Python For Loops

A loop lets one block of code handle many values. The for loop takes each item from an iterable, assigns it to a variable, and runs the body once. This is the backbone of reports, cleaning, and data processing. You will also separate accumulators from built-in aggregations.

Iterate over a collection

The loop variable receives each item in turn. Name it after the item, not after the index. The body is indented and runs once per item.

orders = ["latte", "mocha", "tea"]
for order in orders:
    print(order)

One line for every order

range produces numbers

range(stop) starts at 0 and stops before stop. range(start, stop) begins at start. range(start, stop, step) skips by step. range is lazy: it produces values as the loop asks for them.

print(list(range(3)))
print(list(range(2, 5)))
print(list(range(0, 10, 2)))

Three range shapes

Accumulate a result

Create the accumulator before the loop, update it inside, and print it after the loop. An accumulator can be a number, a list, or a string.

prices = [4.5, 6.0, 3.25]
total = 0
for price in prices:
    total += price
print(f"Total: ${total:.2f}")

Summing with a loop

Keep running state

An accumulator is a variable that carries a result across iterations. Initialize it before the loop, update it inside the loop, and use it after the loop. If the update is outside the loop, you only keep the last value.

prices = [4.5, 3.0, 6.25]
total = 0
count = 0
for price in prices:
    total += price
    count += 1
print(count, round(total, 2), round(total / count, 2))

One loop, three summary values

Use a built-in when the loop only aggregates

If the loop only computes a sum, count, maximum, or a yes/no condition, a built-in may express the intent more clearly. any() stops at the first true value; all() stops at the first false value. Use a loop when you need several results or custom control flow.

prices = [4.5, 3.0, 6.25]
print(sum(prices), len(prices), max(prices), min(prices))
print(any(price > 6 for price in prices))
print(all(price > 0 for price in prices))

Prefer the clearer aggregation

enumerate keeps the line number

enumerate() gives each item and its position, so a skipped line does not shift the error message. Use the start argument to match human line numbers. Do not remove items from a list while iterating over it; build a new list or iterate over a copy. In the receipt project, the line number is what makes an error useful.

lines = ["Latte|2", "", "Tea|1"]
for line_no, line in enumerate(lines, 1):
    if not line:
        continue
    name, qty = line.split("|")
    print(line_no, name, int(qty))

Positions survive skipped rows

Try it yourself

Change the values, predict the output, then run the code. The Python runtime is downloaded the first time you press Run, so the first run can take a few seconds.