If the hostname of a node contains characters that can't be used in a
Python variable name, those characters will be replaced with
underscores in the variable name, so `nodes.machine-a` will be exposed
to Python as `machine_a`.

## Machine objects {#ssec-machine-objects}

The following methods are available on machine objects:

@PYTHON_MACHINE_METHODS@

To test user units declared by `systemd.user.services` the optional
`user` argument can be used:

```py
machine.start()
machine.wait_for_x()
machine.wait_for_unit("xautolock.service", "x-session-user")
```

This applies to `systemctl`, `get_unit_info`, `wait_for_unit`,
`start_job` and `stop_job`.

For faster dev cycles it's also possible to disable the code-linters
(this shouldn't be committed though):

```nix
{
  skipLint = true;
  nodes.machine =
    { config, pkgs, ... }:
    { # configuration…
    };

  testScript =
    ''
      Python code…
    '';
}
```

This will produce a Nix warning at evaluation time. To fully disable the
linter, wrap the test script in comment directives to disable the Black
linter directly (again, don't commit this within the Nixpkgs
repository):

```nix
{
  testScript =
    ''
      # fmt: off
      Python code…
      # fmt: on
    '';
}
```

Similarly, the type checking of test scripts can be disabled in the following
way:

```nix
{
  skipTypeCheck = true;
  nodes.machine =
    { config, pkgs, ... }:
    { # configuration…
    };
}
```

## Failing tests early {#ssec-failing-tests-early}

To fail tests early when certain invariants are no longer met (instead of waiting for the build to time out), the decorator `polling_condition` is provided. For example, if we are testing a program `foo` that should not quit after being started, we might write the following:

```py
@polling_condition
def foo_running():
    machine.succeed("pgrep -x foo")


machine.succeed("foo --start")
machine.wait_until_succeeds("pgrep -x foo")

with foo_running:
    ...  # Put `foo` through its paces
```

`polling_condition` takes the following (optional) arguments:

`seconds_interval`

:   specifies how often the condition should be polled:

```py
@polling_condition(seconds_interval=10)
def foo_running():
    machine.succeed("pgrep -x foo")
```

`description`

:   is used in the log when the condition is checked. If this is not provided, the description is pulled from the docstring of the function. These two are therefore equivalent:

```py
@polling_condition
def foo_running():
    "check that foo is running"
    machine.succeed("pgrep -x foo")
```

```py
@polling_condition(description="check that foo is running")
def foo_running():
    machine.succeed("pgrep -x foo")
```

## Adding Python packages to the test script {#ssec-python-packages-in-test-script}

When additional Python libraries are required in the test script, they can be
added using the parameter `extraPythonPackages`. For example, you could add
`numpy` like this:

```nix
{
  extraPythonPackages = p: [ p.numpy ];

  nodes = { };

  # Type checking on extra packages doesn't work yet
  skipTypeCheck = true;

  testScript = ''
    import numpy as np
    assert str(np.zeros(4)) == "[0. 0. 0. 0.]"
  '';
}
```

In that case, `numpy` is chosen from the generic `python3Packages`.

## Test Options Reference {#sec-test-options-reference}

The following options can be used when writing tests.

```{=include=} options
id-prefix: test-opt-
list-id: test-options-list
source: @NIXOS_TEST_OPTIONS_JSON@
```