Python range for fast counting and indexing

The python range built-in creates an immutable sequence of integers defined by start, stop, and step. Use it when you need reliable counters or index offsets without allocating a list of every number. It highlights how start, stop, and step control the integers a range produces. For loop patterns that use range, see the Python for loop example.

Python Range Example For Step Counting

Output:

pen: 10 mm
stapler: 30 mm

How This Example Works

1. range(0, len(items), 2) walks list indexes with a step of 2.
2. The stop value is exclusive, so it never reaches len(items).
3. Each index reads an item and prints a value tied to the index (10, 30).

Common Pitfalls with python range

Mistake 1: Expecting the stop value to be included.

for n in range(1, 5):
    print(n)

for n in range(1, 6):
    print(n)

Why it happens: range stops before the stop value, so inclusive sequences need stop + 1.

Mistake 2: Using a positive step while counting down.

for n in range(5, 0, 1):
    print(n)

for n in range(5, 0, -1):
    print(n)

Why it happens: a positive step only moves upward; to count down, the step must be negative or the range is empty.

Mistake 3: Passing non-integers for start, stop, or step.

for rate in range(0, 1, 0.1):
    print(rate)

for basis_points in range(0, 101, 5):
    rate = basis_points / 100
    print(rate)

Why it happens: range arguments must be integers (or implement __index__), and the step cannot be zero.

python range vs list(range): Which to Use

Rule of thumb: reach for range for iteration and offsets; materialize a list only when you truly need all values in memory. When you need index + value pairs, prefer enumerate — see the Python enumerate example.

Performance Considerations

range stores only start, stop, and step, so it uses O(1) memory even for huge sequences. Membership checks like n in range(...) are constant time for integers, which is faster than scanning a list. If you call list(range(...)), you pay O(n) memory and time to allocate every value, so do it only when you need a real list.

More Patterns for python range

Count down retries with a negative step.

for remaining in range(3, 0, -1):
    print(f"Retrying in {remaining}s")

The negative step makes the sequence move downward, and the stop value is still exclusive. That keeps a countdown short and predictable without extra conditionals.

Generate business hours without storing a list.

for hour in range(9, 18):
    print(f"{hour}:00")

range(9, 18) yields 9 through 17, which matches a 9-to-5 schedule with an exclusive stop. This is a clean way to produce fixed slots without prebuilding the numbers.

When to Use python range

• Use it for counters, offsets, or fixed-step loops where the data is an integer sequence.
• Use it when you want a memory-efficient sequence rather than a prebuilt list.
• Avoid it for non-integer steps; scale to integers or use other numeric tools.
• Avoid list(range(...)) unless you need to store or mutate the values later.