len() and range()

len()

len() returns the number of elements in a container.

numbers = [1, 2, 3, 4]
print(len(numbers))   # 4

text = "Python"
print(len(text))      # 6

Works with any sized container: lists, tuples, strings, dictionaries, and sets.

range()

range() generates a sequence of integers. It is lazy — the numbers are produced on demand rather than stored all at once.

Three forms:

range(stop)             # 0 up to (not including) stop
range(start, stop)      # start up to (not including) stop
range(start, stop, step)# start up to stop, stepping by step

for i in range(5):
    print(i)   # 0 1 2 3 4

for i in range(2, 10, 2):
    print(i)   # 2 4 6 8

range() produces integers only. To iterate over a list by index, combine it with len():

fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(i, fruits[i])

Output:

0 apple
1 banana
2 cherry

In practice this pattern is rare — enumerate() is almost always clearer. See enumerate() and zip().

Key Ideas

len() measures a container; range() generates integers for iteration. range() is lazy and memory-efficient — range(1_000_000) uses no more memory than range(5). Avoid range(len(seq)) when you need both index and value — use enumerate() instead.