Property-based testing concept in Kotlin - Step-by-Step Execution

Concept Flow - Property-based testing concept

Define properties

↓

Generate random inputs

↓

Run test with inputs

↓

Check if property holds

↓

Next input

↓

Repeat until inputs exhausted or failure

Property-based testing runs many random inputs through a property to check if it always holds true.

Execution Sample

Kotlin

import io.kotest.property.checkAll

checkAll<Int> { x ->
  (x + 0) == x
}

This test checks that adding zero to any integer x returns x, using many random x values.

Execution Table

Step	Input x	Property Checked	Result	Action
1	5	(5 + 0) == 5	True	Continue testing
2	-3	(-3 + 0) == -3	True	Continue testing
3	0	(0 + 0) == 0	True	Continue testing
4	999	(999 + 0) == 999	True	Continue testing
5	-1000	(-1000 + 0) == -1000	True	Continue testing
...	...	...	...	...
N	random x	(x + 0) == x	True	All tests passed

💡 All random inputs tested, property holds for all, test passes

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	...final
x	undefined	5	-3	0	999	-1000	various random ints
propertyResult	undefined	true	true	true	true	true	true

Key Moments - 3 Insights

Why do we test many random inputs instead of just one?

Property-based testing checks that a property holds for all inputs, not just one example. The execution_table shows multiple inputs tested to catch edge cases.

What happens if the property fails for one input?

If the property fails (result false), testing stops and failure is reported immediately, as shown in the 'No' branch in concept_flow.

Are the inputs truly random or fixed?

Inputs are randomly generated each run to explore many cases. The variable_tracker shows different values of x for each test step.

Visual Quiz - 3 Questions

Look at the execution_table, what is the property result at step 3?

A False

B True

C Undefined

D Error

💡 Hint

Check the 'Result' column at step 3 in execution_table

At which step does the testing stop if the property fails?

A After all inputs tested

B After 5 inputs

C Immediately at failure

D Never stops

💡 Hint

Refer to concept_flow where 'No' branch leads to 'Report failure'

If we change the property to (x + 1) == x, how would the execution_table change?

A Some results become False

B All results remain True

C No inputs tested

D Property always passes

💡 Hint

Think about propertyResult values in variable_tracker and execution_table for a false property

Concept Snapshot

Property-based testing runs many random inputs to check a property.
Define property, generate inputs, test property, report failures.
Stops on first failure or passes if all hold.
Good for catching edge cases beyond example tests.

Full Transcript

Property-based testing means writing a rule (property) that should always be true for many inputs. The test runs many random inputs through this rule. Each input is checked if the property holds. If it fails for any input, testing stops and reports failure. If all inputs pass, the test passes. This helps find bugs that example-based tests might miss. The execution table shows inputs and results step by step. The variable tracker shows how inputs and results change. Key moments explain why many inputs are tested and what happens on failure. The quiz checks understanding of these steps.