Conditions and loops

reading the flow of control

What it is

Control flow is the order in which lines execute. Top to bottom by default, but conditions and loops change that: some lines are skipped, some are repeated.

To read code like this you need to be able to answer one question: on what data does this line execute? The rest follows.

What it looks like in code

orders = [
    {"id": 1, "status": "paid", "total": 250},
    {"id": 2, "status": "pending", "total": 0},
    {"id": 3, "status": "paid", "total": 90},
]

total = 0
for order in orders:
    if order["status"] != "paid":
        continue
    if order["total"] <= 0:
        print(f"order {order['id']}: paid, but the amount is zero")
        continue
    total += order["total"]
print("total:", total)

Output:

total: 340

How to read it

Indentation is scope. Everything shifted right of a for or an if belongs to it. In Python indentation is not formatting but syntax: move a line and you change the meaning of the program.

continue means "skip the rest and take the next item". The two of them in the example are filters: unpaid orders and zero-value orders never reach the addition.

break means "leave the loop entirely". Confusing the two is a common reading error: continue skips one step, break ends the iteration.

A loop's else is a rare construct, but it does turn up. It runs if the loop finished without a break.

List comprehensions

The same meaning fits on one line:

paid = [o["total"] for o in orders if o["status"] == "paid"]
print(sum(paid))     # 340

It reads, in meaning, right to left: take o from orders where the status is paid, and collect their o["total"]. The AI writes these readily — they are short, but with two or three conditions they become unreadable.

The ternary expression

status = "paid" if order["total"] > 0 else "pending"

Reads as: status equals "paid" if the condition holds, otherwise "pending". The same construct in JavaScript and Dart is written differently:

const status = order.total > 0 ? "paid" : "pending";

The ? before the first option, the : before the second — same thing, other syntax.

What to tell the AI

On what data does line 14 execute? Give a concrete example
of input that gets us there.

Can this loop run zero times? On what data?

The second question matters more than it looks. A loop over an empty list simply does nothing — silently, with no error. If the code below expects the variable to have accumulated something, it gets zero or emptiness.

What to look for

Deep nesting. Three or more levels of if inside for inside if is hard to read and easy to break. A sign that some conditions could become an early continue or move into a function of their own.

A loop that might not run. An empty list, an empty API response — and the whole block is skipped. Check that the code below accounts for that.

Modifying a list while iterating it. Removing items from the same list the for is walking gives you skipped items. It looks harmless and works incorrectly.

A condition that is always true or always false. This happens after edits: if status != "paid" or status != "pending" is true for every value. Substituting any value is enough to spot it.

Cheat sheet