From Question Dictionaries to Question Objects

The Chapter 3 project processed complete attempt dictionaries through several functions. That design worked because every record used the keys those functions expected.

Question dictionaries follow the same rule. A familiar quiz question looks like this:

question = {
    "prompt": "What does len('cat') return?",
    "answer": "3",
}

This design works. Dictionaries are not a mistake, and a class is not automatically better. In this chapter, we will examine what happens when a growing program depends on every question dictionary having exactly the same shape.

Compare these questions:

questions = [
    {
        "prompt": "What does len('cat') return?",
        "answer": "3",
    },
    {
        "text": "Which keyword starts a function?",
        "answer": "def",
    },
]

The second dictionary uses text where the program expects prompt. Python can create that dictionary without complaint. The problem appears later, when another part of the program tries to use the missing key.

The behavior lives somewhere else

A function can check an answer:

def is_correct(question, response):
    return response == question["answer"]

The question data lives in one place, while the rule for working with that data lives in another. As the program grows, a change to the dictionary may also require changes in every function that uses its keys.

That is the design pressure we will address. We want a dependable kind of value that:

• always represents one quiz question;

• carries the information a question needs;

• provides the operations that belong to a question.

The next lesson introduces Python’s way to define such a value. First, check that the problem is clear.

Try the failure

The editor contains two question dictionaries and a function that displays their prompts. Run it, read the final line of the error, and then repair the inconsistent key.

Repair only the inconsistent key for now. First make the existing design work again. A useful abstraction should solve a problem you understand, not hide one you have not examined.