Python random.randrange() Function: Usage and Examples

Select a random number from an integer range with an optional step.

What It Does and Why It Matters

random.randrange(start, stop[, step]) follows range-style boundaries: stop is excluded and step cannot be zero. Modern Python rejects noninteger numeric arguments instead of silently converting floats.

An empty integer range raises ValueError; validate the bounds before selecting.

Pseudorandom examples are suitable for simulation and tests. Use secrets instead for tokens, passwords and authentication.

Step-by-Step Example

Start with the complete example below. Read the inputs and predict what the final print statement will show before you run it.

import random
random_generator = random.Random(42)
selected_value = random_generator.randrange(10, 20, 2)
print(selected_value in range(10, 20, 2))
print(selected_value % 2 == 0)

How the Code Works

The imports make these standard-library tools available: random.
The name random_generator receives the value of random.Random(42). Changing this input lets you explore a different case.
The print calls display the results. Compare these results with the method’s documented return type and the input used.
The dedicated Random instance isolates the generator state from unrelated random calls. A fixed seed is useful here for repeatable demonstrations but must not be reused to generate secrets.

Expected output:

True
True

Each displayed line comes from the corresponding calculation or check. The output helps verify the return values before you adapt the example to your own input.

What to Notice

Pay attention to the accepted input types, the returned value and the boundary behavior. Predict what changes if you replace the example input with an empty, zero, negative or missing value, where that value is meaningful for this operation.

If the task uses a library function, inspect its documented exceptions and return type before reusing it inside a larger function.

Where You Would Use This

Use this in calculations, data summaries, simulations or validation when the numerical assumptions match the task. Check input domains, units and precision before interpreting a result. A single example output does not prove a statistical conclusion.

Practice Exercise

Generate random odd numbers between 11 and 29 using a step of two.

Before running your edited example, write down the result you expect. Then test at least one valid boundary or deliberately invalid value, using appropriate exception handling where necessary.

Reference: official Python documentation for the complete API contract and version notes.

Common Beginner Mistakes

Assuming the stop value is inclusive or passing a zero step.
Overlooking the documented condition: random.randrange(start, stop[, step]) follows range-style boundaries: stop is excluded and step cannot be zero.

Key Takeaways

Select a random number from an integer range with an optional step.
random.randrange(start, stop[, step]) follows range-style boundaries: stop is excluded and step cannot be zero.
Generate random odd numbers between 11 and 29 using a step of two.
Pseudorandom examples are suitable for simulation and tests. Use secrets instead for tokens, passwords and authentication.