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Property-based testing relies on specifying some property of code, unlike unit tests, which specify the expected output in response to some inputs. In this course, we'll cover the concepts we need to feel confident about using even the most advanced features of PropEr with Elixir.

We'll start with the basic and foundational principles of property-based testing, see what the framework offers us to get started, make our way through thinking in properties, write our own custom data generators, and then learn more advanced concepts. We'll also see how you can use property-based testing in a realistic project.

By the time we finish this course, we should feel comfortable testing real-world applications using properties.

Property-Based Testing with PropEr in Elixir

Which callbacks for stateful tests belong in the real interaction with the system but not in the abstract phase?

Which callbacks for stateful tests belong in the real interaction with the system but not in the abstract phase?

The actual system will be started and stopped before every single test iteration.

The actual system will be booted once before the tests run. The system instance will be shared for all iterations, before being shut down after the whole run. 

The actual system will be started, then an abstract representation of the test case will be created. After this, the system will be stopped. The test itself will not have run once at that point, so by the time the framework would try to execute the test, the actual system would have already shut down.

What happens to the system in the following property?

```elixir
property "example" do
 forall cmds <- commands(__MODULE__) do
 ActualSystem.start_link()
 {_history, _state, result} = run_commands(__MODULE__, cmds) 
 ActualSystem.stop()
 result == :ok
 end 
end
```

What happens to the system in the following property?
<pre><code class="language-elixir">property &quot;example&quot; do
 forall cmds &lt;- commands(__MODULE__) do
 ActualSystem.start_link()
 {_history, _state, result} = run_commands(__MODULE__, cmds) 
 ActualSystem.stop()
 result == :ok
 end 
end
</code></pre>

The actual system will be started, then an abstract representation of the test case will be created. After this, the system will be stopped. But the test itself will not have run once at that point, so by the time the framework would try to execute the test, the actual system would have already shut down.

What happens to the system in the following property?


```elixir
property "example" do 
 ActualSystem.start_link()
 forall cmds <- commands(__MODULE__) do
 {_history, _state, result} = run_commands(__MODULE__, cmds)
 result == :ok 
 end
 ActualSystem.stop()
end
```

What happens to the system in the following property?
<pre><code class="language-elixir">property &quot;example&quot; do 
 ActualSystem.start_link()
 forall cmds &lt;- commands(__MODULE__) do
 {_history, _state, result} = run_commands(__MODULE__, cmds)
 result == :ok 
 end
 ActualSystem.stop()
end
</code></pre>

Solve a short quiz to test your understanding of the concepts of stateful properties.

Foundations of Property-Based Testing

Writing Properties

Thinking in Properties

Custom Generators

Responsible Testing

Properties-Driven Development

Shrinking

Stateful Properties

Case Study: Bookstore

State Machine Properties

Conclusion

Quiz Yourself on Stateful Properties