range() & while

range() produces a sequence of numbers — how you count in Python:

range(5)         # 0, 1, 2, 3, 4        (stop excluded!)
range(1, 6)      # 1, 2, 3, 4, 5        (start, stop)
range(0, 10, 2)  # 0, 2, 4, 6, 8        (start, stop, step)
range(10, 0, -1) # 10, 9, ..., 1        (counting down)

range(stop) never includes stop — the half-open rule again, same as slicing. Combine with list() to see it: list(range(4)) → [0, 1, 2, 3].

enumerate: value AND position

for i, fruit in enumerate(fruits):
    print(i, fruit)     # 0 apple, 1 pear, 2 plum

zip: walk two lists together

names = ["a", "b"]
scores = [9, 7]
for name, score in zip(names, scores):
    print(name, score)

while: loop while a condition holds

count = 3
while count > 0:
    print(count)
    count -= 1
print("liftoff")

while re-checks the condition each round. If nothing inside makes the condition false, the loop never ends — the infinite loop, the classic beginner trap. When a loop must end, something in the body must move toward the exit (decrement a counter, change a flag...).