Property Based Testing ELI5

Assume you know about regular unit testing, possibly you’ve heard about the common test. There is another way to check your system, different from ordinary testing methods. It is called property based testing.

To make a test one should define types and ranges for the input data, which the system will take under the test. Definition of input data looks like a tuple with type and range argument descriptions. They are data, and you build this data before the test.

Then the test engine will call your code with random combinations of defined input data. Test engine tries to observe if the system under test still shows the defined properties.

If at some moment the required property is not observed, test will stop and engine will try to shrink the input. This is an attempt to find simplest input data which still breaks the test. Test engine will try to reduce complexity for as many parameters as possible.

See also

Libraries which offer property based testing: PropEr and Quviq QuickCheck (commercial) (note that QuickCheck also exists for other languages!)


Imagine that your boss asked you to write a fizz_buzz function. From the documentation it is clear, that a good fizz_buzz takes an integer argument between 1 and 100. Also it will return fizz for arguments that are divisible by 3, buzz for arguments that are divisible by 5, fizbuzz for arguments divisible by both 3 and 5.

So the input range: it is an integer between 1 and 100.

And 3 properties we want to observe:

  1. When X rem 15 =:= 0 we want to see fizzbuzz as return value.

  2. When X rem 3 =:= 0 we want to see fizz.

  3. When X rem 5 =:= 0 we want to see buzz.


Please consult actual documentation for the selected test library. The syntax of the library calls differs.

Test engine will apply random combinations of values from the defined input range. Test engine will ensure that all 3 properties are observed during the run.


You can test any black box system this way, without knowing its internals. The system under test can be a C program or even a real piece of hardware!

It is also possible to run parallel tests (ensure that a parallel system doesn’t break when things happen in parallel) and state machine tests (ensure that a state machine doesn’t break under the random input).