Assertion

Q: What happens when an assertion fails?

The assertion throws an error, such as AssertionError in Python or JavaScript's node:assert module. In a test, the runner marks that test as failed and shows the expected and actual values; in a program, execution stops unless the error is caught.

Updated September 30, 20262 min read

In short

An assertion is a statement in code declaring that a condition must be true at that point, stopping the test or program with an error if it is false.

What is an assertion in programming and testing?

An assertion is a line of code that states something must be true, such as total == 100 or that a list is not empty. If the condition holds, execution continues silently. If it doesn't, the assertion fails, usually by throwing an error that reports what was expected and what actually happened.

Assertions have two main uses. In tests, they are the assert step of Arrange, Act, Assert: they compare the actual result with the expected one, and a test without assertions only proves that the code didn't crash. Test frameworks and assertion libraries provide helpers such as assert.equal, expect(x).toBe(y), deep equality for objects, and checks that a function throws an error. In production code, runtime assertions document assumptions that must always hold, called invariants, and catch programming mistakes close to where they happen.

An assertion is like a checkpoint on a hiking trail, where the guide confirms everyone is still there before the group continues. Good test assertions are specific, comparing exact values instead of just checking that something is truthy, and they produce failure messages that explain the problem without extra debugging.

Assertions are often confused with input validation. An assertion guards against bugs in your own code, conditions that should be impossible if the program is correct, while validation handles bad data from users or other systems, which is expected and must always be checked. Never rely on assertions to validate user input, because many languages let you switch them off; running Python with the -O flag, for example, removes assert statements entirely.

Key takeaways

An assertion states a condition that must be true and fails loudly when it isn't.
In tests, assertions compare actual results with expected results.
In production code, assertions document invariants and catch bugs early.
Specific assertions with clear messages make failures easy to diagnose.
Assertions are not a substitute for validating user input.

Example

A runtime assertion and test assertions in Pythonpython

def apply_discount(price, percent):
    # Runtime assertion: documents an assumption about our own code
    assert 0 <= percent <= 100, f"percent out of range: {percent}"
    return round(price * (1 - percent / 100), 2)

def test_apply_discount():
    # Test assertions: compare the actual result with the expected one
    assert apply_discount(80.0, 25) == 60.0
    assert apply_discount(19.99, 0) == 19.99

Readers ask

What happens when an assertion fails?

The assertion throws an error, such as AssertionError in Python or JavaScript's node:assert module. In a test, the runner marks that test as failed and shows the expected and actual values; in a program, execution stops unless the error is caught.

Should assertions be used in production code?

Yes, for conditions that indicate a bug in your own code, such as an impossible state. Don't use them for user input or other expected errors, which need proper validation and error handling because assertions may be disabled.