Read and Adapt a Complete Program

Reading code is a separate skill from writing it. A complete unfamiliar program can look dense even when every individual feature is familiar.

Use a repeatable reading order.

1. Find the data

Consider:

scores = [
    {"name": "Ari", "points": 12},
    {"name": "Bo", "points": 19},
]

This is a list of consistently shaped dictionary records. Before reading any loop, you know what one item looks like.

2. Read function contracts from the outside

def highest_score(scores):
    pass

The name suggests one selected result. The single parameter receives the collection.

Look at the return statements next. Does the function return a record, a number, a string, or None?

3. Trace one small call

For a selection loop:

best = None

for score in scores:
    if best is None or score["points"] > best["points"]:
        best = score

Trace two records:

The best is None condition gives the first record a place to start. Python checks the left side of or first. When it is true, Python already knows the whole condition is true, so it skips the right side. On the first pass, that means it never tries to read best["points"] while best is None.

On later passes, best holds a record, so the left side is false and Python evaluates the comparison on the right. A later record replaces best only when its points are greater. This left-to-right skipping is called short-circuit evaluation.

4. Follow the callers

If a formatting function calls highest_score, identify which returned fields it uses.

This reveals the program’s chain:

records -> selection function -> selected record -> formatted report

Retrieve a familiar report boundary

A formatting function may keep its complete lines in a list and return them as one string:

lines = [f"Busiest: {busiest['name']}"]
return "\n".join(lines)

Before reading on, explain the call to yourself. Why is "\n" before .join(), why is lines the argument, and what kind of value comes back?

The same rule from the refactoring lesson applies here: the string before the dot is the separator, the argument supplies a list of strings, and the result is one string. When you extend this report, add another complete string to lines and preserve that boundary.

5. Establish a baseline before editing

Run the starter exactly as it is. Record its current output.

When a new requirement arrives, preserve that baseline unless the requirement explicitly changes it.

Make the smallest accurate extension

Suppose the program must also report the lowest score. A focused extension is:

• add a second selection function with the same record contract;

• call both selectors from the report;

• keep the existing highest string in lines unchanged;

• add one new lowest string before the list is joined.

A complete rewrite is unnecessary. Accurate reading lets you change the smallest responsible area.