What is a for loop?

Run the same lines once for every item in a list, and add the results up as you go.

A for loop runs the same block of code once for every item in a list. You write the block once and Python repeats it, handing you one item at a time.

I show four things here: the loop variable, indentation, zip for walking two lists in step, and a running total.

Step 1. Print each close

Three closing prices. One loop.

closes = [185.4, 187.2, 184.9]  # three closing prices, oldest first

for c in closes:                # c becomes 185.4, then 187.2, then 184.9
    print("close:", c)
# -> close: 185.4
# -> close: 187.2
# -> close: 184.9
close: 185.4
close: 187.2
close: 184.9

c is the loop variable. The name is yours to pick. The line for c in closes: ends in a colon, and the indented line below it is the body: the part Python repeats.

Indentation decides what belongs to the loop. Move a line out and it runs once, after the loop has finished.

for c in closes:
    print("inside :", c)      # indented, so runs three times
print("outside:", c)          # not indented, so runs once, after the loop
# -> inside : 185.4
# -> inside : 187.2
# -> inside : 184.9
# -> outside: 184.9
inside : 185.4
inside : 187.2
inside : 184.9
outside: 184.9

The last line prints 184.9 because c keeps the value it held on the final pass.

Step 2. Two lists in step

zip walks two lists together and hands you one item from each.

dates  = ["Mon", "Tue", "Wed"]    # one label per close, same order
closes = [185.4, 187.2, 184.9]    # both lists must be the same length

for d, c in zip(dates, closes):   # first pass: d = "Mon", c = 185.4
    print(d, c)
# -> Mon 185.4
# -> Tue 187.2
# -> Wed 184.9
Mon 185.4
Tue 187.2
Wed 184.9

Two loop variables now, d and c, filled position by position.

Step 3. Add things up as you go

To accumulate a total, create it before the loop and add to it inside.

total = 0.0                       # created before the loop
for c in closes:
    total += c                    # shorthand for: total = total + c
    print("running total:", round(total, 1))
# -> running total: 185.4
# -> running total: 372.6
# -> running total: 557.5

print(total / len(closes))        # -> 185.83333333333334
running total: 185.4
running total: 372.6
running total: 557.5
185.83333333333334

Move total = 0.0 inside the loop and it is reset on every pass:

for c in closes:
    total = 0.0               # reset on every pass
    total += c                # so only the last close ever survives
print(total)                  # -> 184.9
184.9

Step 4. Value a portfolio

Three holdings, each with a price and a share count. The value of one holding is price times shares.

tickers = ["AAA", "CCC", "DDD"]
prices  = [185.40, 410.20, 128.75]  # price per share, same order as tickers
shares  = [10, 4, 20]               # shares held, same order again

total = 0.0                         # the portfolio value builds up in here

for tkr, px, n in zip(tickers, prices, shares):   # zip handles three lists too
    value = px * n                                # one holding: price times shares
    total += value                                # running sum across holdings
    print(tkr, value)
# -> AAA 1854.0
# -> CCC 1640.8
# -> DDD 2575.0

print("total", total)             # -> total 6069.8
AAA 1854.0
CCC 1640.8
DDD 2575.0
total 6069.8

Add a fourth stock to all three lists and the loop values it too. Nothing else changes.

Your turn

Print each holding’s weight: its value divided by the portfolio total. The total has to exist before you can divide by it, so this takes two passes.

total = sum(px * n for px, n in zip(prices, shares))   # a one-line version of the loop above

for tkr, px, n in zip(tickers, prices, shares):
    print(tkr, round(px * n / total, 3))
# -> AAA 0.305
# -> CCC 0.27
# -> DDD 0.424

Before rounding, the weights sum to 1, so each one is the share of the portfolio held in that stock.