Python Built-in Functions: range, reversed, iter, and next – Usage and Best Practices

range([start,] stop[, step]) generates an integer sequence and is commonly used with for loops. It accepts up to three arguments: start (default 0), stop (exclusive), and step (default 1).

range(stop)
range(start, stop[, step])

Examples:

# Single argument: 0 to 5 (exclusive)
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# Two arguments: 1 to 5 (exclusive)
for i in range(1, 5):
    print(i)  # 1, 2, 3, 4

# Three arguments: 0 to 10 (exclusive) step 2
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

reversed(seq) returns a reverse iterator, allowing traversal of a sequence in opposite order without modifying the original.

reversed(sequence)

Examples:

my_list = [1, 2, 3, 4]
for item in reversed(my_list):
    print(item)  # 4, 3, 2, 1

my_string = "hello"
for char in reversed(my_string):
    print(char)  # o, l, l, e, h

iter(obj[, sentinel]) creates an iterator from an iterable object or from a callable with a sentinel value.

iter(object)
iter(callable, sentinel)

Example:

my_list = ['a', 'b', 'c']
iterator = iter(my_list)
print(next(iterator))  # a
print(next(iterator))  # b
print(next(iterator))  # c
# Next call raises StopIteration

next(iterator[, default]) retrieves the next item from an iterator, optionally returning a default value instead of raising StopIteration when the iterator is exhausted.

next(iterator)
next(iterator, default)

Example:

my_list = ['x', 'y', 'z']
iterator = iter(my_list)
print(next(iterator))  # x
print(next(iterator))  # y
print(next(iterator))  # z
print(next(iterator, 'no more items'))  # no more items

Best Practices and Tips

Use range() for clear index‑controlled loops instead of manual counters.

Employ reversed() to iterate backwards without altering the original sequence.

Combine iter() and next() for custom iteration, especially with infinite streams or event‑driven data sources.

Handle potential StopIteration gracefully by providing a default value to next() or catching the exception.