actions, and check the state after each one, with the answers worked out by hand.

The refused actions are the most important rows. The balance must not change when an action is refused. Does it? In withdraw, the check comes before the change, so yes.

Attributes can be read from outside

In Python you can write account.balance = 1000000, and the rules are skipped. Python trusts the programmer. Many programmers mark an attribute that outsiders should not touch with an underscore, _balance, as a friendly warning. More advanced tools can protect it. For now, the habit is: change the state through the methods, and only read the attributes directly.

Common mistakes

• Changing the state before checking the rule. The object is broken even though the error was raised.
• A method that changes the state and also returns an answer, and nobody knows which job it does.
• Forgetting self. before the attribute in self.balance = self.balance + amount.
• Repeating the rule in the main program. The class should own it.
• Writing to an attribute from outside and skipping the rules.

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 Make a counter. You write a class with a state that changes.
2. Build. Open the lab Bank account. The checker tries the good and bad actions.
3. Stretch. Open the problem Add a transfer and write a method that moves money from one account to another.

Remember these

• The state of an object is the values of its attributes. Methods read it and change it.
• A command changes the state. A question returns an answer. Keep them apart.
• The rules of a thing belong inside its class. Check before you change.
• Test a class by following the state after each action, including the refused ones.

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 balance after opening with 500, depositing 250 and withdrawing 100?

2. Why does withdraw check the balance before it changes it?

3. What does show() change?