that run Nix operations that modify the store.

For more options, see the module
[`qemu-vm.nix`](https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix).

The test script is a sequence of Python statements that perform various
actions, such as starting VMs, executing commands in the VMs, and so on.
Each virtual machine is represented as an object stored in the variable
`name` if this is also the identifier of the machine in the declarative
config. If you specified a node `nodes.machine`, the following example starts the
machine, waits until it has finished booting, then executes a command
and checks that the output is more-or-less correct:

```py
machine.start()
machine.wait_for_unit("default.target")
t.assertIn("Linux", machine.succeed("uname"), "Wrong OS")
```

The first line is technically unnecessary; machines are implicitly started
when you first execute an action on them (such as `wait_for_unit` or
`succeed`). If you have multiple machines, you can speed up the test by
starting them in parallel:

```py
start_all()
```

Under the variable `t`, all assertions from [`unittest.TestCase`](https://docs.python.org/3/library/unittest.html) are available.

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