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`.

## Overriding a test {#sec-override-nixos-test}

The NixOS test framework returns tests with multiple overriding methods.

`overrideTestDerivation` *function*
:   Like applying `overrideAttrs` on the [test](#test-opt-test) derivation.

    This is a convenience for `extend` with an override on the [`rawTestDerivationArg`](#test-opt-rawTestDerivationArg) option.

    *function*
    :   An extension function, e.g. `finalAttrs: prevAttrs: { /* … */ }`, the result of which is passed to [`mkDerivation`](https://nixos.org/manual/nixpkgs/stable/#sec-using-stdenv).
        Just as with `overrideAttrs`, an abbreviated form can be used, e.g. `prevAttrs: { /* … */ }` or even `{ /* … */ }`.
        See [`lib.extends`](https://nixos.org/manual/nixpkgs/stable/#function-library-lib.fixedPoints.extends).

`extendNixOS { module = ` *module* `; specialArgs = ` *specialArgs* `; }`
:   Evaluates the test with additional NixOS modules and/or arguments.

    `module`
    :   A NixOS module to add to all the nodes in the test. Sets test option [`extraBaseModules`](#test-opt-extraBaseModules).

    `specialArgs`
    :   An attribute set of arguments to pass to all NixOS modules. These override the existing arguments, as well as any `_module.args.<name>` that the modules may define. Sets test option [`node.specialArgs`](#test-opt-node.specialArgs).

    This is a convenience function for `extend` that overrides the aforementioned test options.

    :::{.example #ex-nixos-test-extendNixOS}

    # Using extendNixOS in `passthru.tests` to make `(openssh.tests.overrideAttrs f).tests.nixos` coherent

    ```nix
    mkDerivation (finalAttrs: {