Writing Tests

Last updated on 2026-01-27 | Edit this page

Overview

Questions

How can I make my programs more reliable?

Objectives

  • Explain what a test is.
  • Explain what an assertion is.
  • Use tests and assertions to check my code.

Why Testing Matters

When we write code, we want to be confident that it produces the right results.
Additionally, it should keep working when we make changes. And even if we make mistakes,
they should be caught early.

Julia provides two simple ways to test your code:

  1. @test (from the Test standard library).
  2. @assert (built-in macro).

Using @test

The @test macro is part of Julia’s Test standard library.
It checks whether an expression evaluates to true.
If not, the test fails but your program keeps running.

JULIA 

using Test  # load the testing tools

@test 2 + 2 == 4

OUTPUT 

Test Passed

JULIA 

@test sqrt(9) == 3

OUTPUT 

Test Passed

JULIA 

@test 10 ÷ 3 == 3

OUTPUT 

Test Passed

JULIA 

@test 10 / 3 == 3

OUTPUT 

Test Failed at REPL[5]:1
  Expression: 10 / 3 == 3
   Evaluated: 3.3333333333333335 == 3

ERROR: There was an error during testing

@test reports failures without stopping execution. You usually use it for larger projects where you want to test many things at once. For these cases, there is another structure we can use: @testset.

JULIA 

@testset "Math tests" begin
    @test 2 + 2 == 4
    @test 3^2 == 9
    @test sqrt(16) == 4
end

OUTPUT 

Test Summary: | Pass  Total  Time
Math tests    |    3      3  0.1s
Test.DefaultTestSet("Math tests", Any[], 3, false, false, true, 1.75629781203e9, 1.756297812095e9, false)

The @assert Macro

The @assert macro is built into Julia. It also checks if an expression is true, but if the check fails, it immediately throws an error and stops the program.

JULIA 

@assert 2 + 2 == 4

JULIA 

@assert 10 / 3 == 3

OUTPUT 

ERROR: AssertionError: 10 / 3 == 3
Stacktrace:
 [1] top-level scope
   @ REPL[8]:1

You can also provide a custom error message:

JULIA 

x = -1
@assert x ≥ 0 "x must be non-negative!"

OUTPUT 

ERROR: AssertionError: x must be non-negative!
Stacktrace:
 [1] top-level scope
   @ REPL[10]:1

When to Use What?

  • Use @test when writing test files or checking lots of conditions at once.
  • Use @assert inside your program to enforce assumptions (like “input must be positive”).
Challenge

Test

  1. Write a function is_even(n) that returns true if n is even.

  2. Add a testset that checks:

    • is_even(2) is true.
    • is_even(3) is false.

  3. Add an assertion in your function that throws an error if n is not an integer.

JULIA 

function is_even(n)
    @assert isa(n, Integer) "Input must be an integer"
    return n % 2 == 0
end

@testset "Even number tests" begin
    @test is_even(2) == true
    @test is_even(3) == false
end