for: Every Item in Turn

Once information is in a list, you usually want to do the same thing to every item: print it, check it, add it to a total. Writing the same line once per item would work for three items and fall apart at three hundred. A loop runs a block of lines once for every item, and for is Python's everyday loop.

Think of a teacher marking a stack of papers. The same steps for every paper, a running tally kept on the side, and the final number announced when the stack is done. That is almost exactly the shape of the most common loop in programming.

for Each Item

A for line reads almost like English: for temp in temps. The name after for, here temp, points at each item in turn. The indented block runs once for each item, and then the program carries on after the loop. You choose the name; pick one that reads well with the list's name.

temps = [18, 22, 25, 19, 30, 27, 21]
for temp in temps:
    print(f"{temp} degrees")
print("That was the week.")

18 degrees
22 degrees
25 degrees
19 degrees
30 degrees
27 degrees
21 degrees
That was the week.

Counting with range

When you need to repeat something a number of times rather than walk a list, range produces the numbers for you. range(5) gives 0, 1, 2, 3 and 4: five numbers, starting at zero and stopping before 5, just like a slice. range(1, 4) gives 1, 2 and 3.

for n in range(3, 0, -1):
    print(n)
print("Go!")

3
2
1
Go!

The third number in that range is the step: minus one counts down. Start at 3, stop before 0, step by minus one.

The Running Total

Adding up a list is the pattern behind sums, counts and averages everywhere, and it has exactly three parts in three places. Before the loop, set the total to zero, once. Inside the loop, add the current item to it. After the loop, print or use the result.

temps = [18, 22, 25, 19, 30, 27, 21]
total = 0
for temp in temps:
    total = total + temp
print(f"Average: {total / len(temps):.1f}")

Average: 23.1

The classic mistake is to put the print inside the loop by indenting it one level too far. The program then prints a partial total after every item, three lines instead of one. The difference is only four spaces, which is why indentation deserves your attention.

temps = [18, 22, 25]
total = 0
for temp in temps:
    total = total + temp
    print(f"Total: {total}")

Total: 18
Total: 40
Total: 65

Counting and Finding

The same shape answers other questions. To count, start at zero and add one when an item matches, such as every day above 25 degrees. To find, walk the list until an item matches and stop there, which the next topic's break makes possible.

temps = [18, 22, 25, 19, 30, 27, 21]
hot = 0
for temp in temps:
    if temp > 25:
        hot = hot + 1
print(f"{hot} hot days")

2 hot days