🔁 Python `range()` — Comprehensive Guide + Practice Editor

Learn how range() really works (start/stop/step, negatives, slicing, indexing, membership tests, performance) and run code instantly using Pyodide.

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What it is

An iterable of ints (lazy)

Common use

Loop counts / indices

Memory

Doesn’t store all numbers

Stop rule

Stop is excluded

1) What is `range()`?

range() creates a special object called a range object. It represents an arithmetic progression of integers:

It is iterable (you can loop over it).
It is lazy (it does not build a full list of numbers in memory).
It supports length, indexing, slicing, and membership tests.

Think of it as: “a compact recipe for numbers”, not a stored list.

Quick demo: range is NOT a list

r = range(5)
print(r)                 # range(0, 5)
print(type(r))           # <class 'range'>
print(list(r))           # [0, 1, 2, 3, 4]

2) Syntax (three forms)

A `range(stop)`

Starts at 0, increases by 1, stops before stop.

range(stop)

print(list(range(5)))    # 0..4

B `range(start, stop)`

Starts at start, increases by 1, stops before stop.

range(start, stop)

print(list(range(2, 7))) # 2..6

C `range(start, stop, step)`

Starts at start, adds step each time, and stops before stop.

range(start, stop, step)

print(list(range(1, 10, 2)))  # odds: 1,3,5,7,9

Golden rules (must remember)

Stop is excluded: range(5) ends at 4.
Step cannot be 0: range(1, 10, 0) is an error.
If step is positive, numbers go up. If step is negative, numbers go down.
If your step direction doesn’t “move toward” stop, you get an empty range.

Empty range examples

print(list(range(5, 1)))      # [] (step defaults to +1, but start>stop)
print(list(range(1, 5, -1)))  # [] (step is -1, but start<stop)

3) Understanding stop-excluded with real examples

“Stop excluded” is extremely useful because it makes ranges fit naturally with:

Counting n items: range(n) gives exactly n values.
Indexing: list indices are 0 to len(list)-1.
Slicing: slices are also stop-excluded, like a[2:7].

Counting and indexing examples

items = ["a", "b", "c", "d"]

# 1) exactly len(items) indices
for i in range(len(items)):
    print(i, items[i])

# 2) last index is len(items) - 1
print("Last item:", items[len(items) - 1])

4) Negative steps (reverse counting)

Use a negative step to count down. A simple pattern is: range(start, stop, -1) where stop is usually 0 or -1 depending on inclusion.

Countdown patterns

# 10 down to 1
print(list(range(10, 0, -1)))

# 10 down to 0 (include 0)
print(list(range(10, -1, -1)))

5) How many numbers will a range produce?

You can get the size using len(range(...)). This is fast because range is compact.

len(range(...))

print(len(range(10)))          # 10 numbers: 0..9
print(len(range(2, 10)))       # 8 numbers: 2..9
print(len(range(1, 10, 2)))    # 5 numbers: 1,3,5,7,9
print(len(range(10, 0, -1)))   # 10 numbers: 10..1

6) range as a “sequence” (indexing, slicing, membership)

A range acts like an immutable sequence:

r[0] gives the first value
r[-1] gives the last value
r[2:10:2] returns another range
x in r checks membership efficiently

Indexing + slicing + membership

r = range(3, 20, 3)  # 3,6,9,12,15,18

print(r[0])          # 3
print(r[-1])         # 18
print(list(r[1:4]))  # [6, 9, 12]

print(12 in r)       # True
print(14 in r)       # False

Common beginner mistakes

Forgetting stop is excluded: range(1, 5) gives 1,2,3,4 (not 5).
Using the wrong stop when reversing: to include 0 you need stop = -1.
Step direction mismatch: range(1, 5, -1) becomes empty.
Step = 0 is invalid: range(1, 10, 0) raises an error.

7) Practical recipes (copy-ready)

✅ Recipe: 1 to n (inclusive)

1..n inclusive

n = 10
for i in range(1, n + 1):
    print(i)

✅ Recipe: Even numbers up to n

Evens up to n

n = 20
for i in range(2, n + 1, 2):
    print(i)

✅ Recipe: Reverse loop over indices

Reverse indices

items = ["x", "y", "z", "w"]

for i in range(len(items) - 1, -1, -1):
    print(i, items[i])

✅ Recipe: Multiplication table (nested loops)

Table 1..5

for i in range(1, 6):
    for j in range(1, 6):
        print(i * j, end="\t")
    print()

8) Try it yourself — Pyodide Python Editor

This editor runs Python directly in your browser using Pyodide (Python compiled to WebAssembly). No installation required.

Python code

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Tip: Use print() to see output. Use input() carefully (see note).

Output stdout / errors

About `input()` in browser

In this page, input() is redirected to a simple prompt popup.

9) Mini practice tasks (range-focused)

Print numbers from 100 to 1 (reverse) using range.
Print multiples of 7 from 7 to 70.
Print squares of numbers from 1 to 10.
Take n and print all odd numbers up to n.
Print the index + character of a string using range(len(s)).