47 lines
2.1 KiB
ReStructuredText
47 lines
2.1 KiB
ReStructuredText
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.. _test-anatomy:
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Anatomy of a test
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=================
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In the simplest terms, a test is meant to look at the result of a particular
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behavior, and make sure that result aligns with what you would expect.
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Behavior is not something that can be empirically measured, which is why writing
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tests can be challenging.
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"Behavior" is the way in which some system **acts in response** to a particular
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situation and/or stimuli. But exactly *how* or *why* something is done is not
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quite as important as *what* was done.
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You can think of a test as being broken down into four steps:
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1. **Arrange**
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2. **Act**
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3. **Assert**
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4. **Cleanup**
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**Arrange** is where we prepare everything for our test. This means pretty
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much everything except for the "**act**". It's lining up the dominoes so that
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the **act** can do its thing in one, state-changing step. This can mean
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preparing objects, starting/killing services, entering records into a database,
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or even things like defining a URL to query, generating some credentials for a
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user that doesn't exist yet, or just waiting for some process to finish.
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**Act** is the singular, state-changing action that kicks off the **behavior**
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we want to test. This behavior is what carries out the changing of the state of
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the system under test (SUT), and it's the resulting changed state that we can
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look at to make a judgement about the behavior. This typically takes the form of
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a function/method call.
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**Assert** is where we look at that resulting state and check if it looks how
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we'd expect after the dust has settled. It's where we gather evidence to say the
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behavior does or does not aligns with what we expect. The ``assert`` in our test
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is where we take that measurement/observation and apply our judgement to it. If
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something should be green, we'd say ``assert thing == "green"``.
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**Cleanup** is where the test picks up after itself, so other tests aren't being
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accidentally influenced by it.
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At its core, the test is ultimately the **act** and **assert** steps, with the
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**arrange** step only providing the context. **Behavior** exists between **act**
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and **assert**.
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