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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 the simplest input data which still breaks the test. Test engine will try to reduce complexity for as many parameters as possible.


Imagine that your interviewer has 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.

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.

Last modified: 10 June 2024