for loops and range

A for loop runs a block once for each item in a sequence. It's the workhorse of automation, where you almost always have a list of servers, files, or records to process the same way.

Looping over a list

servers = ["web-01", "web-02", "db-01"]
for server in servers:
    print("checking", server)

On each pass, the variable after for (here server) holds the next item. The indented block runs once per item. The loop ends when the list runs out.

The loop variable is yours to name

server is just a name you pick. Choose something that describes one item, so the body reads clearly. for s in servers works but for server in servers is easier to follow.

range: loop a fixed number of times

When you want to repeat something a set number of times, range produces a sequence of integers to loop over:

for i in range(3):
    print("attempt", i)

That prints 0, 1, 2. range(3) counts from 0 up to but not including 3, so it yields three numbers.

range with a start and step

range takes up to three arguments, start, stop, and step:

range(1, 4)      # 1, 2, 3
range(0, 10, 2)  # 0, 2, 4, 6, 8

The stop value is always excluded. This off-by-one is deliberate and becomes natural quickly. Reach for range when you need counting; reach for a plain for item in collection when you already have the items.