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For loops in Python: forecast a year in six lines

What will the first year look like if sales grow 15% a month? You could work out twelve months by hand. Or you could describe one month and let a loop do the other eleven.

The big idea

A for loop runs the same block once for each value in a sequence, handing your code that value through a loop variable.

See it in code

1The basics

range(1, 4) produces 1, 2, 3. The loop runs its indented block once per value, with month holding each in turn:

python
for month in range(1, 4):
    print("Month", month)
Run it. One pass per value:
Month 1
Month 2
Month 3

Three values in, three lines out. The loop variable is the hook everything else hangs on.

2A step further

A loop's body can do several steps, and it can change variables that live outside it. Here each pass reports the month, then grows sales for the next one:

python
price = 38
units = 300
growth = 0.15

for month in range(1, 4):
    revenue = units * price
    print(f"Month {month}: {units} tees, ${revenue:,}")
    units = round(units * (1 + growth))
Run it. Three months of 15% growth:
Month 1: 300 tees, $11,400
Month 2: 345 tees, $13,110
Month 3: 397 tees, $15,086

units carries over from one pass to the next, so each month starts where the last one ended. The order matters: report this month first, then grow.

3At Evergreen

Change one number in range and the same body forecasts a full year. A second loop shows the other common form, iterating directly over a list:

python
price = 38
units = 300
growth = 0.15
monthly_revenue = []

for month in range(1, 13):
    monthly_revenue.append(units * price)
    units = round(units * (1 + growth))

for quarter_end in [3, 6, 9, 12]:
    print(f"Month {quarter_end}: ${monthly_revenue[quarter_end - 1]:,}")
print(f"Year one: ${sum(monthly_revenue):,}")
Run it. Twelve months, then the quarter-ends:
Month 3: $15,086
Month 6: $22,990
Month 9: $34,960
Month 12: $53,200
Year one: $331,398

The first loop builds twelve values with range. The second reads four of them by looping over a list. Monthly revenue more than quadruples across the year.

How for i in range(6) drives the loop A flow chart for a for loop. Start sets i to 0, then runs the loop body, then moves i to the next value, then checks whether more values remain in range(6). If yes, it loops back to the body with i = 1, 2, 3 and so on; if no, the loop ends. Below, the loop variable i takes each value 0 through 5. yes — i = 1, 2, 3 … no i = 0 run the loop body i moves to next value more in range(6)? loop ends i takes each value: 0 1 2 3 4 5
The loop variable steps through every value in the range, running the body once per value, then exits.
The same idea, everywhere

Nothing here is specific to sales. Swap the body and the same loop walks the codons of a DNA strand, the bars of a price series, or the cards a web page needs to render.

Try it yourself

Change growth to 0.10 and guess how far year one falls before you run it. A third? Five percent? The real drop is about 27%, because growth compounds. Then print every month instead of only the quarter-ends.

The common mistake

Misreading where range stops. range(1, 12) yields 1 through 11, which is eleven months, not twelve: the stop value is excluded. For months 1 to 12, write range(1, 13).

What it unlocks

Learn the three-argument form of range(), keep a running figure with an accumulator, and when you don't know the number of passes in advance, use a while loop.

Want the simpler version? Read the Kids version →

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